Induced Electron Emission Spectrometer Using Plural Radiation Sources

Abstract

An induced electron emission spectrometer wherein different sources are employed to irradiate the sample from different energy sources to permit the operator to distinguish the photoelectron emission lines from the Auger electron emission lines. In one embodiment, the X-ray source employs two different X-ray energy emitting materials with means for selectively energizing one or the other of said materials to irradiate the sample under investigation with selective X-ray energies. In another embodiment the sample is first irradiated with X-rays and thereafter irradiated with electrons. In another embodiment, an X-ray source and then an ultraviolet radiation source are used.

Description

BACKGROUND OF THE INVENTION

Induced electron emission spectrometers are presently utilized to perform non-destructive, direct qualitative and quantitative analysis of samples, including chemical analysis, measurement of electron binding energies, and structure determinations. One typical form of IEE spectrometer is shown and described in United States Patent Applications Ser. No. 763,691 entitled Apparatus For Performing Chemical Analysis By Electron Emission Spectroscopy filed on Sept. 30, 1968 by J.C. Helmer et al and Ser. No. 825,680 entitled Induced Electron Emission Spectrometer Having A Unipotential Sample Chamber filed on May 19, 1969 by J.C. Helmer et al, both of which are assigned to the assignee of this application, and also in the Journal of Applied Physics Letters, Vol. 13, Pages 226 - 268, (1968).

In operation, electrons emitted from a thermionic cathode are directed onto the surface of an anode of a suitable material such as aluminum or magnesium to produce soft X-rays therefrom. The X-rays, which are of a precisely known energy (e.g. 1486eV for aluminum and 1353eV for magnesium), irradiate the surface of the sample under analysis to produce photoelectrons therefrom. The photoelectrons pass through an analyzer section where the energy range of the photoelectron emission is determined, the photoelectrons being counted in an electron multiplier detector as the energy range is scanned. The photoelectron energy lines are recorded on an X-Y plotter or the like as the IEE spectrum. The energy range is a function of the element under analysis, and the photoelectron energy is also a function of the chemical environment of the particular atom.

In addition to the photoelectrons of interest, the X-ray irradiation results in Auger electrons being emitted from the sample due to the well known Auger effect, and these electrons are also counted and appear as Auger lines in the IEE spectrum. An experienced operator is able to distinguish the Auger lines from the photoelectron lines by their relative positions in the spectrum. However, less experienced operators often confuse the Auger lines with the photoelectron lines of interest and the spectrum is misinterpreted.

Auger electrons may also be produced from the sample by irradiating the sample by electron bombardment, and Auger electron analysis is often performed in this manner.

BRIEF SUMMARY OF THE PRESENT INVENTION

The present invention provides a fast and simple method and apparatus for distinguishing energy lines due to photo-electrons from energy lines due to Auger electrons in an IEE spectrum.

In one embodiment of the invention, two sources of X-rays are provided in the X-ray source section of the spectrometer, these sources being made of different materials emitting X-rays of different energies, for example aluminum and magnesium. The photoelectron line positions in the spectrum are dependent on the energy of the particular impinging X-ray, i.e. the photoelectrons have a kinetic energy E equal to the photon energy hv minus the binding energy .phi. of the electrons of the element in the sample. Therefore the detected photoelectron line positions obtained with one X-ray material will be shifted relative to the photoelectron line positions obtained using the other X-ray material. However, the Auger electron lines are independent of the X-ray energy and their positions will remain the same for the two different X-ray sources.

Therefore, by employing two successive analyzer scans using the two different X-ray sources, a shift in any observed spectrum line will immediately signify to the operator that the particular detected line is a photoelectron energy line whereas a non-shifted position line can be interpreted as an Auger electron line.

In one form of the invention, the anode which emits the X-rays in response to the electron bombardment is made in annular form with two half-circle sections, one half-circle section having an X-ray emitting surface of one material, e.g. aluminum, and the other half-circle section having an X-ray emitting surface of a different material, e.g. magnesium. One thermionic filament is positioned to direct electrons onto one of said anode sections and a seperate thermionic filament is positioned to direct electrons onto the other anode section. By selectively energizing the two different filaments, the operator may select X-rays of either of the two energy levels.

In another form of the invention, two annular, concentrically positioned anodes are utilized, each of a different X-ray emitting material. A single thermonic filament is utilized to provide electrons for bombarding the annular shaped anodes, with means for selectively directing the electrons onto one or the other of the two anodes to obtain X-rays of one or the other energy levels.

In another embodiment of the invention, the sample is irradiated with X-rays to produce photoelectrons and Auger electrons therefrom. The sample is then irradiated with electrons so that only Auger electrons are emitted from the sample. The absense of the photoelectron lines will serve the operator in distinguishing the photoelectron emission lines from the Auger emission lines.

In another embodiment, a source of ultraviolet radiation is used in conjunction with an X-ray source, the photoelectron lines from the ultraviolet radiation being shifted relative to the X-ray induced lines to thereby distinguish the Auger lines.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an induced electron emission spectrometer employing the present invention.

FIG. 2 is a face view of one form of X-ray source having two half-circle anode sections, each with a different X-ray material surface.

FIG. 3 is a face view of another form of X-ray source employing two concentric anode sections.

FIG. 4 is a schematic view of still another form of X-ray and electron beam source for practising the present invention.

FIG. 5 is a face view of the structure of FIG. 4.

FIG. 6 is a schematic diagram showing an ultraviolet radiation source used with an X-ray source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, the induced electron emission spectrometer comprises an X-ray source including an annular anode 11 provided with a surface 12 of good X-ray emitting material, for example aluminum or magnesium, and a thermionic cathode 13 and focus electrode 14 encircling the anode 11 and adapted to provide an annular shaped stream of electrons 13' bombarding the anode surface 12. The anode 11 is provided with a central channel 15 for cooling by water flow.

The X-rays emitted from the anode surface 12 are directed onto the sample 16 to be analyzed, and the photoelectrons emitted from the sample due to this irradiation pass into the analyzer section including the spherical-shaped condenser structure 17, focus control electrodes 18, cylindrical condenser 19, and electron multiplier unit 21. A sweep potential applied between the sample and the analyzer entrance permits energy selection of the emitted photoelectron admitted into the analyzer. The photoelectron count registered by the electron multiplier 21 may be plotted on an X-Y recorder 22 as a function of the analyzing energies.

Since Auger electrons emitted from the sample 16 will be focused on and counted by the electron multiplier 21 as well as the photoelectrons, lines will appear in the IEE spectrum due to the Auger electrons. From the relative positions of these lines in the spectrum, an experienced operator is able to distinguish the Auger electron lines from the photoelectron lines. Less experienced operators are easily confused by the lines.

To provide the operator with a simple, fast technique for line distinction, the X-ray anode 11 is provided in two half-circle sections 23 and 24 as shown in FIG. 2, with one half section 23 having a surface of one X-ray material, e.g. aluminum, and the other half section 24 having a surface of another X-ray material, e.g. magnesium. The thermionic filament cathode 13 is provided in two half-circle sections 25 and 26, one for each anode section. A simple switch 27 permits the operator to energize either of the cathode sections 25 or 26, thus selectively bombarding one or the other of the anode surfaces 23 or 24 to provide one or the other of the two values of X-ray energy irradiating the sample. The photoelectron lines in the spectrum will be shifted for the two different X-ray energies, whereas the Auger electron lines will remain unchanged. One sweep of the analyzer with one X-ray source energized, followed by a second sweep using the other X-ray source will quickly inform the operator whether a particular line is one due to photoelectrons or to Auger electrons.

Although a cylincrical sample is shown, other shapes such as a flat sample can be employed. The sample can also be rotated so the same surface of the sample is irradiated by both X-ray energies.

FIG. 3 shows another embodiment of the present invention wherein the anode portion of the X-ray source includes two concentric annular members 27 and 28, each having a different X-ray material on its face surface. A single thermionic cathode 29 is provided encircling the two anode sections. The operator, by selecting the potential applied to the focus electrode 14 positioned between the cathode 29 and the two anodes, can direct the electrons to bombard the X-ray emitting surface of one or the other of the anodes 27 and 28, and thus chose the X-ray energy as with the X-ray source described above with reference to FIG. 2.

It should be understood that the X-ray source may take other forms and that the X-ray materials may be those other than aluminum and magnesium without departing from the scope of this invention.

Referring now to FIGS. 4 and 5 there is shown a source of X-rays for bombarding the sample which comprises a semicircular shaped anode 31 of X-ray emitting material, and semicircular shaped thermionic cathode 32 and focus electrode 33 for providing a beam of electrons to bombard the anode and produce the X-rays as described above. An apparatus is provided for producing a semicircular beam of electrons for bombarding the sample 16 which comprises a thermionic cathode 34, repeller electrode 35 and focus electrode 36.

When cathode 32 is energized, the sample 16 is irradiated with X-rays and, during the scan by the analyzer, both photoelectron and Auger electron lines are observed. When the cathode 34 is energized in lieu of cathode 32, a beam of electrons is provided from the electron beam source comprising cathode 32 and electrodes 35, 36 to irradiate the surface of the sample to thereby emit Auger electrons therefrom but not photoelectrons. The Auger electron lines obtained may be utilized to distinguish the photoelectron lines from the Auger electron lines obtained with the X-ray irradiation.

As shown in FIG. 6, a source of ultraviolet radiation 37 may be used with the source of X-rays 38 so that the operator can switch from one to the other. The photoelectron lines emitted due to the ultraviolet radiation are shifted substantially relative to the X-ray induced line, so that the Auger lines may be readily recognized.

Claims

1. In an induced electron emission spectrometer wherein cathode means are provided for producing electrons and anode means are bombarded by said electrons to produce X-rays for irradiating a sample to be analyzed, said sample emitting charged particles, a charged particle detector, and energy selecting means for directing charged particles of selected energy onto said charged particle detector, the improvement comprising anode means comprising at least two different X-ray irradiating materials and cathode means for selectively bombarding one or the other of said irradiating

2. The spectrometer of claim 1 wherein said two different X-ray irradiating

3. The spectrometer of claim 1 wherein said anode comprises at least two sections and wherein said cathode means comprises a portion for each anode

4. The spectrometer of claim 3 wherein said two different x-ray irradiating

5. The spectrometer of claim 3 wherein said anode is annular shaped, with each section in the form of a half-circle, and wherein said cathode is in two half-circle sections, one for each anode section, and means for

6. The spectrometer of claim 5 wherein said two different X-ray irradiating

7. The spectrometer of claim 1 wherein said anode comprises at least two sections and wherein means are provided for selectively directing the electrons from said cathode onto one or the other of said anode sections.

8. The spectromter of claim 7 wherein said two different X-ray irradiating

9. The spectrometer of claim 7 wherein said anode sections are concentric annular anodes and wherein said cathode is annular shaped and concentric with said two anode sections, said latter means selectively directing electrons from the cathode onto one or the other of said anode sections.

10. The spectrometer of claim 9 wherein said two different X-ray

11. In an induced electron emission spectrometer wherein a sample under analysis emits electrons upon being irradiated with X-ray radiation, means for selectively bombarding with electrons a first target to produce X-rays having a first energy and a second target to produce X-rays having a second energy, said second energy being different from said first energy, means for irradiating said sample with said X-rays of said first energy to produce a first induced electron emission spectrum from said sample and means for irradiating said sample with X-rays of said second energy to produce a second induced electron emission spectrum from said sample, detector means for detecting said electrons emitted from said sample, and energy selecting means for directing emitted electrons of selected energies onto said detector means, THE IMPROVEMENT WHEREIN said means for selectively bombarding with electrons said first and second targets comprises a focusing electrode whereby said bombarding electrons can be selectively focused to bombard either of said targets.