US3621241A

US3621241A - Electrostatic analyzers with auxiliary focusing electrodes

Info

Publication number: US3621241A
Application number: US864458A
Authority: US
Inventors: David W Turner
Original assignee: PerkinElmer Ltd
Current assignee: PerkinElmer Ltd
Priority date: 1969-10-07
Filing date: 1969-10-07
Publication date: 1971-11-16
Anticipated expiration: 1988-11-16

Abstract

An electrostatic analyzer of the energy spectrum of charged particles is provided comprising spaced apart analyzer plates adapted for operation in a velocity-focusing mode of operation and auxiliary electrode means adjoining an inner face of an analyzer plate. Circuit means apply a potential to each analyzer plate and a potential of opposite polarity to an associated auxiliary electrode means for establishing an electrostatic field between the plates which in a zone of influence of the auxiliary electrode means is modified to direct peripheral particle trajectories towards a focus substantially common with that of paraxial trajectories.

Description

United States Patent David W. Turner l leadington, England Oct. 7, 1969 Nov. 16, 197 l Perkin-Elmer Limited Beaconsfield, Buckinghamshire, England Inventor Appl. No. Filed Patented Assignee ELECTROSTATIC ANALYZERS WITH AUXILIARY FOCUSING ELECTRODES 8 Claims, 3 Drawing Figs.

US. Cl 250/4L9 ME, 250/49.5 P

Int. Cl H01 j 39/34 Field oi Search 250/4 1 .9 ME, 49.5 P

[56] References Cited UNITED STATES PATENTS 2,533,966 12/1950 Simmons, Jr. 250/4l.9 2,667,582 l/l954 Backus 250/41.9

Primary Examiner-William F Lindquist Att0rneyEdward R. Hyde, Jr.

ABSTRACT: An electrostatic analyzer of the energy spectrum of charged particles is provided comprising spaced apart analyzer plates adapted for operation in a velocity-focusing mode of operation and auxiliary electrode means adjoining an inner face of an analyzer plate. Circuit means apply a potential to each analyzer plate and a potential of opposite polarity to an associated auxiliary electrode means for establishing an electrostatic field between the plates which in a zone of influence of the auxiliary electrpde means is modified to direct peripheral particle trajectories towards a focus substantially common with that of paraxial trajectories.

PAIENIEnuuv 1s IHII SHEET 2 BF 2 ELECTROSTATHC ANALYZERS WITH-ll AUXILIARY FOCUSING ELECTRODES This invention relates to electrostatic analyzers for observing the energy spectrum of charged particles. The invention relates more particularly to Photoelectron Spectroscopy and to apparatus embodying electrostatic analyzers.

In photoelectron spectroscopy, a beam of photoelectrons is ejected from a sample gas within a target chamber by highenergy photons. The beam fans out over a small solid angle from a small aperture in the chamber into a space between analyzer plates which are maintained at a selected potential difference. In passing through the analyzer the photoelectrons undergo velocity focusing, and in order to detect the different energies that may be present the potential difference between the analyzer plates is scanned over a selected range. The photoelectrons of successive energy values are thereby brought successively to a focus at a discrete exit aperture, behind which a detector such as a particle multiplier is positioned.

Prior electrostatic analyzers have exhibited inherent aberrations which degrade the resolution of the observed spectra. Spherical aberration is particularly prominent, and since it is brought about by the fact that the paraxial rays and the peripheral rays do not have a common focus, it can be reduced by providing a smaller solid angle of radiation. This, however, cannot be carried far enough in practice to be fully effective without at the same time reducing the detector signal to an extent that cannot be tolerated. It is preferable to minimize spherical aberration while retaining a reasonably large angle of acceptance.

It is an object ofthis invention to provide an improved energy analyzer arrangement for a spectrometer.

Another object of this invention is to provide an analyzer arrangement for modifying the electrostatic field between the analyzer plates of a spectrometer in order that paraxial and peripheral rays of particles ofa given energy will be brought to a substantially common focus.

In accordance with features of the present invention an electrostatic analyzer of the energy spectrum of charged particles is provided comprising spaced apart analyzer plates adapted for a velocity-focusing mode of operation and auxiliary electrode means adjoining an inner face of an analyzer plate. Circuit means apply a potential to each analyzer plate and a potential of opposite polarity to an associated auxiliary electrode means for establishing an electrostatic field between theplates which in a zone of influence of the auxiliary electrode means and is modified to direct the peripheral particle trajectories toward a focus substantially common with that of the paraxial trajectories.

The auxiliary electrode means in one embodiment comprises a longitudinally extending electrode recessed in an associated plate, the surface of the electrode facing a median plane of the analyzer and shaped for causing the desired redirection effect without substantially interfering with the continuity of the inner face of the plate. In an alternative embodiment the auxiliary electrode means comprises a plurality of electrodes for enabling the shape of the modified field to be controlled by the application of different potentials to the individual electrodes.

Additionally, means are provided for reducing an adverse effect caused by fringing fields existing at an exit or entry region of an electrostatic analyzer upon the resolution limit of the energy spectra. Eringing is reduced by arranging an arcuate conductive path each side of the exit aperture in the form, for example, of metal sheeting flaring out from the aperture towards the detector station at a constant or conveniently changing radius.

The invention will now be described, by way of example, in its application to a cylindrical condenser configuration, of which the spherical condenser is a well-known variant in electrostatic analyzers. In the description which follows, reference shall be had to the attached diagrammatic drawings, wherein:

FIG. 1 is a plan view of a cylindrical electrostatic analyzer embodying the present invention; and

FIG. 2 is an electrical diagram of the analyzer energization.

FIG. 3 is a sectional view taken on the line 3--3 of FIG. i.

For a clearer understanding of its mode of operation and because we are particularly concerned with the application of our invention to photoelectron spectroscopy, we will assume that the electrostatic analyzer to be described forms part of a photoelectron spectrometer, although the skilled in the art will readily appreciate that its usefulness extends beyond the analysis of photoelectron energies.

FIG. ll shows a vacuum tank l with the top removed, and within it, a target chamber 2 and an electron multiplier 3 adjoining the entry and the exit, respectively, of the analyzer 4. The target chamber 2 is a generally cylindrical tube in which a slit, say 1 cm. in length, is formed at the apex of two cuspshaped wall portions 2A. The slit defines the area through which photoelectrons ejected from a sample gas in the target chamber 2 by photons emanating from a source not shown will fan out into a solid angle of radiation and represents in fact the entry aperture of the analyzer 4. The exit aperture of the analyzer 4 is defined by juxtaposed antifringing cusp-shaped members 5. One of these may be moved through the micrometric adjustment means 6 reiative to the other, which is fixed. In this manner the spacing between the edges of the members at the apex of the cusp may be increased or decreased to alter the size of the exit aperture. Alternatively, a mechanism may be included for moving both members simultaneously and symmetrically in opposite directions. It is advantageous to include means for enabling theslit to be moved in any direction and optionally means for controlling the cusps geometry.

The cylindrical plates forming the main electrodes of the analyzer 4 are shown at 4A and 4B. They are supported in spaced relation but insulated from each other by a base plate 4C mounted on the bottom of tank I and by a top plate (not shown). From both the baseplate and the top plate a middle cylindrical flange, such as 4D, extends towards the electrostatic axis of the analyzer 4. The flanges terminateshort of said axis and the spacing between the two opposed cylindrical edges represents approximately the height of the effective analyzer volume, of which the width is represented by the spacing between

plates

4A and 4B. The object of the flanges is to straighten the electrostatic field at the top and bottom of said effective volume. Plate 4A is slotted at 4E in a direction perpendicular to the plane of the paper and within the slot there is supported a discrete electrically insulated auxiliary electrode means in the form of a narrow conductive strip 4F, to which an electrical potential may be applied from outside the tank] through insulated terminal 7. An alternative to the slot 4E is a cavity formed in the plate 4A in which the strip 4F is supported with a spacing from the cavity wall. Slot 4G, strip 4H and insulated terminal 8 are the counterparts of 4E, 4F and 7, respectively, in relation to plate 48.

In operation, a suitable potential difference scanned over a predetermined range is applied between plates 4A and 48 through insulated terminals 9 and 10 and a suitable and simultaneously scanned potential difference of opposite sign is applied between auxiliary electrode means 4F and 4H. If the potentials are properly adjusted in relation to the range of energies expected in the photoelectrons issuing from the target chamber 2, the well of potential created by each auxiliary electrode means produces a bulge in the distribution of the electrostatic field existing between the analyzer plates which tends to redirect the peripheral photoelectrons towards a focus substantially common with that of the paraxial photoelectrons.

The effect of the two auxiliary electrode means should fall off rapidly and ideally reduce to zero or nearly so at the median plane of the analyzer. For this reason the width of the slot in which the auxiliary electrode means is located and the width of the active surface of the auxiliary electrode means, i.e., that facing the median plane, are both made small compared with the spacing between the plates 4A and 48 so as to achieve what might be termed a point source effect. This is the meaning intended in our reference to discrete" auxiliary electrode means.

In a particular embodiment of the invention a slot width of 3 mm. was associated with an electrode width of 1 mm. arld a plate spacing of 2 cm. The active surface of the auxiliary electrode means may be given the same curvature as that of the associated plate and may be mounted flush.

The internal surfaces of the analyzer likely to be struck by the photoelectrons may be treated to give low emission of secondary electrons. We have used benzene soot and a colloidal graphite solution with equally good results.

Each auxiliary electrode means may comprise an array of two or more electrically individual elongated electrodes to each of which a potential of suitable value may be applied in relation to the potentials applied to each of the remaining electrodes. In using a multielectrode structure with each analyzer plate, one would strive to achieve by geometry and selective energization of electrodes a field distorting effect which vanished to zero at the median plane and at any other point intermediate between the electrode array and the median plane was of the right value to produce the right amount of redirection of the peripheral photoelectrons to cause them to focus at substantially the same plane as the paraxial photoelectrons.

The two potentials for the analyzer plates and the opposite potentials for the auxiliary electrode means are derived from a common supply. The four potentials are swept together in a manner mechanically slaved to the chart recorder. In FIG. 2, block 30 represents a DC power supply (e.g., 50 volt supply), R1 and R6 are ganged rheostats for setting the scan datum, VR3 is the slaving potentiometer driven by the chart-advancing mechanism of the recorder, VRI and VR2 are potentiometers for setting the ratio between plate potential and auxiliary electrode means potential, R7 and R8 are resistors of convenient value for the energization of the plates, and VR is a potentiometer for balancing to ground. The analyzer plates are indicated at 31 and 32 and the respective auxiliary electrode means at 33 and 34. Finally R25 are scale expansion resistors. In operation, R1 and 6 are adjusted in accordance with the requirements of the analysis to be carried out, but VRl, VR2 and VR3 are essentially preset controls.

Representative values for the components shown of FIG. 2 are as follows:

Rldt R6 1,0001] The FIG. 2 arrangement is slightly modified to give independent adjustment of the potentials to ground extended to electrodes 33 and 34 while still ensuring that they are swept as the analyzer plate potentials are swept. This modification VR and VR in FIG. 2 are replaced with fixed resistors of equal and suitable value and connecting two potentiometers in series across the chain comprising R-,, VR;,' and R the sliders of the two potentiometers being routed to the auxiliary electrodes in the manner shown for VR and VR in FIG. 2.

Whatever the potential existing across the chain comprising VR,, R VR R and VR for any setting of the ganged potentiometers VR and VR there will be a given ratio of potential applied to an auxiliary electrode to that applied to the analyzing plate.

Potentiometers VR balances when its slider is exactly midway and VR and VR are electrically and mechanically identical. Thus with their sliders ganged, when the slider of VR is at the bottom of the VR, winding and the slider of VR is at the top of the VR winding then plate 32 will receive a positive potential to ground which is the potential drop across R-, and half of VR;,' in series and the same positive potential will be applied to the auxiliary electrode 33. Similarly, plate 31 will receive a negative potential to ground equal in value to the potential drop across R and the other half of VR;,. This negative potential will also be extended to auxiliary electrode 34. The ratio of an auxiliary electrode potential to the potential of the adjoining plate will therefore be l:l. Now, as shown on FIG. 2, the sliders of VR, and VR, are ganged so that they move in opposite directions. If we increase the setting of the ganged sliders, the positive potential received by auxiliary electrode 33 will be augmented by the potential drop appearing between the slider of VR, and the resistor R At the same time the negative potential received by auxiliary electrode 34 will be augmented by the potential drop appearing between the slider of VR and the resistor R Therefore the ratio of the auxiliary electrode potential to the plate potential is clearly greater than l:l.

If we turn the gang to its maximum setting, the potential increase received by the auxiliary electrodes will be the total potential drop across the respective potentiometer windings. At this setting, the ratio of auxiliary electrode potential to plate potential is found by dividing the series resistance of VR, plus R, plus half of VR;, by the series resistance of R, plus half of VR:,. Substituting the values given in the specification we have 500+47+50/47+50=597/97 which gives a ratio of approximately 6:1.

The ratio of auxiliary electrode potential to plate potential may be adjusted therefore in the range 1:1 to 611. Since the sweep potential provided by the slave potentiometer VR; is applied across the ends of the chain comprising VR R VR R and VR it follows that any ratio selected in the range 1:1 and 6:1 is maintained during a potential sweep of the analyzer plates.

It has been found that in so far as the potentials applied to the two auxiliary electrodes control the spherical aberrations of the analyzer, the fine setting of these potentials may be used with advantage for the final adjustment of energy concentration at the exit slit of the analyzer.

In photoelectron spectroscopy, the chamber 2, the photomultiplier 3 and the analyzer 4 are conveniently arranged for continuous vacuum operation, hence the need for vacuum tank I intended to be connected to a vacuum pump, not shown.

In an alternative embodiment to the analysis of the electron energy spectrum by scanning the voltage applied to the analyzer plates in the manner referred to, the plates are maintained at a suitable fixed potential and the voltage applied to an apertured electrode, placed between the effective electron source which in the embodiment described is the slit at the apex of the two cusp-shaped wall portions 2A and the entry to the analyzer is scanned. The voltage is alternatively an accelerating or a decelerating voltage and the aperture through which the electrons are either accelerated or decelerated is shaped for electron-optical purposes.

An improved electrostatic analyzer arrangement for reducing aberrations in an electron spectrometer has been described. While this particular embodiment of the invention has been illustrated and described, it will be understood that various modifications may be made therein without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. An electrostatic analyzer of the energy spectrum of charged particles comprising a pair of spaced apart analyzer plates adapted for the velocity-focusing mode of operation and auxiliary electrode means adjacent the inner face of each of said plates, means to electrically insulate each auxiliary electrode from its adjacent analyzer plate, means for applying a potential of first polarity to each plate and a potential of opposite polarity to the associated auxiliary electrode means for modifying an electrostatic field between the plates in the zone of influence of the auxiliary electrode means to direct peripheral particle trajectories towards a focus substantially common with that of paraxial trajectories.

2. An analyzer as claimed in claim 1, wherein the auxiliary electrode means comprises an elongated electrode means recessed in said inner face at a location intermediate between the entry and the exit of the analyzer.

3. An analyzer as claimed in claim 2, wherein a surface of iliary electrode means extends in length so as to span the two the elongated electrode means facing the median plane of the flanges. analyzer is substantially flush with said inner face. 6, A al a laimed i n claim 4, wherein antifringing 4. The analyzer of claim 1 wherein said plates define a cylinmeans is provided at the exit of the analyzer.

drica] Fondfmser and the length the auxillaty electrode 5 7. An analyzeras claimed in claim 6, wherein said antifring- 3: 2:gzt'zg gigf ggf gg around much the ing means is a pair ofcus p-shaped conductors. i

5. An analyzer as claimed in claim 4, comprising antifring- P as claimed m Clam 4 where, amf'mgmg ing cylindrical flanges lying in the median plane symmetrically means provlded at the entry oflhe analyzer spaced from the electrostatic axis of the analyzer and the aux- 10

Claims

3. An analyzer as claimed in claim 2, wherein a surface of the elongated electrode means facing the median plane of the analyzer is substantially flush with said inner face.

4. The analyzer of claim 1 wherein said plates define a cylindrical condenser and the length of the auxiliary electrode means is oriented parallel to the axis around which the condenser is geometrically generated.

5. An analyzer as claimed in claim 4, comprising antifringing cylindrical flanges lying in the median plane symmetrically spaced from the electrostatic axis of the analyzer and the auxiliary electrode means extends in length so as to span the two flanges.

6. An analyzer as claimed in claim 4, wherein antifringing means is provided at the exit of the analyzer.

7. An analyzer as claimed in claim 6, wherein said antifringing means is a pair of cusp-shaped conductors.

8. An analyzer as claimed in claim 4 wherein antifringing means is provided at the entry of the analyzer.