US3806728A

US3806728A - Electron impact spectrometer with an improved source of monochromatic electrons

Info

Publication number: US3806728A
Application number: US00344116A
Authority: US
Inventors: L Asbrink; C Lindholm
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-05-27
Filing date: 1973-03-23
Publication date: 1974-04-23
Anticipated expiration: 1991-04-23

Abstract

An electron source for generating a high intensity electron beam, the electrons of which have a limited number of discrete kinetic energies is provided. The electron source which could be used e.g. in an electron impact spectrometer, comprises a monoor di-chromatic light source, e.g., a neon plasma the light of which injected into a gas, preferably a noble gas, in order to generate photo electrons having a limited number of discrete kinetic energies.

Description

United States Patent 11 1 Lindholm et al.

1 ELECTRON IMPACT SPECTROMETER WITH AN IMPROVED SOURCE OF MONOCHROMATIC ELECTRONS [76] Inventors: Carl Einar Lindholm, Sigynvagen 5,

182 64 Djursholm, Leif Gosta A 7 Asbrink, Arbetargatan 24A, 11245 Stockholm, both of Sweden [22] Filed: Mar. 23, 1973 [21] Appl.'No.: 344,116

Related US. Application Data [63] Continuation of Ser. No. 137,162, April 26, 1971,

abandoned.

[30] Foreign Application Priority Data May 27, 1970 Sweden 7286/70 [52] US. Cl 250/305, 250/306, 250/307 [51] Int. Cl G0ln 23/00, G01t 1/36 [58] Field of Search 250/281, 305, 306, 307,

[56] References Cited UNlTED STATES PATENTS 2,457,530 12/1948 Coggeshall et a1 250/281- 1451 Apr. 23, 1974 Omura ct al 250/423 X Hillier 250/305 OTHER PUBLICATIONS High Resolution, Low Energy Electron Spectrometer, by J. A. Simpson from The Review of Scientific.

Instruments, Vol. 35, No. 12, Dec., 1964, pages Primary Examiner-William F. Lindquist 57] ABSTRACT An electron source for. generating a high intensity electron beam, the electrons of which have a limited number of discrete kinetic energies is provided. The electron source which could be used e.g. in an electron impact spectrometer, comprises a monoor dichromatic light source, e.g., a neon plasma the light of which injected into a gas, preferably a noble gas, in order to generate photo electrons having a limited number of discrete kinetic energies.

4 Claims, 3 Drawing Figures FECORDE z +20V 132 I i M/L T/PL/EF ELECTRON IMPACT SPECTROMETER WITH AN IMPROVED SOURCE OF MONOCHROMATIC ELECTRONS This application is a continuation of Ser. No. 137,162, filed Apr. 26, 1971, now abandoned.

Apparatus for irradiating a sample with electrons having a limited number of discrete kinetic energies.

The present invention refers to an apparatus for irradiating a sample with electrons having a limited number of discrete kinetic energies, e.g., in an electron impact spectrometer.

When analyzing and identifying molecules or atoms in a sample the energies of the different electron orbits in the molecules of the sample can be investigated. It is, e.g., possible to study the ionization of atoms and molecules by means of photoelectron spectrometry. The sample is then irradiated by X-ray quanta or ultraviolet light. Due to this irradiation photoelectrons having a certain kinetic energy are detached from the atoms. The kinetic energy of the photoelectron will then be determined by a difference between the energy of the radiation and the binding energy. The kinetic energy of the photoelectron is determined by feeding the electrons to an analyzer, i.e., a device through which only electrons having a certain predetermined energy are able to pass. The analyzer may consist of two concentric spherical or cylindrical electrodes between which the photoelectrons pass. If the voltage across the electrodes is chosen in a suitable way and apertures are arranged at the input and output of the analyzer only electrons having a predetermined kinetic energy will pass through the analyzer. By varying the potentials of the electrodes this energy can be changed and the number of electrons as a function of their kinetic energies can be determined. This photoelectron spectrometer is described in details e.g. in Analytical Chemistry, 42 No. 1 Jan. 1970 pp. 2OA-4OA.

Supplementary investigations of the structure of atoms and molecules can be performed by means of excitation. The excitation can be observed optically by studying the absorption in the sample of ultraviolet light of different wavelengths or by using electron impact spectrometry. Optical investigations are advanta geous in that the energy resolution easily could be made very high whereas they suffer from the drawback that the intensity is difficult to define and investigations of higher excitation energies than 7 eV requires expensive and complicated vacuum spectrographs. Electronic investigations by using electron impact spectrometry do not suffer from these drawbacks.

The intensity determination is very reliable and the complete energy range can be investigated in one sweep without any limitations. In chemical and molecule physical studies the electron impact spectrometer is thus a very useful tool. The only important drawback ofthis instrument has up until now been that the energy resolution is not as good as in the optical spectrometers.

In an electron impact spectrometer the sample is exited by electrons having a certain kinetic energy. When the electrons meet the sample they lose part of their kinetic energy and excite the atom. By determining the loss of energy of the electrons the exitation energies of the sample may be determined. In order to obtain mono-energetic electrons one has hitherto used a conventional electron gun, consisting of a glow cathode and an acceleration electrode and thus generated electrons having kinetic energies within a certain range. This range will then be at least some 250 meV. In order to obtain mono-energetic electrons a mono-chromator must thus be used. This device could, e.g., consist of two concentric cylindrical or spherical electrodes and it could be designed as the above described electron energy analyzer. Since a mono-chromator having a high resolution will have a very low degree of transmission, the number of electrons obtained from the'monochromator will decrease rapidly with a decreasing bandwidth of the mono-chromator.

From the description above it is obvious that it is desirable to provide a device in which the sample is excited by electrons having a high intensity as well as a high degree of energy uniformity. The purpose of the present invention is then to provide such a device. The characteristics of the invention will appear from the enclosed claims.

The'invention will now be described in detail in connection with the enclosed drawing in which:

FIG. 1 shows an electron impact spectrometer known per se;

FIG. 2 shows an electron source used in the arrangement according to the invention; and

FIG. 3 shows an electron impact spectrometer in which the arrangement according to the invention is utilized.

FIG. 1 shows an -electron impact spectrometer d esc ribed e. g. by Lassettre (J. Chem. Phys, 48, 5066 (1968)). The spectrometer consists of an electron source comprising a glow cathode l and an accelerating electrode 2. In the electron source electrons emit-' ted from the glow cathode are accelerated by the electrode 2 to an energy which is suitable for analysis in a mono-chromator. Because of the fact that the initial energy of the electrons emitted from the glow cathode are spread within a fairly wide range a corresponding spread of the accelerated electrons will be obtained. In

order to obtain the mono-energetic electrons the electrons from the gun are therefore supplied to a monochromator consisting of two cylindrical or

spherical electrodes

3, 4. The electrode 3 is then negative in relation to the electrode 2 and the electrode 4 is positive in relation to the same electrode. Electrons injected into the mono-chromator will thus be deflected in the space between the electrodes. Two

apertures

5, 6 are arranged at the input and output of the monochromator respectively which implies that only electrons which are located within a certain energy range will pass through the mono-chromator. The monochromator could then be designed so as to make this energy range as small as required. However, if the energy range in the mono-chromator is decreased, the transmission will likewise decrease, i.e., the number of electrons passing through the mono-chromator will be very small. The electrons obtained from the monochromator are accelerated to a suitable velocity by means of an electrode 7 and they are supplied to an impact chamber 8 in which the sample to be investigated is introduced. The impacting electrons will then lose some of their energy when they meet the gas molecules in the chamber and thus leave the chamber with a reduced energy. The electrons leaving the chamber will then pass through an aperture in an electrode 9, the potential of which determines the velocity .of the electrons supplied to an analyzer of the same design as the monochromator and consisting of two

electrodes

10, 11. By varying the potentials of the electrodes of the analyzer electrons having different kinetic energies will pass between the electrodes. The output of the analyzer is connected to a detector 12, preferably an electron multiplier, provided with an aperture 13, the output of the multiplier being connected to a recorder 14, e. g. a plotter. In this plotter one could thus obtain a curve that defines the number of electrons as a function of the energy loss and in-this way the sample in the chamber 8 could be analyzed as to quantity as well as to quality. The drawback of the above described device is that the resolution of the instrument is limited by the energy spread of the electrons that are injected into the sample. It is thus very essential to provide an electron source that irradiates electrons within a very narrow energy range.

In FIG. 2 an electron source used in the apparatus according to the invention is shown, this electron source making it possible to obtain electrons within a much narrower energy spectrum than previously possible. In FIG. 2 reference 15 denotes a container containing, e.g., neon. The neon is supplied to a microwave cavity 17 via a pressure reducing valve 16, the microwave cavity being connected to a microwave generator 18, e.g'., a magnetron. The magnetron generates electromagnetic waves of a frequency of, eg. 2.5 GHz. The neon will thus then be exited and generate'photones having the energies 17.85 and 16.67 eV. The energy uniformity of the generated radiation will then be extremely high. The microwave cavity 17 is connected to a vacuum tank 20 via a tube 19, the tank in turn being connected to a brass cylinder 22 via a tube 21. The neon will then be evacuated from the vacuum tank 20 whereas the radiation passes into the brass cylinder. The other end of the brass cylinder is connected to a gas container 24, containing, e.g., argon, via a reducing valve 23. The neon light supplied to the cylinder will then generate photoelectrons from the argon, these photoelectrons having extremely well defined energy levels. Thus photoelectrons having the energies 0.73 eV, 0.91 eV, and 1.09 eV are obtained, the spreading within the respective energy levels being about 2 meV, i.e., a very high energy uniformity will be obtained. These electrons leave the brass cylinder via an aperture 25 surrounded by a brass plate 26. ()utside the brass plate 26 another brass plate 27 is arranged, a retarding voltage being supplied to this latter plate. The voltage obtained from a voltage source 28, having its positive terminal connected to the plate 26, is then preferably somewhat higher than 1 eV so that only electrons having the energy 1.9 eV will pass the plate 27, the energy of these electrons, thus being reduced to about 0.09 eV. By using the device defined above extremely mono-energetic electrons could thus be obtained. One could e.g. easily obtain an intensity sufficient for a spectrometer while keeping the electron energies within a range which is less than meV. In order to obtain the above mentioned energy uniformity is, however, very essential that no disturbing fields are present in the container 22. It is thus very essential that no surface charges are present on the inside of the container. During experiments it has been found that these surface charges could be avoided if the surface is pro vided with a thin layer of colloidal graphite. It is also obvious that the mono-chromator could be designed in accordance with the mono-chromator'shown in FIG. 1

but having a much lower resolution (0.1 eV) and consequently a higher transmission. A magnetic monochromator could also possibly be used but such a mono-chromator would probably give rise to disturbing fields in the brass cylinder.

In FIG. 3 an electron impact spectrometer using the electron source of FIG. 2 is shown. The spectrometer comprises the brass container 22, the aperture 25, the brass plate 26 and the retarding electrode 27. The electrode 27 is then connected to earth whereas the container and the aperture 26 have the potential 1 V. The spectrometer further comprises an impact chamber 8 in which the sample to be analyzed is introduced.

The impact chamber has a potential of +300 V. Consequently the electrons from the .electron source will have a high velocity within the chamber where they meet the molecules of the specimen. Some electrons will then lose an amount of energy corresponding to the excitation energies of the sample. After the chamber a brass plate 29 with an converging aperture 30 is arranged, the plate having a potential of +20V. The kinetic energy of the electrons leaving the aperturej30. is thus 20 eV minus the loss of energy deriving from the excitation in the chamber. It is thus essential that the loss of energy of the electrons passing the chamber is less than 20 eV as otherwise the electrons will not pass the plate 29. However, most substances have excitation energies below 15 eV and thus 20 eV is a suitable voltage'as it will give a veryhigh resolution of the different energy levels of the specimen. After passing the aperture 30 the electrons are transferred into an analyzer consisting of two spherical or

cylindrical electrodes

10, 11, the analyzer having essentially the same design as the analyzer of FIG. 1. Electrons leaving the analyzer pass an aperture 13 having the same potential as the aperture 30, i.e., no further acceleration is obtained in the analyzer, and pass into an indicating apparatus 12, suitably an electron multiplier. The output of the indicator is connected to a recorder 14, e.g., a plotter. The potentials of the

electrodes

10 and 11 are preferably symmetrical to the potential of the aperture 30 and furthermore these potentials can be varied in order to obtain the spectrum in the plotter 14 of electrons having different energies. It is of course also possible to obtain a spectrum by varying the potential of the plate 29. Thepotential drop between the plate 29 and the electrodes l0, l1 and 13 should than be constant.

In connection with this it should also be emphasized that it is not necessary to make the electrons generated in the brass cylinder pass a mono-chromator before they are supplied to the chamber 8. It is namely also possible to evaluate the energy losses of the electrons even if they originally have more than one discrete kinetic energy. It should also be pointed out that the above described device could be used as a photo electron spectrometer by removing the chamber 8 and introducing the specimen in thebrass cylinder.

According to the invention an electron source from which low energy electrons having a high degree of energy uniformity is provided. The use of such an electron source is of course not limited to an electron spectrometer. Mono-chromatic electrons .could, e.g., be used in mass spectrometers where ionized molecules or parts of molecules formed in electron impacts are in vestigated. Mono-chromatic electrons could also be used in diffraction investigations of crystals.

We claim:

1. Apparatus for bombarding a sample with electrons having a limited number of discrete kinetic energies, characterized in that the apparatus comprises a gas discharge plasma light source generating a limited number of spectral lines, a gas container devoid of surface charges and disturbing fields containing a gas, means for injecting the light from said plasma light source into said gas container to generate photoelectrons having a plurality of energy levels from the gas molecules contained therein, said gas container also being provided with an aperture through which said generated electrons leave the container, an impact chamber for a sample to be analyzed, said impact chamber being located to position said sample to be bombarded by generated electrons leaving said aperture, energy discriminating means located between said aperture and said impact chamber for limiting the passage therethrough of electrons having only a single kinetic energy, and means for measuring the change in energy or diffraction of said generated electrons subsequent to impact with a sample.

2. Apparatus according to claim 1, characterized in that the inner surface of the gas container is provided with a layer of colloidal graphite for eliminating the forming of field generating surface charges.

3. Apparatus according to claim 1, characterized in that said light source is a neon plasma.

4. Apparatus according to claim 1, characterized in that said gas is argon.

Claims

4. Apparatus according to claim 1, characterized in that said gas is argon.