US3836775A

US3836775A - Electron impact spectrometer of high sensitivity and large helium tolerance and process of characterizing gaseous atoms and molecules by the energy loss spectrum

Info

Publication number: US3836775A
Application number: US00339544A
Authority: US
Inventors: S Ridgway
Original assignee: Princeton Applied Research Corp
Current assignee: Princeton Applied Research Corp
Priority date: 1973-03-08
Filing date: 1973-03-08
Publication date: 1974-09-17
Anticipated expiration: 1991-09-17

Abstract

Process of and apparatus for measuring the energy loss of electrons in collisions with gaseous material, e.g., gaseous sample atoms and molecules for the purpose of characterizing (identifying, discriminating between or determining the molecular structure of) the atoms or molecules by the electron energy loss spectrum are provided. A current of low energy electrons is introduced into a region defined by being enclosed in a fine wire grid. The sample atoms or molecules are also in this region. The sample may or may not be diluted with helium gas. Electrons that have collided with the sample or with helium attempt to diffuse from this region out toward a collector electrode whose potential may be varied during the course of obtaining a spectrum. In general, if the kinetic energy of an electron is sufficient for it to surmount the potential hill (in the gravitational plot sense) or electrostatic potential between the collector and the grid, it will eventually be absorbed by the collector, since the area of the collector is purposely made enormously greater than that of the grid. If the kinetic energy of the electron is insufficient to reach the collector, i.e., is insufficient to surmount the potential hill or electrostatic potential between the potential of the grid and the potential of the collector, the electron is eventually absorbed on the grid. Thus the current to the grid is made up of those electrons that have lost sufficient kinetic energy in collisions with the sample a atoms or molecules that they cannot surmount the electrostatic potential between the grid and collector and reach the collector. The measure of this current as a function of the collector potential, e.g., the derivative of the current to the grid with respect to the collector potential, provides a spectrum of energy loss of the electrons particularly useful in characterizing, i.e., identifying, discriminating between, or determining the molecular structure, of the sample gaseous atoms or molecules.

Description

llnite States Patent Ridgway Sept. 17, I974 [75] Inventor: Stuart L. Ridgway, Princeton, NJ.

[73] Assignee: Princeton Applied Research Corporation, West Windsor Township, NJ.

221 Filed: Mar. 8, 1973 21 Appl.No.:339,544

[52] US. Cl. 250/305, 250/306 [51] Int. Cl. G0ln 23/02 [58] Field of Search 250/305, 306, 307

[5 6] References Cited UNITED STATES PATENTS 6/1972 Golden 250/305 OTHER PUBLICATIONS Electrical Differentiation For Energy Loss Analysis, Curtis, Journal of Physics E. (Great Britain) Vol. 3, Nov. 1970, 250305.

A New Method to Study Molecules By Electron Impact Spec, Knoop, Chemical Physics Letters, Vol. 5, No. 8, 6/70, 250-305.

Primary Examiner-James W. Lawrence Assistant Examiner-C. E. Church Attorney, Agent, or Firm-Popper, Bain, Bobis, Gilfillan & Rhodes [57] ABSTRACT Process of and apparatus for measuring the energy loss of electrons in collisions with gaseous material, e.g., gaseous sample atoms and molecules for the purpose of characterizing (identifying, discriminating between or determining the molecular structure of) the atoms or molecules by the electron energy loss spectrum are provided. A current of low energy electrons is introduced into a region defined by being enclosed in a fine wire grid. The sample atoms or molecules are also in this region. The sample may or may not be diluted with helium gas. Electrons that have collided with the sample or with helium attempt to diffuse from this region out toward a collector electrode whose potential may be varied during the course of obtaining a spectrum. In general, if the kinetic energy of an electron is sufficient for it to surmount the potential hill (in the gravitational plot sense) or electrostatic potential between the collector and the grid, it will eventually be absorbed by the collector, since the area of the collector is purposely made enormously greater than that of the grid. If the kinetic energy of the electron is insufficient to reach the collector, i.e., is insufficient to surmount the potential hill or electrostatic potential between the potential of the grid and the potential of the collector, the electron is eventually absorbed on the grid. Thus the current to the grid is made up of those electrons that have lost sufficient kinetic energy in collisions with the sample a atoms or molecules that they cannot surmount the electrostatic potential between the grid and collector and reach the collector. The measure of this current as a function of the collector potential, e.g., the derivative of the current to the grid with respect to the collector potential, provides a spectrum of energy loss of the electrons partic-' ularly useful in characterizing, i..e., identifying, discriminating between, or determining the molecular structure, of the sample gaseous atoms or molecules.

55 Claims, 6 Drawing Figures i I I I 1 I 4 r ANALYSIS SPACE I ELECTRODE & 25

- l0

SHIELDASAMPLE

1 ,17 HEATER PI ELECTRON SOURCE a EELECTRON LENS SYSTEM 0 s3 INJEcToRQ I ELECTROD pi, 92 F," asPAcE GRID ZCOSLPLAEEMQ I 9 COLLECTOR S P E R SYSTEM IB INSULATION ,NACUU M ENCLOSURE l3 HELIUM GAS CHROMATOGRAPH PATENIEU SEP 1 1974 sum 3 [If 5 A w. GEO

wmddm OP xUOJm PATENIEDSEFI mm SHEET Q 0F 5 FIGA ELECTRON IMPACT SPECTROMETER OF HIGH SENSITIVITY AND LARGE HELIUM TOLERANCE AND PROCESS OF CHARACTERIZING GASEOUS ATOMS AND MOLECULES BY THE ENERGY LOSS SPECTRUM BACKGROUND Prior art electron impact spectrometers for the purpose of measuring the energy loss spectrum in collisions between an electron beam and atoms and molecules have generally used the principle of preparing a moderately or highly monoenergetic beam of electrons in a high vacuum electron monochromator, then impacting them on a gas target in a suitable differentially pumped target box, and then selecting some, or a range of angles from those electrons emerging from the target box, with a specific energy. The energy acceptance of the emergent electron analyzing device is scanned to provide the energy loss spectrum. Although high spectral energy resolution is available in this manner, the current of analyzed electrons is small, and a long time is required to acquire a spectrum. Furthermore, the presence of an inert gas such as helium in the sample causes a strong scattering of the electrons out of the beam, and further weakens the signal available. This reduces the potential utility of the device for qualitative chemical analysis of the molecules that would emerge from a gas chromatograph, where dilution with helium is a most common feature of such samples.

SUMMARY The present invention provides a novel process for characterizing gaseous material, gaseous atoms and molecules, and a novel electron impact spectrometer, and which novel process and apparatus are particularly useful with samples that are carried in a large excess of helium. Further, such novel process and apparatus provide sufficient sensitivity so as to enable the acquisition of a spectrum in a fraction of a minute and with sufficient resolution to enable the spectra obtainable to be sufficiently characteristic of the particular sample molecule or atom so that discrimination can be made between quite similar atoms and molecules.

DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic representation of a spectrometer system embodying the present invention and showing those elements that prepare the electron current, provide for its interaction with the sample, and for the collection of the electrons after their interactions;

FIG. 2 is a schematic representation of a signal processing block showing, for example, those elements that apply the appropriate voltages to the various electrodes and structures of the present invention;

FIG. 3 is a diagrammatic representation of a gas chromatograph in connection with which the present invention is particularly useful;

FIG. 4 is a diagrammatic representation of means, according to the present invention, for providing energy selection on the electrons to be impacted on the sample atoms or molecules to be characterized, and

FIGS. 5 and 6 are shown diagrammatic representations of space grids.

DESCRIPTION OF THE INVENTION In the description of the present invention, it will be understood by those skilled in the art that the motion of the electrons is considered to be in substantially constant electrostatic fields, i.e., the fields are varied extremely slowly relative to the transit time of the electrons through the apparatus of the present invention, or the time of propagation of a light signal across the apparatus. Furthermore, there are no substantial time varying magnetic fields. In this situation the electric field possesses a potential, whose gradient is the electric field. The normal unit of potential difference is the volt, and this will be used. The potential energy of a change q at a place where the electrostatic potential is qS is qzb. A field for which the potential energy function exists is called a conservative field, and is a field for which the conservative of energy holds true as the particle moves through the field. In this apparatus, therefore, as long as the electron moves in the electrostatic fields, and does not undergo inelastic collisions with molecules or surfaces, the total energy is a constant of the motion where the total energy is defined by the well known relation E=T V, where E is the total energy, T the kinetic energy 1/2 mv and V the potential energy qdJ. The common simplification that is frequently used in electron optics will be adopted by choosing as l the unit of energy the electron volt which is the energy change in displacing one electron charge through a potential difference of one volt. Kinetic energy will be measured in electron volts, and the energies of the ex cited states of molecules will be also so measured. Since the charge on the electron is negative, the electron potential energy and the conventional electrostatic potential have, by the above conventions, equal magnitudes and opposite signs. Thus it will be said that the potential of a region is X volts, whereas the potential energy of an electron in such a region would be X electron volts. The choice of the zero of the electrostatic potential is arbitrary, and the only physical measurable quantities are potential differences. It is ponderous and confusing to continuously refer to potential differences relative to some origin, so therefore in this specification and claims when the terms potential or potential energy are used without reference to a difference it will be understood that these potentials are referred to the reference zero of potential energy such that the average electron emitted from the electron source would have zero kinetic (and total energy) at this reference zero of potential. The thermionic cathode is approximately at this so defined zero of potential, and it would be exactly so if the electrons emitted from it were emitted at zero velocity. The small thermal velocity at which the electrons are actually emitted correspond to an energy of about /2 electron volt, so the cathode potential is thus /2 volt positive relative to the previously defined zero of potential, and the electrons leaving the cathode have /z electron volt of potential energy, and /z electron volt of kinetic energy. By this definition of the zero of potential the electron total energy is zero after emission from the cathode, and remains so during the conservative motion through the apparatus. When it loses energy in an inelastic collision with a molecule or atom the totaI energy becomes negative. The kinetic energy can never be less than zero. In all regions accessible to the original electrons the electrostatic potential is positive, and the electron po tential energy is negative. Once an electron has lost some energy in a collision, e.g., 6 electron volts, the total energy of this electron would be 6 electron volts. Only those portions of the apparatus that have an electrostatic potential more positive than 6 volts, and a corresponding potential energy for electrons more negative than 6 electron volts, are accessible to such an energy degraded electron.

Referringnow to FIG. 1 and the spectrometer system shown therein, there is shown an electron source 1 at a predetermined potential, the electron source may be for example, a resistance heated tungsten or similar filament that emits thermionic electrons. These electrons are introduced into a collision space 24 by the assistance of an electron lens system 2 and an injector electrode 20. The potential relative to the potential of the electron source is established by a surrounding space grid 8 which is at a predetermined potential with respect to the potential of the electron source. Upon being introduced into the collision space 24, the electrons are provided with kinetic energy substantially equal in electron volts to the potential in volts of the space grid with respect to the potential of the electron source 1.

The space grid 8 is substantially surrounded or enclosed by a collector electrode 9 which may have metal structures as shown formed upon it to form pockets for the entrapment of electrons. The collector electrode is also at a predetermined potential with respect to the potential of the electron source and, as taught in detail infra, the relative potentials of the space grid and collector electrode may be varied. The space between the collision space and the collector is called the analysis space 25. It is in this space that the electrons are classified according to whether their kinetic energy is sufficient that they may surmount the electrostatic potential (sometimes referred to by those skilled in the art as a hill in the sense of a gravitational plot) between the potential of the collector 9 and the potential of the space grid 8 relative to the potential of the electron source 1. The collector 9 and the space grid 8 are both enclosed in a nearly completely closed conducting structure 10 referred to as the shield and sample can, since it serves both the function of shielding the spectrometer active region from exter al electric fields, and also suffices to contain the sample. A sample of gaseous material, e.g., gaseous atoms or molecules, is introduced through the sample inlet tube 6, and escapes from the sample can 10 by way of the electron entrance orifice 33 in the injector electrode 20. If the escape of a sample after analysis is not sufficiently fast through this orifice 33, the valve 26 may be opened by energizing solenoid 30 which attracts iron piece 28 against the resistance of spring 29, the actuation being transmitted by rod 27. On release of the energization of solenoid 30, the spring 29 recloses valve 26.

The spectrometer system is evacuated by vacuum pump system 14. Pressures in the shield and sample can 10 may range between 10 torr to .l torr. The spec trometer system is enclosed in a vacuum enclosure 13. Heating means 17 enclosed by insulating means 18 is provided for convenience in cleaning and outgassing the spectrometer system, and for running the spectrometer at an elevated temperature when it is desirable for minimizing the absorption of the sample molecules on the surfaces interior to the shield and sample can 10. Pressures in the vacuum enclosure 13 should be sufficiently low to get good operation of the electron source 1 and the electron lens 2, and should be better than about 10 torr, and are typically in the range of 10 torr to 2 X 10 torr depending on the sample loading.

With regard to the general operation of the spectrometer system of FIG. 1, it will be assumed that the potential of the electron source 1 is zero volts and that the potential of the space grid 8 relative to the electron source 1 is a fixed or constant +20 volts, a relative voltage greater in volts than the expected loss of kinetic energy in electron volts to be experienced by the electrons in colliding with the sample gaseous atoms or molecules. Thus, the electrons from source 1 that have passed through the electron lens system 2 and orifice 33 and which are inside grid 8, and which have suffered no energy loss in collision with a sample atom or molecule in the collision space 24, will be provided with kinetic energy of +20 electron volts. It will also be assumed that the potential of the shield and sample can 10 is approximately 4 volts relative to the electron source 1, thus no electron from the collision space 24 will have sufficient kinetic energy to reach it. The bottom of the shield and sample can 10 may be provided with a slightly inclined portion generally opposite the entrance of the electron beam to form an electron mirror 12. Thus the electrons may make a multitude of oscillations up and down through the collision space 24 being reflected by the shield and sample can 10, and not escaping through the potential valley or pass provided by the injector electrode 20 for the entrance of the electron beam. Upon coming close to the space grid 8, or being scattered by a sample gas atom or molecule, the path of the electron is changed, and it may be directed toward the collector electrode 9. If the potential of the collector electrode 9 is above (more positive) +20 volts, all electrons make it to the collector electrode on the first try. As the potential of the collector electrode relative to the potential of the electron source 1 is decreased (becomes less positive), the number of tries that an electron must make in order to find that its motion is sufficiently directed against the electrostatic potential or potential barrier between the space grid 8 and the collector 9, increases; thus, it will be understood by those skilled in the art that the potential of the collector electrode is a retarding potential. In general, each time the electron reenters the collision space 24, it may be scattered in a new direction to enable it to make it to the collector electrode on the next try. In the absence of collisions with sample atoms or molecules to be characterized, close passes of the grid wires comprising the space grid 8 serve the same function. It is important for good energy resolution that the wires comprising the space grid 8 be very fine, and openly spaced in order that most of the electrons that are collected or adsorbed by the space grid are those that are energetically incapable of making it to the collector 9; i.e., electrons that have insufficient kinetic energy to surmount the electrostatic potential or potential barrier between the space grid- 8 and collector electrode 9. It is also important for good energy resolution that the grid be supported in a manner that the area of conducting material at the potential of the space grid be not increased substantially over that of the space grid, thus the metal that supports the grid in the embodiment shown in FIG. 6 is maintained at a negative potential; and the grid in the embodiment shown in FIG. 5 is supported on high quality insulating material.

It has been found to be particularly advantageous in the achievement of the desired energy resolution in the practice of the present invention, to structure the space grid 8 such that the probability of the capture or collection of an electron by the space grid 8 in a single pass of the electron from the analysis space 25 through the collision space 24 and on to the analysis space 25 again, be less than .03. The current signal provided by collecting the electrons could be taken as the collector electrode current, or as the current to the space grid, or as the difference of the two.

It is preferable, although not essential to take the signal from the space grid. There are two advantages to taking the signal from the space grid. The first is that the shot noise of the base current is the major source of noise in the system, and the noise that limits the ultimate sensitivity of the system. In the spectral region where the interesting lines of many compounds lie, the current to the space grid is small, and the current to the collector electrode is large. Since the shot noise power is proportional to the current, the signal is available with less accompanying shot noise at the space grid. The second advantage of using the space grid as the signal output electrode is that the capacity of this electrode to ground, and to other electrodes is very small. This minimizes the contribution of the voltage noise of the amplifier to the output noise level. Accordingly, as shown in FIG. 2, the space grid is connected via lead 16 to the input of low noise amplifier 35. A feedback resistor R, is connected from the output of this amplifier to the input in the well known virtual ground configuration. The amplifier is floated, the other terminal being connected to the voltage source that is set or controlled to the potential to which it is desired to set the space grid 8.

It will be further understood'by those skilled in the art that if the sample characterizing current is provided by electrons collected at the collector electrode 9, such current will be comprised of electrons having sufficient kinetic energy to surmount the electrostatic potential barrier, between the space grid 8 and collector electrode 9, including electrons having sufficient residual energy after colliding with the gaseous sample atoms or molecules. Further, if the sample characterizing current is provided by electrons collected at the space grid 8, such current will be provided by electrons having insufficient residual kinetic energy after colliding with the sample gaseous atoms or molecules to surmount the electrostatic potential barrier between the space grid and the collector electrode upon the difference in potential in volts between the collector electrode and the electron source potential being less than the kinetic energy in electron volts lost by the electrons in collisions with the sample.

To further understand the operation, further reference is made to FIG. 2 commencing with a suitable programmer 46. This programmer may be coupled, for example, to the peak detector of a gas chromatograph. Once it is apparent that there is sufficient material from the chromatograph to enable a spectrum to be run, and the sample flow time from the thermal conductivity detector and the charging time of the spectrometer volume have been properly allowed for, a signal is sent to a suitable sweep source 37. The potential outputs of the sweep source are referenced to the electron source via lead 23 to the spectrometer. The sweep voltage is transmitted to a suitable display device, such as a suitable x-y plotter 39, and to further signal processing devices such as a suitable computer 42 via a multiplexer 40 and a suitable analog to digital converter 41. The sweep is also sent to the excitation control 44.

One mode of operating the spectrometer is to keep the potential of the space grid 8 fixed or constant while the collector electrode voltage is being swept or varied to scan the spectrum; for example, and in accordance I with the present invention, the potential of the collector electrode relative to the electron source potential is swept between a value more positive than the potenial of the space grid relative to the electron source potential, to a value negative with respect to the electron source potential. It has also been found advantageous to sweep or vary the voltage of the space grid 8 as the voltage of the collector electrode 9 is swept or varied to scan the spectrum. One mode is proportional varying or sweeping in that the space grid potential relative to the electron source potential is made some constant factor times the collector electrode potential relative to the electron source potential, this factor usually being between l.l and 2.0. The advantage of sweeping or varying the space grid potential is that in general for wide classes of compounds the lower energy lines of the molecules are better excited with electrons of lower energy than is necessary to excite higher energy levels. Another mode of excitation of considerable utility is to sweep the potential of the space grid 8 in parallel with that of the collector electrode 9, with the potential .of the space grid being maintained at a predetermined'constant difference of at least one volt, typically between l and 6 volts, above (more positive) that of the collector electrode. In this mode, the resolution of the energy analyzer is constant, since one is always looking at those electrons that have residual kinetic energy after the energy loss collision with sample atoms or electrons equal to the difference in potential between the space grid and the collector potential. It will be understood by those skilled in the art that such relative varying of the potential of the space grid 8 also varies the kinetic energy provided to the electrons accordingly.

The gaseous sample is characterized by the current of collected electrons, in the various alternative modes taught above, and more significantly in accordance with the present invention, the gaseous material is characterized by being measured as a. function of the electrostatic potential established by difference in potential between the respective potentials of the space grid 8 and collector electrode 9 with respect to the potential of the electron source. Still more particularly, the gaseous material may be characterized by measuring the current as a function of the electrostatic potential by taking the derivative of such current with respect to the potential of the space grid 8 or the collector electrode 9 with respect to the potential of the electron source 1.

In one embodiment of the invention, and still referring to FIG. 2, the derivative of the space grid current with respect to the varying collector electrode potential was taken to characterize the sample gaseous material and obtained by an ac signal. technique. A suitable oscillator 43 operating at a frequency f provides a small ac modulating voltage of approximately 50 mV rms which is added to the sweep voltage supplied to the collector electrode 9. To understand this method of modulation, it will be understood that at a particular collector to space grid potential difference, electrons of greater energy than this potential difference will collect on the collector electrode with high probability, while electrons with less energy than this potential difference are compelled to be collected on the space grid. If there is a transition in the sample atoms or molecules such that there is a class of electrons with energy equal to the potential difference between the space grid electrode and the collector electrode (reference value), this class of electrons will be partitioned between the space grid and collector electrodes. If. however. the potential difference between the space grid and the collector electrode is slightly lowered, the electrons, after the energy loss collision, have energy which will be slightly greater than this potential difference and most of the electrons of this collision class will make it to the collector electrode. Similarly, let it be assumed that the potential difference between the space grid and the collector electrode be raised above the original reference value, i.e. above the difference in potential between the space grid and collector electrode. Under this condition the electrons cannot reach the collector electrode, and will be collected by the space grid electrode. Thus, it will be understood that upon the modulation of the space grid collector electrode potential difference, there will be an alternating current to the space grid equal to the product of the slope of the space grid vs. collector voltage function and the amplitude of the space grid-collector electrode potential difference modulation.

Thus, it will be understood that the ac modulation voltage provided by the oscillator 43 applied to the sweep voltage applied to the collector electrode 9 alternately raises and lowers the space grid-collector potential difference above and below the above-noted reference value. Upon such ac modulation, the electron provided current to the space grid 8 will have an ac component substantially equal to the product of the modulation amplitude applied to the collector electrode and the derivative of the space grid current with respect to the collector voltage. This is the desired electron energy loss spectrum. The space grid current signal is converted to a voltage signal by low noise amplifier 33. This output signal is transmitted to amplifier 34, which further amplifies the signal, and removes from the output of amplifier 33 that component of the output that is due to the sweeping of the potential of the space grid 8. If collector voltage modulation is used, then the output signal from amplifier 34 may be processed in suitable lock-in

amplifiers

34 and 45. Lock-in amplifier 35 has a reference for its mixer that is the modulating signal developed by the oscillator 43. Further sharpening and enhancement of detail in the spectra may be obtained by the use of further lock-in amplifiers whose mixer reference is a 2nd, 3rd, or higher harmonic in coherent phase relation with the fundamental modulating signal applied to the collector. The use of the higher harmonics gives spectra that are a higher derivative of the space grid current versus collector voltage function. A lock-in amplifier 45 operating on the third harmonic reference is shown. Depending upon the type of display device available, the outputs of the two lock-ins may be displayed alternatively or simultaneously.

Alternatively, the output signal from amplifier 34 may be amplified in amplifier 36, and the appropriate derivatives of the space grid current with respect to the collector electrode potential may be obtained by means of filter 38, and the signal transmitted to the display device 39. Alternatively, the signal from amplifier 34, and the sweep from the sweep source 37 may be conducted to the digital computer 42 by way of the time division multiplexor 40 and the analog to digital convertor 41. The digital computer may process the spectral information, display the spectra in analog devices, match them with other spectra stored in memory, and provide signals to control the programmer and the excitation control, and other components of the system.

As noted above, although a gas chromatograph is not necessary as a sample source for the system, the present invention has utility separate and apart from chromatography, one of the major advantages of the present invention is that it is uniquely able to process samples that are obtained from a gas chromatograph. One method of coupling the present invention to a gas chromatograph is illustrated in the block diagram of FIG. 3. The sample to be characterized, which sample in general will be a mixture of materials that it is desired to separate by the chromatograph column, is injected into the column at the sample inlet 60. The flow of helium from the helium cylinder 20 sweeps the sample through the column, and sweeps different materials with different affinities for the material of the column at different rates. The components emerge from the column separated in time carried by the helium gas. The presence of a component in the carrier gas is detected by a suitable thermal conductivity detector 47. ln order to enhance the sensitivity for components present in low concentration, some of the helium may be removed from the stream by a suitable separator 48 such as commonly used for the same purpose with mass spectrometer peak identifiers. The discarded helium is removed by a suitable mechanical vacuum pump 49. Between the output of the separator 48 and the spectrometer inlet 6 may be provided

suitable reservoirs

50, 51 and 52 for holding the various components as they emerge from the column and separator, and for scheduling them into the spectrometer inlet 6 after a previous peak has been analyzed. These reservoirs can serve an additional function of providing a reserve of material so that the concentration of analyte in the spectrometer chamber chamber 10 may be maintained constant during the analysis of a sample.

With regard to the general operation of a cycle of the chromatograph of FIG. 3, it will be assumed that the shield and sample can has been emptied through valve 26, that the reservoirs are empty by having been evacuated through

valves

61, 62 and 63, and that the output of the separator 48 has been going to waste through valve 59. On the appearance of a signal from the terminal conductivity conductor 47, programmer 46 (F162) allows for the transit through the separator and opens valve 53 to admit sample to reservoir 50, and a valve 61 is closed. When a substantial amount of the peak has passed into reservoir 50, valve 56 is opened full briefly to allow the transfer of about half the material to the spectrometer shield and sample can 10 (FIG. 1), and then closed to a steady flow rate to maintain the concentration at a steady value. As the peak passes the thermal conductivity detector 47 the valve 53 is closed and valve 59 opened to send the helium to waste. As a new peak arrives as sensed by the thermal conductivity detector 47, valve 62 is closed, valve 59 is closed and valve 54 is opened to admit the material in the next peak to the reservoir 51. Upon completion of the analysis of the material in reservoir 50, and the charging of the reservoir 51 with the material in the second peak from the gas chromatograph, valve 57 is cycled as was previously cycled valve 56 to admit the sample to the spectrometer shield and sample can 10 (FIG. 1), and the analysis of this sample is commenced. As new peaks come off of the gas chromatograph, the

reservoirs

50, 51 and 52, and their valves are operated to service the peaks. Three reservoirs have been chosen for illustration, it will be understood that more or fewer can be chosen depending upon the amount'of close bunching of peaks to be anticipated, which close bunching would require a sample queue of greater holding capacity.

For increased resolution it becomes highly desirable and advantageous to provide energy selection means on the electrons emitted from the electron source 1 of FIG. 1 so as to provide a stream of electrons to the collision space 24 having a predetermined kinetic energy distribution about a predetermined energy mean or average. For example, if the electron source 1 is a heated tungsten filament operating at a suitable temperature, such as 2600Kelvin, the width of the energy distribution of the thermionic electrons is of the order of .5 volts, aand limits the attainable resolution of the present invention. It is also not necessary to provide the shield and sample can 10 in that the collector electrode 9 can be structured to provide for the'containment of the sample to be characterized. These alternatives, in accordance with the present invention, are illustrated in FIG. 4. The electron pre-analysis means or energy selection means 63 may include a cylindrical electrostatic energy analyzer, comprised of cylindrical attractor electrode 70, and cylindrical repeller electrode 69, mounted by insulating means (not shown) upon a suitable support or frame 64, which frame supports, or contains spectrometer entrance slit 65, and spectrometer exit slit 66. Electrons from an electron source, e.g., electron source 1 of FIG. 1, either directly, or through an electron imaging system (not shown), enter the spectrometer at the entrance slit 65. Electrons of the correct and selected energy transverse a circular orbit, and exit the spectrometer via the exit slit. Electrons of higher than the selected energy traverse the analysis region between the

plates

69 and 70 with a larger radius of curvature, and do not exit via the exit slit. Some ap preciable fraction of the higher energy electrons will be collected upon beam steering signal electrode 67. Electrons of lesser energy and thus radius of curvature will fall short of the exit slit, and a substantial fraction of them will be collected by beam steering signal electrode 68. The length of the arc of the electron orbit is chosen to be approximately 127 to obtain the well known single focusing of cylindrical electron energy analyzers. At the price of a small increase in cost and complexity, the well known double focusing schemes using 180 deflection in an inverse square law electrostatic field using concentric hemispheres could be used. Typical operating parameters for the cylindrical electron energy analyzer with equilibrium radius of curvature 2.4 inches, repeller electrode inner radius of curvature 2.6 inches, and attractor electrode outer radius of curvature 2.2 inches were electron source potential 0.0 volts, attractor potential +l5.0 volts, repeller potential +l0.7 volts, and frame potential +l3.5 volts. The frame potential sets the potential of the entrance aand exit slits, and whether the electrons are retarded or accelerated as they traverse the region from the slit to the analysis region between the electrodes 69'and 70. Deviation of the frame potential from a value exactly intermediate to the attractor potential allows minor adjustments to be made in the focusing properties of the spectrometer, and the spectrometer may be tuned for increased intensity. Pre-analyzed electron streams to be provided to the collision space 24, in accordance with the teaching of the present invention and as may be provided by the electron pre-analysis means 63, may advantageously be provided with a kinetic energy distribution of approximately 100 mV full width at half maximum. Higher resolutions are available at a lesser current.

The potentials on the electrodes will be chosen such that electrons of 13 volts kinetic energy exactly traverse the equilibrium orbit, between the attractor and repeller electrodes, of 2.4 inches radius of curvature. In order to provide such electrons the electron source must be different in potential from the electrostatic potential of the equilibrium orbit by 13 volts less the average kinetic energy of thermal emission of the electrons, which may be a few tenths of a volt. As cathode conditions change, due to source aging, absorption or desorption of sample material or other gas in the vacuum system, or temperature change, the energy acceptance of the spectrometer may not be centered upon the electron energy distribution of the electron source. This will cause a loss of intensity of the beam, and a small shift in the average energy of the analyzed electrons, and a consequent change in the position of the spectral features observed by the instrument. These effects can be adequately allowed for in the method of spectra analysis, in that the collector voltage be scanned across the energy of the noninteracted component of the electron beam to measure the initial beam energy. Then the positions of the spectral features may be measured with respect to this zero energy loss peak. However, it is a great convenience to have automatic means for keeping the position of this zero energy loss peak fixed, and to keep the beam intensity maximized. If the average energy of the electrons provided by the electron source is greater than that for which the energy analyzer is set, then the current to electrode 67 is enlarged, and that to electrode 68 is diminished. Thus the current signals from these electrodes are conducted to differential amplifier 91 shown in FIG. 5, electronic service block via leads 82 and 83. The output of this amplifier, which is a small negative or positive voltage depending upon whether the current to electrode 68 or to electrode 67 is larger, is added to the electron source potential pro vide via lead 15.

The introduction of the electron energy analyzer of FIG. 4 makes an alternative method of modulation attractive.

The alternative method of modulation (i.e., alternative to the above-mentioned modulation of the spacegrid collector electrode potential difference) is to apply the ac modulating voltage developed by oscillator 43 (FIG. 2) to the potential difference between the attractor and the

repeller

69 and 70 electrodes of FIG. 4. To understand this method of modulation, it will be understood that at a particular collector to space grid potential difference, electrons of greater energy than this potential difference will collect on the collector electrode with high probability, while electrons with less energy than this potential difference are compelled to be collected on the space grid. If there is a transition in the sample atoms or molecules such that there is a class of electrons with energy (reference value) equal to the potential difference between the space grid electrode and the collector electrode, this class of electrons will be partitioned between the space grid and collector electrodes. If, however, the energy of the incident electron beam or stream exiting the pre-analyzer 63 is slightly raised above the reference value, the electrons, after the energy loss collision, will have their energy also slightly raised by the same amount, because of the discreteness of the energy acceptances by the target molecules; most of the electrons of this collision class will make it to the collector electrode.

Similarly, let it be assumed that the energy of the incident electron stream of beam be lowered below the original reference value. Under this condition the electrons cannot reach the collector electrode, and will be collected by the space grid electrode. Thus, it will be understood that upon the modulation of the energy of the incident electron beam, there will be an alternating current to the space grid equal to the product of the slope of the space grid vs. collector electrode voltage function and the amplitude of the electron energy modulation. The advantage of this method is that the stray capacitances between the space grid and the energy analyzing electrodes can be made very small, so that there is very little background current in the signal channel. It has been found that the amount of ac modulation may be chosen advantageously such that the amount of modulation in the energy of the electron beam exiting from the electron energy pre-analyzer 63 is approximately between 50 and 150 millivolts.

The ac components of the electron provided current to the space grid is substantially equal to the product of the amplitude of the energy modulation of the electron beam consequent to the modulation applied at the pre-analyzer repeller and attractor electrodes, and the derivative of the space grid current with respect to the collector voltage. This also provides the desired energy loss spectrum.

It will be still further understood by those skilled in the art that the changes in current to the space grid and to the collector electrode are transfers of current from one electrode to another, and that the changes in each current are of opposite sign at the two electrodes. These currents may be conducted to a differential amplifier to gain a larger signal, and a reduction of certain types of noise that might be riding on the total electron beam. In the case of electron beam, energy modulation taught above, the output of the differential amplifier will be proportional to the algebraic sum of the ac currents to each electrode individually, which individual currents depend upon slopes and modulation as described above.

For the best functioning of the electron energy analyzer of FIG. 4 in the ordinary magnetic field of a typical laboratory a magnetic shield-80 is provided. It is a cylinder closed at one end, and extending approximately one diameter or more beyond the electron energy analyzer in the direction of the open end. A magnetic shielding material of high permeability such as molybdenum permalloy should be used, and preferably in the fully annealed condition. For a nine inch diameter cylinder a thickness of .050 incheshas been found to be satisfactory.

It is important for the functioning of the spectrometer of the present invention as an instrument of good resolution and high sensitiwity, that the ratio of the area of the space grid electrode 8 for interception of the electrons diffusing through the collision be as small as possible. In FIG. 5 is shown one embodiment which realizes this requirement, and in FIG. 6 there is shown a second embodiment which also realizes this requirement. In FIG. 5 there is shown the space grid electrode wound upon an insulating framework substantially cubical in outline. Typical dimensions which have been found to be satisfactory and which are presented merely by way of example, would be for the cube to be assembled of 2 mm dia pyrex or Corning grade 7070 glass rods. The length of the cube edges is about 10 cm. Four of the rods, forming

edges

91, 92, 93 and 94 are extended to form insulating supPorts of the structure upon the collector or upon the shield and sample can. These supports are shown in FIG. 4 as being from the collector, and in FIG. 1 as being from the shield and sample can 10. The space grid electrode may be wound of very fine wire, e.g. .0003 inch diameter. In the form of the grid winding shown in FIG. 5 the space grid wire 103 is tied to the frame edge 97, is carried up to frame edge 100, then over to frame edge 101, then down to edge 95, and then over to the beginning edge 97. The winding is continued in this manner with a spacing between turns of approximately .2 inches, more or less. When the four faces have been covered by the winding, the last turn is brought up from frame edge 97 to frame edge 100 in the neighborhood of its intersection with frame edge 94, the wire is then brought down over frame edge 102 to frame edge 98, then over to frame edge 96, up to frame edge 99 and back over to frame edge 102. The winding is continued in this manner until the two faces that were uncovered are covered. In this process the top and bottom faces are covered a second time with wire that is approximately perpendicular in direction to the first winding over these faces. In FIG. 5 the winding is interrupted at position 104 for improved clarity. The last partial turn of this winding is shown starting at position 105, carried over frame edge 96, and up the intersection of frame edge 92 and frame edge 92 and frame edge 99. Here it is tied to frame edge 92, where it is then carried up through a penetration in the sample can that has small gas conductance and good insulating value to become space grid signal lead 16. When the winding is brought from frame edge 100 to frame edge 102, it may be tied to frame edge 94 for additional security. The wire that is best for the space grid is too fine for convenience as a signal lead, and it has been found convenient to splice the grid wire 103 to a larger diameter lead wire 16 to ease the bringing out of the lead through the container that holds the gas sample. This structure provides a large volume for interaction of electrons and the sample gas and the interior of this structure is mostly at a relatively constant electrostatic potential, namely that of the space grid itself.

In FIG. 6 there is shown an alternative embodiment for providing the space grid electrode. In this embodiment the wire forming the space grid electrode is wound in a single spiral over the four insulating

rods

110, 111, 112 and 113. As before, these rods may be 2 mm dia pyrex or Corning grade 7070 glass. The rods are supported by a rectangular sheet metal frame composed of

end pieces

106 and 107, and

side pieces

108 and 109 the frame is made of suitable electrically conductive metal, e.g., copper. End piece 107 has a hole 117 in it to allow the entrance of the electron beam along axis 114. Clearance for insulating purposes is provided between the winding and the

side pieces

108 and 109. The winding is shown starting at 103, brought over to insulating rod 113, and up to insulating rod 112. The winding is continued in this manner until the faces are covered, and then tied to the insulating rod 112, and then continued as space grid connection lead 16. The metal frame is maintained at a potential below that of the electron source 1, so that it cannot accept significant electron current. The potential distribution in the .box formed of the frame, and the grid wound over its open faces has a maximum in the center. The electron beam is introduced off the center of this box. The potential distribution provides radial electrical forces that tend to cause an orbiting of the injected beam around the geometrical center of the box. This arrangement provides a high probability for interaction of the beam with the sample gas molecules or atoms, and a simple method of winding the space grid electrode.

What is claimed is:

1. The process of characterizing gaseous material, comprising the steps of:

surrounding said gaseous material in a region of predetermined potential with a predetermined electrostatic potential barrier;

impacting said gaseous material with electrons of predetermined total energy;

collecting electrons having a predetermined relationship between their kinetic energy and the potential of said predetermined their kinetic energy and the potential of said predetermined electrostatic potential barrier to provide a current, said current characterizing said gaseous material; and measuring said current.

2. The process according to claim 1 wherein said collected electrons providing said current are related to the potential of said electrostatic potential barrier by having insufficient residual kinetic energy after colliding with said gaseous material to surmount said electrostatic potential barrier.

3. The process according to claim .1 wherein said collected electrons providing said current are related to the potential of said electrostatic potential barrier by having sufficient kinetic energy to surmount said electrostatic potential barrier and wherein said current includes electrons having sufficient residual kinetic energy after colliding with said gaseous material to surmount said electrostatic potential barrier.

4. The process according to claim 1 including the further step of processing said current to characterize said gaseous material.

5. The process according to claim 4 including the further step of modulating one of the potentials which establishes said predetermined electrostatic potential barrier and wherein said processing comprises the step of measuring said current as a function of said modulated potential.

6. The process according to claim 5 wherein said processing comprises the step of taking the derivative of said current with respect to said potential which is modulated.

7. The process according to claim 1 wherein said electrostatic potential barrier is varied over a predetermined range.

8. The process according to claim 1 including the further step of modulating the energy of said electrons over a predetermined energy range. i i i i 9. The process according to claim 1 wherein said electrostatic potential is provided by two potentials and wherein one of said potentials is varied in a predetermined manner with respect to the other.

10. The process according to claim 9 wherein said one potential is varied proportionally with respect to said other potential.

11. The process according to claim 9 wherein said one potential is varied to provide a constant predetermined potential difference between said one potential and said other potential.

12. The process according to claim 1 wherein said predetermined potential of said region is a substantially uniform potential.

13. The process according to claim 1 wherein said predetermined potential of said region is a varying potential.

14. The process according to claim 1 including the further step of pre-analyzing the kinetic energy distribution of said electrons impacting said gaseous material so as to impact said gaseous material with a stream of electrons having a predetermined energy distribution about a predetermined energy average.

15. The process according to claim 14 wherein said energy distribution is approximately millivolts full width at half maximum.

16. The process according to claim 1 including the further step of modulating said varying retarding potential.

17. The process according to claim 16 wherein said energy distribution is approximately 100 millivolts full width at half maximum.

18. The process of characterizing gaseous material, comprising the steps of:

providing a stream of electrons from an electron source at a predetermined potential;

providing a collision space at a predetermined potential relative to said potential of said electron source, said predetermined relative potential being greater in volts than the kinetic energy lost by said electrons in electron volts upon said electrons colliding with said gaseous material;

substantially surrounding said collision space with a variable retarding potential at a predetermined potential realtive to said potential of said electron source;

evacuating the region occupied by said collsion space and said variable retarding potential to a predetermined pressure;

introducing said gaseous material into said collision space;

introducing said electrons into :said collision space to provide said electrons with predetermined kinetic energy substantially equal in electron volts to said predetermined potential of said collision space and impacting said gaseous material with said electrons provided with said predetermined kinetic energy;

varying the predetermined potential of said variable retarding potential over a predetermined potential range;

providing a current by collecting electrons having insufficient residual kinetic energy after colliding with said gaseous material to surmount the potential difference between the potential of said collision space relative to said electron source and the varying potential of said variable retarding potential relative to said electron source upon the potential diffference between said variable retarding potential and said potential of said electron source being at least less in volts than the kinetic energy lost by said electrons in electron volts upon said electrons colliding with said gaseous material; and

measuring said current as a function of said varying retarding potential.

19. The process according to claim 16 including the further step of varying the potential of said collision space relative to said potential of said electron source to vary the kinetic energy provided said electrons over a predetermined kinetic energy range.

20. The process according to claim 19 wherein the potential of said collision space relative to said potential of said electron source is varied in a predetermined manner with respect to said varying potential of said surrounding retarding potential relative to said potential of said electron source so as to vary said kinetic energy provided said electrons in a predetermined manner with respect to said varying relative potential of said surrounding retarding potential.

21. The process according to claim 20 wherein said potential of said collision space relative to said potential of said electron source is varied proportionally with respect to said varying potential of said surrounding retarding potential with respect to said potential of said electron source so as to vary the kinetic energy provided said electrons proportionally with respect to said varying potential of said surrounding retarding potential with respect to said potential of said electron source.

22. The process according to claim 20 wherein said potential of said collision space relative to said potential of said electron source is varied to provide a predetermined constant potential difference with respect to said varying potential of said surrounding retarding potential with respect to said potential of said electron source so as to provide said electrons with varying kinetic energy in electron volts which is at a predetermined constant difference with respect to said varying potential of said surrounding retarding potential with respect to said potential of said electron source in volts.

23. The process according to claim 22 wherein said predetermined constant potential difference is at least one volt.

24. The process according to claim 22 wherein said predetermined constant potential difference is between 1 and 6 volts.

25. The process according to claim 18 wherein the potential of said collision space relative to said potential of said electron source is a predetermined relative positive potential and wherein the potential of said surrounding variable potential relative to said potential of said electron source is varied between a relative potential more positive than said relative positive potential of said collision space and a relative potential which is negative with respect to said potential of said electron source.

26. The process according to claim 18 wherein said step of measuring said current as a function of said varying retarding potential comprises the step of taking the derivative of said current with respect to said varying retarding potential.

27. The process according to claim 18 including the further step of pre-analyzing the kinetic energy distribution of said electrons from said electron source to provide a stream of electrons for impacting said gaseous material having a predetermined energy distribution about a predetermined average.

28. The process according to claim 27 including the further step of modulating the energy of said stream of electrons.

29. The process of characterizing gaseous material, comprising the steps of:

providing a source of electrons, each of said electrons from said source having approximately the same energy; providing a first region of space over which the potential energy of an electron relative to the reference zero of potential energy, which reference zero of potential energy is defined as that potential energy for electrons at which the average energy electron from said electron source would possess zero kinetic energy, is substantially negative over a substantial portion thereof, said first region of space containing said gaseous material; providing a second region of space surrounding said first region of space and over which second region of space the potential energy of electrons increases with separation from said first region of space;

bounding said second region of space with a surface at a predetermined variable potential relative to said reference zero of potential, upon said electrons from said source being introduced into said first region of space and impacting said gaseous material, certain of said electrons passing through said surface and providing a current depending in magnitude upon said predetermined variable potential, said dependence characterizing said gaseous material directing electrons from said source into said first region; and measuring said current as a function of said variable potential.

30. Apparatus for characterizing gaseous material, comprising:

means forproviding a stream of electrons;

means for surrounding said gaseous material with an electrostatic potential at a predetermined potential, for providing said electrons with predetermined kinetic energy and for impacting said gaseous material with electrons provided with predetermined kinetic energy; and

means for measuring current provided by electrons having a predetermined relationship between their kinetic energy and said predetermined potential of said electrostatic potential, said current characterizing said gaseous material.

31. Apparatus according to claim 30 wherein said current measuring means is for measuring current provided by electrons that are related to said predetermined potential of said electrostatic potential by having insufficient residual kinetic energy after colliding with said gaseous material to surmount said electrostatic potential.

32. Apparatus according to claim 30 wherein said current measuring means is for measuring current provided by electrons that are related to said predetermined potential of said electrostatic potential by having sufficient kinetic energy to surmount said electrostatic potential and wherein said measured electrons include electrons having sufficient residual kinetic energy after colliding with said gaseous material to surmount said electrostatic potential.

33. Apparatus according to claim wherein said current measuring means comprises means for measuring said current as a function of said predetermined electrostatic potential.

34. Apparatus according to claim 30 wherein said current measuring means comprises means for taking the derivative of said current with respect to said predetermined electrostatic potential.

35. Apparatus according to claim 30 further including means for varying said electrostatic potential over a predetermined range.

36. Apparatus according to claim further includ ing means for varying said kinetic energy provided said electrons over a predetermined kinetic energy range.

37. Apparatus according to claim 36 wherein said means for measuring said electron current to said grid means measure said electron current as a function of said varying potential of said electrode means, said function characterizing said gaseous material.

38. Apparatus according to claim 36 wherein said means for providing said electrons with varying kinetic energy provides said electrons with kinetic energy which is varied in a predetermined manner with respect to said varying electrostatic potential.

39. Apparatus according to claim 38 wherein said means for varying said kinetic energy provided said electrons varies said kinetic energy to provide a constant predetermined potential difference between said varying kinetic energy and said varying electrostatic potential.

40. Apparatus according to claim 38 wherein said means for measuring said electron current as a function of said varying potential of said electrode means is for taking the derivative of said electron current with respect to said varying potential of said electrode means.

41. Apparatus according to claim 38 wherein said means for varying said kinetic energy provided said electrons varies said kinetic energy proportionally with respect to said varying electrostatic potential.

42. Apparatus according to claim 41 wherein said derivative is the second derivative of said electron current with respect to said varying electrode means potential.

43. Apparatus according to claim 41 wherein said derivative is the third derivative of said electron current with respectto said varying potential of said electrode means.

44. Apparatus for characterizing gaseous material, comprising the steps of:

means for providing a stream of electrons from an electron source at a predetermined potential;

means for providing a collision space at a predetermined substantially uniform, potential relative to said potential of said electron source, said predetermined relative potential being greater in volts than the kinetic energy lost by said electrons in electron volts upon said electrons colliding with said gaseous material, and said collision space containing said gaseous material;

means for introducing said electrons into said collision space to impact said gaseous material;

means for substantially surrounding said collision space with a variable retarding potential at a predetermined variable potential relative to said potential of said electron source and for varying said surrounding predetermined retarding potential over a predetermined potential range;

means for evacuating the region occupied by said collision space and said variable retarding potential to a predetermined pressure;

upon said electrons being introduced into said collision space, said electrons acquiring predetermined kinetic energy substantially equal in electron volts to said relative predetermined potential in volts of said collision space whereby said gaseous material is impacted with said electrons of said predetermined kinetic energy;

said means for providing said collision space also for collecting electrons having ins'ufficient residual kinetic energy after colliding with said gaseous material to surmount the potential difference between said predetermined potential of said collision space relative to said electron source and the varying potential of said variable surrounding retarding potential relative to said electron source upon the potential difference between said variable surrounding retarding potential and said potential of said electron source being at least less in volts than the kinetic energy lost by saidelectrons in electron volts upon said electrons colliding with said gaseous material, said collected electrons providing a current; and

means for measuring said current as a function of said varying surrounding retarding potential.

45. Apparatus according to claim 44 wherein the potential of said collision space relative to said potential of said electron source is a predetermined relative positive potential and wherein the potential of said surrounding variable potential relative to said potential of said electron source is varied between a relative potential more positive than said relative positive potential of said collision space and a relative potential which is negative with respect to said potential of said electron source.

46. Apparatus according to claim 44 wherein said means for mmeasuring said current as a function of said varying retarding potential comprises means for taking the derivative of said current with respect to said vary ing retarding potential.

47. Apparatus according to claim 44 further including means for modulating said variable retarding potential.

48. Apparatus according to claim 44 further including means for pre-analyzing the kinetic energy distribution of said electrons so as to impact said gaseous material with a stream of electrons having a predetermined energy distribution about a predetermined energy means.

49. Apparatus according to claim 48 wherein said energy distribution is approximately millivolts full width at half minimum.

50. Apparatus according to claim 44 further including means for varying the potential of said collision space relative to said potential of said electron source to vary the kinetic energy provided said electrons over a predetermined kinetic energy range.

51. Apparatus according to claim 50 wherein said means for varying the potential of said collision space relative to said potential of said electron source is for varying such potential in a predetermined manner so as to vary said kinetic energy provided said electrons in a predetermined manner with respect to said varying relative potential of said surrounding retarding potential.

52. Apparatus according to claim 51 wherein said means for varying the potential of said collision space relative to said potential of said electron source is for varying such potential proportionally with respect to said varying potential of said surrounding retarding potential with respect to said potential of said electron source so as to vary the kinetic energy provided said electrons proportionally with respect to said varying potential of said surrounding retarding potential with respect to said potential of said electron source.

53. Apparatus according to claim 51 wherein said means for carrying the potential of said collision space relative to said potential of said electron source is for varying such potential so as to provide a predetermined constant potential difference between such potential and said varying potential of said surrounding retarding potential with respect to said potential of said electron source so as to provide said electrons with varying kinetic energy in electron volts which is at a predetermined constant difference with respect to said varying potential in volts of said surrounding retarding potential with respect to said potential of said electron source.

54. Apparatus for characterizing gaseous material, comprising:

means for providing a stream of electrons;

means for selecting a substream of said stream of electrons having a narrower energy spread than that of said stream of electrons;

grid means for establishing a region of positive potential, said region containing gaseous material to be characterized, and said grid means having a predetermined percentage of transmission for electrons;

electrode means surrounding said grid means, said electrode means for establishing a retarding field region and for accepting an electron current;

means for providing said electrode means with a varying potential;

means for introducing said substream of electrons into said region of positive potential to impact gaseous material received therein; and

means for measuring the electron current to said grid means as a function of the potential of said electrode means, said current characterizing said gaseous material.

55. Apparatus for characterizing gaseous material,

comprising:

means providing a source of electrons, each of said electrons from said source having approximately the same energy;

means providing a first region of space over which the potential energy of an electron relative to the reference zero of potential energy, which reference zero of potential energy is defined as that potential energy for electrons at which the average energy electron from said electron source would possess zero kinetic energy, is substantially negative over a substantial portion thereof, said first region of space containing said gaseous material;

means providing a second region of space surrounding said first region of space and over which second region of space the potential energy of electrons increases with separation from said first region of space;

means providing a surface at a predetermined variable potential relative to said reference zero of potential and which surface bounds said second re gion of space, upon said electrons from said source being introduced into said first region of space and impacting said gaseous material, certain of said electrons passing through said surface and providing a current depending in magnitude upon said predetermined variable potential, said dependence characterizing said gaseous material means for directing electrons from said source into said first region; and means for measuring said current as a function of said variable potential.

Claims

1. The process of characterizing gaseous material, comprising the steps of: surrounding said gaseous material in a region of predetermined potential with a predetermined electrostatic potential barrier; impacting said gaseous material with electrons of predetermined total energy; collecting electrons having a predetermined relationship between their kinetic energy and the potential of said predetermined their kinetic energy and the potential of said predetermined electrostatic potential barrier to provide a current, said current characterizing said gaseous material; and measuring said current.

3. The process according to claim 1 wherein said collected electrons providing said current are related to the potential of said electrostatic potential barrier by having sufficient kinetic energy to surmount said electrostatic potential barrier and wherein said current includes electrons having sufficient residual kinetic energy after colliding with said gaseous material to surmount said electrostatic potential barrier.

8. The process according to claim 1 including the further step of modulating the energy of said electrons over a predetermined energy range.

9. The process according to claim 1 wherein said electrostatic potential is provided by two potentials and wherein one of said potentials is varied in a predetermined manner with respect to the other.

15. The process according to claim 14 wherein said energy distribution is approximately 100 millivolts full width at half maximum.

18. The process of characterizing gaseous material, comprising the steps of: providing a stream of electrons from an electron source at a predetermined potential; providing a collision space at a predetermined potential relative to said potential of said electron source, said predetermined relative potential being greater in volts than the kinetic energy lost by said electrons in electron volts upon said electrons colliding with said gaseous material; substantially surrounding said collision space with a variable retarding potential at a predetermined potential realtive to said potential of said electron source; evacuating the region occupied by said collsion space and said variable retarding potential to a predetermined pressure; introducing said gaseous material into said collision space; introducing said electrons into said collision space to provide said electrons with predetermined kinetic energy substantially equal in electron volts to said predetermined potential of said collision space and impacting said gaseous material with said electrons provided with said predetermined kinetic energy; varying the predetermined potential of said variable retarding potential over a predetermined potential range; providing a current by collecting electrons having insufficient residual kinetic energy after colliding with said gaseous material to surmount the potential difference between the potential of said collision space relative to said electron source and the varying potential of said variablE retarding potential relative to said electron source upon the potential diffference between said variable retarding potential and said potential of said electron source being at least less in volts than the kinetic energy lost by said electrons in electron volts upon said electrons colliding with said gaseous material; and measuring said current as a function of said varying retarding potential.

29. The process of characterizing gaseous material, comprising the steps of: providing a source of electrons, each of said electrons from said source having approximately the same energy; providing a first region of space over which the potential energy of an electron relative to the reference zero of potential energy, which referEnce zero of potential energy is defined as that potential energy for electrons at which the average energy electron from said electron source would possess zero kinetic energy, is substantially negative over a substantial portion thereof, said first region of space containing said gaseous material; providing a second region of space surrounding said first region of space and over which second region of space the potential energy of electrons increases with separation from said first region of space; bounding said second region of space with a surface at a predetermined variable potential relative to said reference zero of potential, upon said electrons from said source being introduced into said first region of space and impacting said gaseous material, certain of said electrons passing through said surface and providing a current depending in magnitude upon said predetermined variable potential, said dependence characterizing said gaseous material directing electrons from said source into said first region; and measuring said current as a function of said variable potential.

30. Apparatus for characterizing gaseous material, comprising: means for providing a stream of electrons; means for surrounding said gaseous material with an electrostatic potential at a predetermined potential, for providing said electrons with predetermined kinetic energy and for impacting said gaseous material with electrons provided with predetermined kinetic energy; and means for measuring current provided by electrons having a predetermined relationship between their kinetic energy and said predetermined potential of said electrostatic potential, said current characterizing said gaseous material.

33. Apparatus according to claim 30 wherein said current measuring means comprises means for measuring said current as a function of said predetermined electrostatic potential.

36. Apparatus according to claim 35 further including means for varying said kinetic energy provided said electrons over a predetermined kinetic energy range.

43. Apparatus according to claim 41 wherein said derivative is the third derivative of said electron current with respect to said varying potential of said electrode means.

44. Apparatus for characterizing gaseous material, comprising the steps of: means for providing a stream of electrons from an electron source at a predetermined potential; means for providing a collision space at a predetermined substantially uniform, potential relative to said potential of said electron source, said predetermined relative potential being greater in volts than the kinetic energy lost by said electrons in electron volts upon said electrons colliding with said gaseous material, and said collision space containing said gaseous material; means for introducing said electrons into said collision space to impact said gaseous material; means for substantially surrounding said collision space with a variable retarding potential at a predetermined variable potential relative to said potential of said electron source and for varying said surrounding predetermined retarding potential over a predetermined potential range; means for evacuating the region occupied by said collision space and said variable retarding potential to a predetermined pressure; upon said electrons being introduced into said collision space, said electrons acquiring predetermined kinetic energy substantially equal in electron volts to said relative predetermined potential in volts of said collision space whereby said gaseous material is impacted with said electrons of said predetermined kinetic energy; said means for providing said collision space also for collecting electrons having insufficient residual kinetic energy after colliding with said gaseous material to surmount the potential difference between said predetermined potential of said collision space relative to said electron source and the varying potential of said variable surrounding retarding potential relative to said electron source upon the potential difference between said variable surrounding retarding potential and said potential of said electron source being at least less in volts than the kinetic energy lost by said electrons in electron volts upon said electrons colliding with said gaseous material, said collected electrons providing a current; and means for measuring said current as a function of said varying surrounding retarding potential.

46. Apparatus according to claim 44 wherein said means for mmeasuring said current as a function of said varying retarding potential comprises means for taking the derivative of said current with respect to said varying retarding potential.

49. Apparatus according to claim 48 wherein said energy distribution is approximately 100 millivolts full width at half minimum.

54. Apparatus for characterizing gaseous material, comprising: means for providing a stream of electrons; means for selecting a substream of said stream of electrons having a narrower energy spread than that of said stream of electrons; grid means for establishing a region of positive potential, said region containing gaseous material to be characterized, and said grid means having a predetermined percentage of transmission for electrons; electrode means surrounding said grid means, said electrode means for establishing a retarding field region and for accepting an electron current; means for providing said electrode means with a varying potential; means for introducing said substream of electrons into said region of positive potential to impact gaseous material received therein; and means for measuring the electron current to said grid means as a function of the potential of said electrode means, said current characterizing said gaseous material.

55. Apparatus for characterizing gaseous material, comprising: means providing a source of electrons, each of said electrons from said source having approximately the same energy; means providing a first region of space over which the potential energy of an electron relative to the reference zero of potential energy, which reference zero of potential energy is defined as that potential energy for electrons at which the average energy electron from said electron source would possess zero kinetic energy, is substantially negative over a substantial portion thereof, said first region of space containing said gaseous material; means providing a second region of space surrounding said first region of space and over which second region of space the potential energy of electrons increases with separation from said first region of space; means providing a surface at a predetermiNed variable potential relative to said reference zero of potential and which surface bounds said second region of space, upon said electrons from said source being introduced into said first region of space and impacting said gaseous material, certain of said electrons passing through said surface and providing a current depending in magnitude upon said predetermined variable potential, said dependence characterizing said gaseous material means for directing electrons from said source into said first region; and means for measuring said current as a function of said variable potential.