U.S. patent number 3,805,068 [Application Number 05/277,571] was granted by the patent office on 1974-04-16 for electron energy analysis.
Invention is credited to Jerald D. Lee.
United States Patent |
3,805,068 |
Lee |
April 16, 1974 |
ELECTRON ENERGY ANALYSIS
Abstract
Method and apparatus for energy analysis of a stream of moving
electrons by effecting electrostatic segregation and counting of an
electron portion having a preselected kinetic energy.
Inventors: |
Lee; Jerald D. (Wilmington,
DE) |
Family
ID: |
26796152 |
Appl.
No.: |
05/277,571 |
Filed: |
August 3, 1972 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
99475 |
Dec 18, 1970 |
|
|
|
|
Current U.S.
Class: |
250/305;
850/21 |
Current CPC
Class: |
H01J
49/488 (20130101) |
Current International
Class: |
H01J
49/00 (20060101); H01J 49/48 (20060101); H01j
037/26 () |
Field of
Search: |
;250/49.5AE,41.9D |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Borchelt; Archie R.
Assistant Examiner: Church; C. E.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. application Ser.
No. 99,475 filed Dec. 18, 1970, in the name of the same applicant
and now abandoned.
Claims
1. A method for determining the energy of a preselected fraction of
electrons contained within a moving stream of charged particles
having different kinetic energies emanating from a common source,
comprising,
a. directing said moving stream of charged particles linearly away
from said source;
b. applying to said moving stream an electrostatic field
preselected to bar forward longitudinal passage of substantially
all of said charged particles having kinetic energies below a first
preselected energy level while permitting forward passage of the
remainder of said charged particles, and to retard the axial
component of the velocity of said remaining charged particles
without substantially altering the radial component of the velocity
of said remaining charged particles;
c. electrostatically enhancing the outwardly directed radial
component of the velocity of said remaining charged particles;
d. segregating by transverse drift that fraction of said remaining
charged particles having kinetic energies within a preselected
range of kinetic energies for which it is desired to analyze by
positioning a collection means coaxial with and radially outward
from the axis of said stream so that those of said remaining
charged particles in the preselected energy range and having a
radially outwardly directed component of velocity will impinge on
the collection means; and
e. determining the number of said electrons impinging on the
collection
3. A cylindrical energy analyzer for a moving stream of charged
particles having different kinetic energies, comprising in
combination:
a. charged particle inlet means;
b. electrostatically charged filter means displaced axially from
said inlet means and maintained at a preselected electrical
potential level to bar forward passage of substantially all charged
particles having kinetic energies below a first preselected energy
level while permitting foward passage of the remaining charged
particles, said filter means comprising at least two grids which
function as a converging lens to impart a convergent transverse
component to the velocity of said charged particles and to retard
the axial component of said charged particles;
c. a field free drift region sustaining the convergent transverse
component imparted to said charged particles;
d. a cut-off means, separated from said filter means by said field
free region and defining a collection region to collect that
portion of said remaining charged particles having a transverse
drift which causes them to impinge on said cut-off means after they
have passed through said field free region so as to segregate by
transverse drift those charged particles having kinetic energies
within the preselected range of analysis; and
e. detection means for determining the number of charged
particles
4. The energy analyzer of claim 3 wherein said charged particles
are
5. The energy analyzer of claim 4 wherein said filter means
comprises two
6. The energy analyzer of claim 5 further comprising a beam stop
positioned on the axis of the analyzer between said inlet means and
said detector to prevent high energy charged particles from flowing
directly from said
7. The energy analyzer of claim 5 wherein said filter means
comprises two pairs of spherical grids, the first pair being
concave and the second pair
8. The energy analyzer of claim 7 further comprising a beam stop
positioned on the axis of the analyzer between said inlet means and
said detector to prevent high energy charged particles from flowing
directly from said
9. A cylindrical energy analyzer for a moving stream of charged
particles having different kinetic energies, comprising in
combination:
a. charged particle inlet means;
b. electrostatically charged filter means displaced axially from
said inlet means and maintained at a preselected electrical
potential level to bar forward passage of substantially all charged
particles having kinetic energies below a first preselected energy
level while permitting forward passage of the remaining charged
particles, and to retard the axial component of the velocity of
said remaining charged particles, without substantially altering
the radial component of the velocity of said remaining charged
particles;
c. collection means, positioned coaxial with and radially outward
from the optical axis of said filter means, to permit collection of
that portion of said remaining charged particles having a radially
outwardly directed transverse drift which causes them to impinge on
said collection means so as to segregate by transverse drift those
charged particles having kinetic energies within the preselected
range of analysis; and
d. detection means for determining the number of charged
particles
10. The energy analyzer of claim 9 wherein said charged particles
are
11. The energy analyzer of claim 10 further comprising enhancing
means to enhance the radial outwardly directed component of the
velocity of said
12. The energy analyzer of claim 11 wherein said enhancing means is
a charged needle positioned coaxially with respect to said
collection means.
13. The energy analyzer of claim 11 further comprising a trap for
those electrons which pass through said filter means and are not
collected by
14. The energy analyzer of claims 10 further comprising an
electrostatically charged prefilter, located before said filter
means, to remove from the stream of electrons those electrons
having kinetic
15. The energy analyzer of claims 10 wherein said filter means
comprises a multiplicity of coaxially disposed metal rings
maintained in mutual electrical isolation one from another at
predetermined electrical potential levels decreasing from positive
to negative in the direction of
16. The energy analyzer of claim 15 wherein the electrons in said
moving stream of charged particles have both axial and radial
velocity components and wherein said filter means also functions as
said enhancing means by retarding the axial component of the
velocity of said charged particles without retarding their radial
components, whereby said electrons drift towards said collection
means through a substantially field free region.
17. The energy analyzer of claims 16 wherein said collection means
comprises a metallic ring collector disposed coaxially with respect
to said filter means and wherein said filter means and said means
to enhance combine to cause said electrons to drift radially
outward to said metallic
18. The energy analyzer of claim 17 wherein said detection means is
an electron multiplier provided with a sensing connection to said
ring collector.
Description
BRIEF SUMMARY OF THE INVENTION
Generally, this invention comprises method and apparatus for energy
analysis of a moving stream of electrons having different kinetic
energies comprising, in seriatim, constraining a stream of
electrons having diverse kinetic energies to a given flow path,
applying to the moving stream an electrostatic repulsion field
barring forward longitudinal passage of substantially all of the
charged particles having kinetic energies below a first preselected
energy level while permitting forward passage of the remainder of
the charged particles, segregating by transverse drift and counting
the fraction of the electrons having a preselected range of kinetic
energies which it is desired to analyze for from the remainder of
the moving stream, and ejecting the remainder of the moving
electron stream.
DRAWINGS
The following drawings depict three embodiments of the invention as
applied to electron spectroscopy, in which:
FIG. 1A is a plot of transmission current v. energy for the
photoelectron stream emitted when a sample is bombarded with
X-rays,
FIG. 1B is a plot of transmission current v. energy characteristic
of a narrow-band energy-pass filter at an arbitrary energy
E.sub.o,
FIG. 2A is a schematic side elevation cross-sectional view of a
typical prior art hemispherical electrostatic analyzer,
FIG. 2B is a plot of transmission current v. energy characteristic
obtained with energy analyzers of this invention,
FIG. 3A is a schematic representation of a first preferred
embodiment of apparatus according to this invention,
FIG. 3B is a diagrammatic representation of the effective regions
of equipotential distribution between paired sections of the
apparatus of FIG. 3A,
FIG. 4A is a transmission v. energy characteristic curve for a
narrow-band energy pass apparatus such as that illustrated in FIG.
3A,
FIG. 4B is the transmission v. energy characteristic curve of FIG.
4A showing, in broken-line representation, the effect of a
prefilter on the narrow band-pass output of FIG. 2B in removing the
broad wing tail of FIG. 4A,
FIG. 5A is a schematic side elevation sectional view of a preferred
design of prefilter,
FIG. 5B is a schematic representation of trajectories of typical
photoelectrons passed by the prefilter of FIG. 5A,
FIG. 6 is a partially schematic side-elevation sectional view, in
detail, of a first preferred embodiment of energy analyzer
according to this invention shown in operational association with
the X-ray-bombarded sample of an electron spectrometer, not
otherwise detailed,
FIGS. 7A and 7B is a typical spectrum records (A) obtained in the
analysis of a gold sample using apparatus constructed according to
the first embodiment of this invention in comparison with a
spectrum record (B) obtained with typical apparatus of the prior
art,
FIG. 8 is a schematic longitudinal section view of a second
embodiment of apparatus according to this invention, and
FIG. 9 is a schematic longitudinal section view of a third
embodiment of apparatus according to this invention.
GENERAL
Electron analysis according to this invention can be applied to a
wide variety of situations; however, it is particularly useful in
the conduct of electron spectroscopy and, accordingly, is
hereinafter described in particular application to this
technique.
Electron spectroscopy for chemical analysis is a comparatively new
procedure which has been described extensively in the article
entitled "Electron Spectroscopy for Chemical Analysis (ESCA)" by
Kai Siegbahn et al., Uppsala University, Uppsala, Sweden (1968)
October (Processed for the Defense Supply Agency by the
Clearinghouse for Scientific and Technical Information with the
identification number AD-844-315).
Briefly, electron spectroscopy is the study of the energy
(velocity) distribution of secondary electrons (photoelectrons)
emitted by a sample upon irradiation of the sample by a primary
energy source, such as a beam of X-rays. The operation is conducted
by an electron spectrometer having a radiation source for exciting
a sample, means for analyzing the velocities (energies) of the
secondary electrons released due to the excitation, and means for
recording electron energy vs. the quantity (current) of electrons
falling within small increments of energy. The apparatus utilizes
high vacuum pumps, a high-voltage source, an X-ray or other emitter
of exciting energy, a sample module or holder, an energy analyzer,
and a readout device such as an X-Y recorder.
ESCA has broad application to the analysis of the full range of
individual chemical elements, even in the presence of other
elements, and is particularly effective in organic chemistry, since
the chief constituent elements carbon, nitrogen, oxygen, etc., are
relatively easy to study. In addition, electron spectroscopy is
better suited than X-ray analysis for studying the atomic structure
of surfaces, because the secondary electrons (as contrasted with
"secondary" X-rays), are emitted only from a surface layer 100A or
less in thickness. Thus, information on composition, bonding states
and the like peculiar to the surface exclusively is readily
obtainable using this tool.
When a sample under analysis is irradiated from a primary source,
the sample emits photoelectrons is essentially random directions
and at velocities (energies) unique to the specific electron-level
structure of the atoms in the sample. To be of value in chemical
analysis, these photoelectrons must be categorized with respect to
their energies and the number of electrons emitted in each energy
category determined over a given interval of time. This
categorizing is effected by an energy analyzer, such as the designs
provided by this invention.
A successful analyzer must (a) provide high resolution, i.e.,
separation of electron fractions of closely adjacent energies and
(b) provide high sensitivity, i.e., measureable and representative
readouts for each small energy increment. Also, the analyzer must
accommodate a high electron throughput, or luminosity, so that
electron energy categorization can be accomplished within a
relatively short time interval.
The energy distribution spectra of photoelectrons produced by X-ray
excitation is such that the electrons which characterize a specific
element lie at a particular energy level and, in general, are
manifested as discrete maxima resting upon a background having a
broad distribution of energy. This background exists because
electrons which would otherwise have discrete energies which
characterize the element (or sample) have undergone collisions
within the sample and thus have lost varying amounts of energy.
Another source of background current is the exciting X-ray
background (bremsstrahlung), which is superimposed upon the desired
exciting X-rays of narrow energy distribution (characteristic
X-rays). A typical energy distribution of electrons emitted by a
sample, including the unavoidable background, is represented by
FIG. 1A.
Since it is desired to measure only the intensity and shape of the
photoelectron peak representing a discrete energy difference, both
the sensitivity and accuracy of measurement are reduced
proportionately if any of the background other than that directly
under the peak is measured. This occurs because the statistical
variation in the number of electrons arising from the broad
background is large. For this reason, it is preferred to measure
the current (representative of the number of electrons in that
narrow energy band) within a photoelectron peak with a narrow-band
energy pass apparatus. FIG. 1B shows the desired transmission
characteristics of such a narrow-band energy pass device at an
arbitrary energy E.sub.o. One such narrow-band pass device of the
prior art which accomplishes this type of discrimination is a
hemispherical electrostatic analyzer, illustrated schematically in
side elevation cross-section in FIG. 2A. Only those electrons with
energy E.sub.o = eER.sub.o /2 can pass both slits, where E is the
electric field produced by the potential difference V, e is the
charge on an electron and R.sub.o is the radius of the hemisphere
including the slits.
DETAILED DESCRIPTION
Conceptually, it is the purpose of this invention to provide means
which affords a transmission v. energy characteristic which can be
visualized as a conjoined filter and cutoff means preselected so
that the energy passed by the filter is precisely at the trailing
edge of the maxima region corresponding to E.sub.o, whereas the
energy level preselected by the cutoff means is precisely at the
leading edge of the maxima region corresponding to E.sub.o, all as
shown in FIG. 2B.
Referring to FIG. 3A, there is shown, for purposes of explanation,
an over-simplified schematic representation of a first embodiment
of apparatus according to this invention comprising two cylindrical
co-axially aligned metallic tube sections 10 and 11, which
collectively constitute an electrostatic high-pass filter, followed
by a collector ring 11a, i.e., the cutoff means, which demonstrates
the principle of the invention. The individual sections are
electrically isolated one from another and are maintained at
preselected electrostatic potentials through taps from the voltage
source 14. In addition, the negative potential side of source 14 is
connected through microammeter 16 to ring 11a.
The course of the moving photoelectron stream is shown as
proceeding from left to right along the line Z in FIG. 3A,
corresponding to the common longitudinal axis of the sections 10
and 11, and ring 11a.
The equipotential distribution in terms of the percent difference
in potential between paired tubular sections, such as sections 10
and 11 of FIG. 3A, is shown diagrammatically in FIG. 3B.
Now, if the electrostatic potential of section 11 is preselected
with respect to section 10 at a negative level repelling all
photoelectrons having energies below a given level, a first
separation corresponding to the action of the filter, i.e., the
high energy pass filter of FIG. 2B, is obtained. Good operation is
obtained if ring 11a is maintained at the same potential as section
11, or within .+-.1/2 percent thereof. Under these circumstances
photoelectrons with energies lying in the maxima corresponding
substantially to E.sub.o are deflected into contact with the metal
wall of ring 11a, and the fraction of photoelectrons of analytical
interest is segregated. The number of photoelectrons in this
segregated fraction is measured as the electrical current passed
through microammeter 16.
The photoelectrons completely escaping ring 11a are ejected from
the apparatus unanalyzed.
From the foregoing, it will be apparent that the width of ring 11a,
i.e., the dimension coparallel with the longitudinal axes of
sections 10 and 11, determines, together with the ring potential,
the energy rnage of the fraction of photoelectrons measured. As
will become more clear as this description proceeds, the incoming
stream of photoelectrons passing from left to right through
sections 10 and 11, as seen in FIG. 3A, is divergent, and the
transverse velocity component thereof, denoted v.sub.t, is the
effective force radially segregating the photoelectron fraction to
be measured. Two other embodiments of apparatus hereinafter
described operate by effectively counterbalancing the divergent
transverse velocity component v.sub.t and substituting therefore a
convergent component v.sub.t ', thereby effecting axial segregation
of the electron fraction of interest.
The transmission v. energy characteristic of a narrow-band energy
pass device such as that of FIG. 3A is shown in FIG. 4A, and it is
seen that there is a relatively broad wing extending forward from
the leading edge. This wing contributes to inaccuracy in the
photoelectron peak measurement, unless it is eliminated.
Referring to FIG. 4B, it is seen that the wing can be largely
eliminated by use of a low-energy pass prefilter having the
transmission characteristic denoted in broken line representation,
which substantially "cuts off" the wing in close adjacency with the
maxima of interest without, however, affecting the latter in any
way.
A prefilter of novel design is detailed in FIG. 5A and consists of
a metal plate 19, maintained at a positive potential (typically,
+300 volts) and apertured via metal screens 19a and 19b to,
respectively, admit and eject the electron stream, such as the
gross photoelectron output from an X-ray sample bombardment chamber
(not shown). This is followed by a coparallel metal screen 20
maintained at a negative potential with respect to plate 19
(typically, +90 volts), spaced at a distance h (typically, 1/4 in.)
from plate 19. Screens 19a, 19b and 20 are all of the same mesh
size (approximately USS No. 28) wherein the nickel wire diameter is
0.00054 inch and the openings 0.04946 inch on a side, providing 97
percent open area. The prefilter is disposed across the beam of
electrons at a bias angle of .theta., typically 45.degree., under
which conditions the fraction of electrons having energies less
than a cutoff value E.sub.o entering this prefilter is "brought to
a focus" by the field between screen 20 and plate 19 at a distance
X.sub.o (which is the center-to-center distance between apertures
19a and 19b) along plate 19 from the point of electron beam entry,
as shown in FIG. 5A. At .theta. = 45.degree., X.sub.o has a maximum
value.
From FIG. 5B it can be seen that the height to which the focused
beam rises is X.sub.o /4. Thus, screen 20 is spaced from plate 19
by an amount h = X.sub.o /4, in which case the trajectories of
electrons having energies E.ltoreq.E.sub.o pass under or just graze
the surface of screen 20. In actuality, the beam entering aperture
19a will always have some divergence d.theta., in which case, the
cutoff will no longer occur at sharply E.sub.o. At .theta. =
45.degree., it can be shown that the cut-off-energy spread dE, from
E.sub.o, that occurs from a beam divergence d.theta. is given by dE
= 2 E.sub.o d.theta.. To accommodate beam sizes passing greater
flux, apertures 19a and 19b can be opened to a width of some value
less than X.sub.o, typically .sqroot.2/2X.sub.o.
The bulk of the high energy (E>E.sub.o) photoelectrons pass
through the meshes of screen 20 and thus are discarded to avoid
disturbance of the subsequent analysis. However, the photoelectron
stream of analytical interest, inclusive of a few high energy
photoelectrons, is deflected to the right and is passed via exit
aperture 19b to the inlet end of the analyzer, i.e., the inlet end
of section 10, FIG. 3A.
Referring to FIG. 6, there is shown, in partial representation
only, an electron spectrometer, evacuated throughout to about
10.sup.-.sup.6 mm. absolute, wherein the sample 25 irradiated is
mounted within a cylinder 37, which can be thin-walled aluminum of,
typically, 3 micron thickness maintained at a positive potential of
0 to 1,500 volts. The exit end of cylinder 37 is covered by a metal
screen 36, which can be the same mesh size as screens 19a, 19b and
20, through which the photoelectrons pass along line Z.sub.1 to the
prefilter, hereinbefore described, denoted generally at 26. In this
design, inlet aperture 19a is covered by metal screen 27, whereas
exit aperture 19b is similarly covered by screen 28.
With prefilter 26 inclined at an angle of 45.degree. towards the
energy analyzer, denoted generally at 30, and with a +90 volt
potential carried on screen 20, the photoelectron stream of
analytical interest is deflected approximately 90.degree. clockwise
to take the horizontal course Z.sub.2, at which it proceeds
generally along the analyzer longitudinal axis. Actually, the
photoelectrons of analytical interest, as well as the residue of
higher energies finally ejected, invariably possess a substantial
transverse velocity component which, over the relatively short
length of tube 34, does not bring them into contact with the tube
sidewalls but does, ultimately, effect segregation of the
photoelectron fraction of analytical interest, all as hereinafter
described. High-energy rejected photoelectrons pass vertically
through screen 20 and are preferably trapped in a metal cup, not
shown.
An electrical field preferably carried between cylinder 37 and the
screen 36, as well as a field between the screens 36 and 27,
adjusts the photoelectron velocities and focuses the photoelectrons
prior to their being channeled through screen 27. Typical operating
potentials are as follows: Sample 25 and cylinder 37 adjustable
between 0 and +1,500 volts, screen 36 potential adjustable between
ground and +1500 volts, support block 31 and screens 27 and 28,
typically at a fixed potential of +300 volts, and screen 20 +90
volts (thereby preselecting the cutoff point of the prefilter at
around 300 electron volts).
At the entrance of analyzer tube 34 there is provided an
electrostatic lens 35, maintained at a typical potential of +100
volts, which can be, if desired, a quadrupole lens of standard
design, but is preferably a simple so-called Einzel lens as shown
(i.e., one wherein a single value of potential determines its
optical characteristics). The concentrated photoelectron stream is
thence passed rightward as seen in FIG. 6 through longitudinally
adjusted centrally apertured plate 38.
The third quarter right-hand length of the analyzer, constituting
the electrostatic filter denoted generally at 66, is made up of a
plurality of coaxially arranged electrically isolated 1-5/16 inches
O.D. .times. 1/2 inch long copper rings, each carried at the
following typical potentials: ring 40 +270 v., ring 41 +210 v.,
ring 42 +150 v., ring 43 +90 v., and ring 44 +30 v. These rings are
disposed between screened apertures 39 on the left and 46 on the
right, so that there is maintained a substantially uniform
potential gradient between the two screens.
The described segmented construction is preferred because there is
thereby obtained enhanced planarity over that of a single field
maintained across the total potential difference, resulting in
improved resolution and sensitivity.
The magnitudes of the resistors 50 and 55 are, typically, 0.5
megohm, whereas resistors 51, 52, 53 and 54 typically have
magnitudes of one megohm each.
As seen in FIG. 6, the opening of aperture 39 is smaller than that
of aperture 46. Both apertures can be conveniently spring-mounted
within their associated tube sections 34 and 45, respectively.
Typically, the cylinder 34-aperture 39 voltage is held at +300
volts, whereas the subassembly 45-46 is held at 0 volts.
The next section in sequence is the cutoff means, denoted generally
at 67, comprising tubular copper skeleton ring 59, which is cut
away peripherally, as indicated at 59a and 59b, over most of its
circumference to permit unimpeded radial passage of the electron
fraction of analytical interest into the coaxially disposed
surrounding shroud casing 60 maintained at -0.8 volts
potential.
The left-hand wall of casing 60 is cut away as indicated at 60a to
present an opening in opposition to the conical flared entrances 63
(maintained at, typically, +90 volts) os the continuous dynode
surface of a conventional electron multiplier detection means,
indicated generally at 64. The voltage along the axis of the dynode
surface increases to, typically, +2,500 volts to accelerate the
segregated electron fraction. Thus, each electron entering the
flared openings 63 collides with the dynode wall surface, producing
a secondary emission of more than one electron. These secondary
electrons, in turn, are accelerated and undergo wall collisions
until the output of a single segregated incoming photoelectron
produces as many as one hundred million electrons, constituting a
gain of 10.sup.8. The output of electron multiplier 64, in turn, is
routed to an amplifier, a counter and an X-Y recorder (not
shown).
In the operation of the analyzer apparatus, the output of electron
multiplier 64 is fed through a rate meter into the Y-axis of the
recorder, while the output of a motor-driven potentiometer, which
is the retarding voltage at cylinder 37, is fed into the X-axis of
the recorder. The resulting display is the electron transmission
versus electron energy.
The right-hand end of the analyzer is provided with trap 68 which
accommodates ejected high energy electrons and comprises co-axially
disposed rings 70, maintained at, typically, -3 volts, 71
maintained at +90 volts, and 72 maintained at 0 volts. There is
also preferably provided a coaxially mounted leftward-extending
1/16 inch dia. needle electrode 72a carried at a retarding
potential of -3 volts to provide assisting radial deflection for
analyzed photoelectrons.
In operation, photoelectrons entering tube 34 at, typically, 303
electron volts (.+-.0.5 volt) can pass radially through the opening
of skeleton ring 59 at ground level. The reason for this is that
high pass filter 66 applies a force leftwards in the direction
Z.sub.2, FIG. 6, which substantially counteracts the longitudinal
force component acting on the photoelectrons of analytical
interest, while leaving the radially outward transverse velocity
components v.sub.t unaffected. This remaining transverse drift,
being substantially the sole remaining force acting on the
photoelectron fraction of interest, propels these photoelectrons
through the opening of skeleton ring 59, so that they can be
measured by electron multiplier 64. Needle electrode 72a assists in
the radial deflection and thus facilitates the segregation.
Photoelectrons of less than 303 volts fall on the succession of
rings 40-45, inclusive, of the high-pass filter. On the other hand,
photoelectrons of energies exceeding 303 electron volts are ejected
and neutralized on the rings 70-72, inclusive, of the trap 68
disposed to the right of ring 59.
The 303 e.v. photoelectrons, now reduced to 3 e.v., bounce around
elastically and inelastically within the shroud casing 60 until
they come within escape distance of the annular opening 60a and are
drawn off therethrough into dynode 64 through confronting openings
63.
FIG. 7A is illustrative of a typical electron energy spectrum
recorded with the first embodiment of the analyzer of this
invention, in which an X-ray source was used with a gold sample,
the irradiation being with aluminum K.alpha. radiation. The X-ray
source input power was 250 ma at 10 KV. The time of scan for this
spectrum was 1.6 min.
The ordinate values of this spectrum are a measure of transmitted
electron current expressed in terms of counts/-second, as
registered by the photoelectron detection means, whereas the
abscissa values are a measure of photoelectron energy expressed in
electron volts. The spectrum obtained is typical of that expected
for a gold sample, showing two characteristic peaks associated with
the N.sub.VI and N.sub.VII energy levels in gold, as well as
certain of the satellite peaks (not labeled) which appear at higher
energies. Complete resolution of the N.sub.VI and N.sub.VII spin
doublet into two lines was obtained.
For comparison, there is shown, in FIG. 7B, a spectrum of gold
obtained with a typical prior-art electrostatic analyzer. It will
be seen that there exists general agreement as regards appearance
and positioning of the photoelectron peaks. The time required to
obtain the spectrum of FIG. 7B is not known. However, the
significant difference is the superiority in count rate obtained
with the analyzer of this invention, i.e., 156,000 counts/sec. as
compared with 9,000 counts/sec. for the N.sub.VII peak.
Thus, at a source current of 250 ma for which my instrument is
designed, the counting rate is better than seventeen times that of
the apparatus producing the spectrum of FIG. 7B.
Referring to FIG. 8 the second embodiment of this invention
utilizes a converging electron lens which is schematically denoted
by the two limiting potential leads V.sub.o, typically +100 volts,
connecting with metal screen 39', and V.sub.1, typically
approximately +50 volts. The electron lens sets up focusing
equipotential lines across the tube 34' .
The sample 25' to be examined is supported within a cylinder 37'
coaxially disposed with respect to tube 34', within which it is
maintained at a potential V.sub.2, typically, +1500 volts. The
sample is irradiated by a beam of X-rays 77 introduced through port
25.sub.a ' covered by window 25.sub.b ' to emit photoelectrons
having characteristic energies, the path of one of which is denoted
by the broken line trace B.
As in the first embodiment, the top end of the analyzer is made up
of a plurality of coaxially arranged electrically isolated copper
rings, each carried at the following typical potentials, ring 40'
75 v, ring 41' 50 v and ring 44' 25 v. These rings are disposed
between screened apertures 39' and 46' (which latter is maintained
at ground potential) so that a substantially uniform potential
gradient is maintained between the two screens, which thus
constitute the equivalent of the electrostatic filter 66 (FIG. 6)
of the first embodiment.
Detector 64' can be of an electron multiplier type (typically, a
Model 4219 marketed by Bendix Corporation) the collector opening of
which is maintained at potential V.sub.3, typically +1,000 v. To
prevent front face bridging of the field created by the +1,000 v
potential applied to detector 64', it is good practice to house the
detector within a grounded metal housing, not shown in FIG. 8. In
this design the detector is disposed substantially coaxially of
tube 34', directly back of a beam stop 75 which can be a circular
metal plate 0.001-0.01 inch thick maintained at potential V.sub.o
(i.e., +100 volts) by attachment to screen 39'. Beam stop 75
extends outward past the periphery of detector 64' approximately
0.1 inch to bar ingress of any high energy photoelectrons which
follow a substantially axial course from sample 25'.
Cylindrical screen 76, constituting, in this design the cutoff
means 67', is coaxially mounted with respect to tube 34', and can
be 10,000 openings/sq,in. nickel wire metal approximately 1 inch
diameter by 1 inch long, maintained at ground potential by
attachment to screen 46', thereby effecting photoelectron
collection by detector 64'.
In operation, assuming that the apparatus is to be employed for the
analysis of 1,500 eV photoelectrons emanating from sample 25', the
typical potentials hereinbefore reported for V.sub.o -V.sub.3
inclusive are adequate. Then the equipotentials maintained between
V.sub.0 and V.sub.1 constitute a converging lens which imparts a
convergent transverse velocity component, v'.sub.t, to each
photoelectron traversing the lens. But v'.sub.t = (2eV.sub.o
/m).sup.1/2 sin .alpha., where m = the mass of the electron and
.alpha. is the angle between a line drawn parallel to the
longitudinal axis of tube 34' and the electron final trajectory A'.
Filter 66', in this instance, applies a homogeneous retarding field
which counterbalances, and thus nullifies, most of the axial
velocity component for photoelectrons at energy .about. eV.sub.o to
be analyzed. The high energy photoelectrons, not being appreciably
deflected by the electric fields, travel essentially straight paths
and, therefore, go past detector 64' without being counted. Low
energy photoelectrons, on the other hand, are repelled by the
homogeneous retarding field. However, detector 64', being under a
positive potential V.sub.3, attracts all photoelectrons which have
entered the cutoff means 67' comprised of collection cylinder
76.
It will be understood that the potential at V.sub.1 must be
experimentally adjusted to yield the proper path direction .alpha.
for any given as-received photoelectron energy input to be
analyzed. Moreover, it is essential that a field-free electron
drift region of substantial magnitude be maintained between the
retarding field of filter 66' and cutoff means 67' in order to
obtain the sharp segregation characteristic of FIG. 1B. The extent
of this region is denoted by line length A.sub.a ' in FIG. 8, which
should be a minimum of one cut-off element diameter (i.e., one
diameter d of cylindrical screen element 76). The lens action
obtained by the potential difference maintained between screen 39'
and tube 34' can have other geometries, such as, for example, a
spherical screen lens.
Thus, referring to FIG. 9, wherein all corresponding elements are
denoted by the same reference numerals as for FIGS. 6 and 8, except
double-primed, the axial velocity retarding field is provided by an
electrostatically charged filter constituting the two-part
spherical metal screen means denoted generally at 80a, 80b having
radii preselected to present the screen wires generally normal to
the paths of incoming electrons. Screens 80a and 80b are concave on
the electron input side and are in electrical connection with
potential taps V.sub.1 and V.sub.2, respectively, thereby
maintaining an equipotential line pattern (not shown) of
substantially uniform repulsion strength. Typically, screens 80a
and 80b can be formed on 10 cm. radii drawn from centers on the
longitudinal axis of tube 34". Electrically isolated rings 40", 41"
and 44" are disposed between screens 80a and 80b and are separated
one from another by resistors 50", 51" and 52", with final
connection to V.sub.2 ground through resistor 55". This
sub-assembly brings the equipotential lines into parallelism with
screens 80a and 80b while, at the same time, reducing field
distortion.
In this third embodiment, segregation of the photoelectron fraction
to be analyzed is by convergence, and this is effected by the
electron lens made up of spherically curved screens 81a, 81b and
the auxiliary ring 85, 86, 87 - resistor 90, 91, 92, 93
sub-assembly. Spherical screens 81a, 81b can be of identical
construction with screens 80a, 80b, except reversed in disposition,
so that their convex sides confront the incoming electrons. As in
the second embodiment, beam stop 75' is interposed between the
retarding field means and the converging means to block direct
electron impingement on detection means 64" which is, in this
design, disposed coaxially of tube 34".
In operation, a typical analyzed photoelectron trajectory is from
sample 25", irradiated as hereinbefore described but not detailed
again here, via line A", during which rejection of lower energy
electrons is effected, thence via A'", which is at only a small
divergent angle with respect to the axis of analyzer tube 34", and
thence sharply convergent along path B' to the collection face of
detector 64".
The cutoff means 67" in this design constitutes aperture plate 95,
covered by metal screen 95a assisting field-free drift maintenance,
disposed coaxially of tube 34". Again, a field-free electron drift
region greater than about one diameter of aperture opening 95 must
be provided, the length of line section B' denoting this
construction feature. Any electrons not segregated by the
convergence cutoff described are removed by the electron trap
denoted generally at 68". This comprises an apertured cylindrical
metal wire screen section 98 disposed concentrically within a solid
metal companionate outer enclosure 99, section 98 being typically
maintained at V.sub.4 (+100v) whereas enclosure 99 can be V.sub.5
(+1,000v).
* * * * *