The electron collecting efficiency of a cylindrical mirror energy analyzer incorporating retardation of the electrons prior to analysis has been determined over the range 0 to 30 eV by two methods. The first method requires the use of a vacuum ultraviolet monochromator to produce monoenergetic electrons of different energies; the second method involves measuring the energy-brightness relationship of the retarding optics and should be applicable to any deflection analyzer with pre-retarding optics. The results of the two methods are compared and the limitations of the latter method are discussed

Gardner, J. L.

Samson, J. A. R.

English

NASA Technical Reports Server

COLLECTING EFFICIENCY OF A CYLINDRICAL MIRROR ELECTRON ENERGY
ANALYZER WITH PRE-RETARDING LENS
J. L. Gardner and James A. R. Samson
Behlen Laboratory of Physics
University of Nebraska
Lincoln, Nebraska 68508 U.S.A.
ABSTRACT
The electron collecting efficiency of a cylindrical mirror energy
analyzer incorporating retardation of the electrons prior to analysis
has been determined over the range 0 to 30 eV by two methods. The
first method requires the use of a vacuum ultraviolet monochromator
to produce monoenergetic electrons of different energies; the second
method involves measuring the energy-brightness relationship of the
retarding optics and should be applicable to any deflection analyzer
with pre-retarding optics. The results of the two methods are compared
and the limitations of the latter method are discussed.
.... 
N74-319 2 5
(NASA-CR-13 9 6 6 2) COLLECTING EFFICIENCY OF
A CYLINDRICAL MIRROR ELECTRON 
ENERGY
'ANALYZER WITH PRERETARDING LENS (NebraskaUnclas
univ.) 20 p HC $14.00 CSCL 14B 3/clasG3L!_ 4770
https://ntrs.nasa.gov/search.jsp?R=19740023812 2020-03-23T04:07:24+00:00Z
2INTRODUCTION
We have previously described a cylindrical mirror electron energy
analyzer with a pre-retarding lens, for use in determining photoelectron
branching ratios.l Included was a brief description of the measurement
of the electron collecting efficiency of the analyzer. Knowledge of this
electron collecting efficiency (the transmission of the analyzer combined
with the efficiency of the detector) is extremely important for accurate
measurement of the relative populations of final states by photoelectron
spectroscopy. In this paper we describe a more accurate determination
of the collecting efficiency of the analyzer extended over the range
0 to 30 eV using the technique previously described, which requires the
use of a number of different photon energies selected with a vacuum
ultraviolet monochromator. In addition, we present a second method for
determining the collection efficiency, which is a simplified version
of the method described by Poole, et al. 2 Only one photon energy is
required and thus an undispersed light source may be used.
The analyzer is shown schematically in fig. 1. Electrons ejected
in a small cone about 540 44' to the axial photon beam pass through a
pair of slits normal to the electron beam direction and may be retarded
or accelerated before passing into the deflection region. The analyzer
is designed for first-order slit to slit focusing on the inner cylinder
and the beam is baffled to accept a 60 cone at the detector end. Spectra
are normally recorded in the constant resolution mode, that is, the
3voltage on the retarding/accelerating slits is swept and the deflecting
field is held constant so that only electrons exiting from the lens
with the required pass energy reach the detector.
CROSS-SECTION METHOD
The geometry of this analyzer is such that electrons of different
angular distributions are not discriminated against, nor are they dis-
criminated against for varying polarization of the incident photon beam.3
Thus, if the gas pressure in the source region is .held constant and
electron scattering from the gas is not significant, it follows that
the rate at which electrons are detected, Nd, is given1 by
Nd a I0ayC(E),
where Io is the incident photon flux, a is the absorption cross section
of the gas, y is the ionization yield, and C(E) is the collection
efficiency of the analyzer. For the rare gases, y is unity in the photon
energy range covered here, (for wavelengths shorter than the onset of
double ionization, the yield is greater than unity). Thus, C(E) is
determined on a relative basis, as a functio iof the initial electron
kinetic energy and for a given pass energy, by recording photoelectron
spectra from a rare gas at a variety of wavelengths of measured intensity
at which the photoabsorption cross section is known.
The calibration results for a number of different ion chambers,
obtained with a spark discharge light source (and corresponding weak
analog signals) have been presented elsewhere. 1 The present calibration
4was performed with a duoplasmatron light source dispersed with a 1/2 m
Seya-Namioka monochromator, producing a number of Ar II emission lines
continuous in time so that electron counting techniques were possible.
The intensity of the light source was monitored on an aluminum photo-
cathode, calibrated against a tungsten standard, positioned at the rear
of the ionization region. Photoelectron spectra were recorded from both
argon and xenon, with relevant cross sections taken from the literature.4
The intensities of the 2P3/2 and 2P1/ 2 peaks in argon were summed at each
wavelength and the electron energy was taken as a mean of the peaks
weighted by the statistical weight for 3/2:1/2 spin-orbit splitting.
For xenon, separate curves were plotted for the 2P3/2 and 2P1/2 peaks.3/22 2
The P3/2 to P1/ 2 intensity ratio in each of the rare gases has been
shown to be constant in the wavelength range covered here. 5  A number
of runs were made for a pass energy of 3 eV and then normalized to give
the data shown in the range 0 to 11 eV in fig. 2.
To extend the energy range of the calibration, a Vodar type grazing
incidence monochromator was used to disperse the radiation from a DC dis-
charge in helium. A number of photoelectron spectra were recorded from
0 0
argon at 584, 537, and 304 A and from neon at 537, 304, and 256 A. The
resulting collection efficiency data were normalized to the 0 to 11 eV
region and averaged to produce the data points at 19, 25, and 27 eV
in fig. 2
The relative collecting efficiency as a function of the initial
electron kinetic energy was determined by the cross section method for
5different pass energies. The curves for the different pass energies
could all be normalized to agree with the 3 eV pass energy data of
fig. 2 for electron energies greater than 0.7 eV. The variation in
collecting efficiency in the low energy region for the different pass
energies was measured by comparing photoelectron spectra recorded from
0
oxygen with dispersed 736 A Ne I radiation. The spectrum provides a
source of electrons of reasonable intensity over the range 0 to 4.8 eV.
The results are plotted in fig. 3 for pass energies of 0.75, 1.0, 1.5,
2.0, and 2.5 eV, all relative to the intensities of spectra recorded at
a pass energy of 3 eV. These results were in agreement with those
obtained by comparing spectra of the B 2 state of 02 at 584 A taken
at.different pass energies. The curves for the different pass energies
depart only for electron energies less than 0.7 eV. The analyzer becomes
much more efficient for low energy electrons as the pass energy is lowered,
with the efficiency peaking about 0.2 eV.
The collecting efficiency was monitored as a function of time by
0
periodically recording the oxygen spectrum at 736 A at a pass energy
of 3 eV. No significant changes in the relative population of the
peaks were detected, indicating that the surfaces of the analyzer, coated
with colloidal graphite, did not change sufficiently to affect the elec-
tron collecting efficiency.
The collecting efficiency of the analyzer for zero acceleration/
retardation in the lens (where the spectrum is scanned by varying the
volatage across the cylinders) was also measured by the cross section
6method. The pressure in the analyzer was held below 10
-4 Torr to reduce
differential scattering effects, which are more serious in this mode
of operation.1 The cone voltage of the channeltron detector was held
at 400 V to minimize effects caused by the changing electron energy at
the detector. Peak heights (or, equivalently, peak areas divided by the
electron energy) were used in recording the photoelectron intensities.
The results for the range 0 to 9 eV are plotted in fig. 4. As determined
previously,l low energy electrons are severely discriminated against in
this mode of operation. The primary cause of the roll-off at low energies
is apparently electrostatic (contact potentials or non-uniform surface
potentials) since small changes in the magnetic field ( but large enough
to effect the resolution) did not significantly alter the shape of the
curve.
PASS CURVE METHOD
Poole, et al.2 recently proposed a technique for measuring the effect
of a retarding lens on the transmission of an energy analyzer, which is
summarized as follows. .Photoelectron spectra were recorded from a number
of gases at a number of undispersed wavelengths. The count rate for a
given initial electron energy at each wavelength was measured as a function
of the pass energy by varying the degree of acceleration or retardation
in the lens. Then the assumption was made that the transmission of the
deflection region of the.analyzer was constant when the electrons had
zero acceleration in the lens and that the efficiency of the detector
7was constant. Thus, a family of curves of collecting efficiency vs. pass
energy for electrons of different initial kinetic energies was generated,
all normalized (to unity) at the point where the pass energy equalled the
initial electron energy. These curves were then interpolated to find the
collecting efficiency as a function of the electron energy for a given
pass energy and this data was used to correct a spectrum recorded at
that pass energy by scanning the voltage on the lens. Kemeny, et al. 6
subsequently derived expressions which explained the transmission data
obtained as just described in terms of the energy-brightness law for
their analyzer geometry.
We wish to show here that the technique of Poole, et al. can be
simplified and that only one source of electrons is required to determine
the transmission of the lens/analyzer combination. Further, the limita-
tions of this technique can be seen by comparing the results obtained
with those from the cross section method.
The collecting efficiency of a lens/analyzer combination will be
a function of both the pass energy E and the initial electron kinetic
energy Ek. From conservation of flux and Abbe's sine law, we see
that the transmission of the lens section is separable into independent
functions of the kinetic energy that the electron has on either side
of the lens. The electrons in this case enter the lens with energy Ek
and exit from the lens with energy Ep. Once out of the lens, the
transmission through the deflection region and the detection efficiency
will depend only on Ep. It follows that the collecting efficiency ofp
8the lens/analyzer combination can be separated into independent functions
of the two variables. That is,
C(EkEp) = g(Ek).f(E). (1)
Ek and Ep (in electron volts) are connected by the relation
Ep = Ek + Var , (2)
where Var is the accelerating potential applied across the lens, and
may be positive or negative.
Consider now the collecting efficiency when Ek = Ep, i.e., when
the voltage across the lens elements is zero. Equation (1) becomes
C(E,E) = f(E).g(E), (3)
so that the function g(E) is given by
g(E) = C(E,E)/f(E). (4)
Inserting equation (4) into equation (1) yields
C(E ,Ek)'f(E )
C(EkE) = f(Ek )  (5)
If the spectrum is recorded at a fixed pass energy, the function f(Ep)
is a constant. Hence
C(Ek,Ek)
C(EkEp) f(Ek) (6)
The function C(Ek,Ek) depends on the transmission through the
analyzer and on the detection efficiency. The transmission through the
analyzer is generally assumed to depend only on the geometry and the
detector efficiency is assumed to be constant. If these assumptions
are true, then C(Ek,Ep) becomes the transmission function of the lens
9alone and essentially represent the change in the brightness of the beam
as the electron.energy changes in the lens.2,7,8 This energy-brightness
relationship depends on the source dimensions and will in general be
difficult to calculate exactly. However, the relationship may be
determined experimentally, at least over a limited energy range.
Assuming C(E,E) to be constant, the function f(E) is readily
measured by plotting a curve of the count rate of photoelectrons from
any one source, at one wavelength, as a function of the pass energy by
0
varying Var. This was done for Xe photoelectrons at 584 A over the
range 0 to 40 eV. The pressure in the analyzer was held below 10-4 Torr
to minimize differential scattering effects. The results at pass energies
greater than 12 eV showed a strong dependence on the voltage on the cone
of the channeltron detector. The electrons focus on the inner cylinder
of the analyzer and the cone voltage deflects them into the detector.
However, little change was observed for cone voltages above 1400 V and the
results taken with an EMI venetian blind multiplier with the front dynode
at 1400 V agreed with the channeltron data. The curve of count rate vs.
pass energy for the 2P3/2 electrons (energy 9.09 eV) is plotted in fig. 5.
The data of fig. 5 was least squares fitted with a power law function,
with the result that the function f(E) has an E 50 dependence over this
energy range. This curve is also plotted in fig. 5. Data were also re-
corded for Xe P1/2 electrons at 920 A (energy 0.04 eV) and the count rate
showed the same functional dependence on the pass energy. We note that
for a line source where the lens produces focusing in one dimension only,
10
the brightness is proportional to E1/2 . The photon beam in our analyzer
enters along the axis and if few off-axis electrons pass the system of
baffles, we might expect the E1/ 2 dependence shown in fig. 5. However,
the count rate data of fig. 5 show a systematic departure from the
E1/2 curve, probably caused by the collection of off-axis electrons. In
a similar analyzer in this laboratory in which the photon beam enters
normal to the axis, the source is smaller and more uniform and the count
rate shows a linear dependence on the pass energy, cf. the results of
Poole, et al. 2 These results are explained by the brightness of the
electron beam being proportional to E for a lens which focuses in two
dimensions. 7
Measurement of the function C(E,E) by the cross section method is
described above. It is clear from fig. 4 that C(EE) is not constant
for electron energies less than 4 eV, but does become constant at
greater energies. Thus, we may reasonably predict from equation (6)
that for electron energies greater than 4 eV, the relative collecting
efficiency of the analyzer at any fixed pass.energy will be given by
the inverse of the function plotted in fig. 5.
COMPARISON OF THE TWO METHODS
The function f(E) plotted in fig. 5 was inverted and normalized
to the relative collecting efficiency for a pass energy of 3 eV (fig. 2)
at.5 eV. The two curves are plotted for comparison in fig. 6. The
agreement between the curves is excellent in the range 2 to 20 eV.
Agreement below 2 eV is not expected since the function C(E,E) from
fig. 4 is clearly not constant in this range. However, multiplying the
functions of figures 2 and 5 together to form C(E,E) by equation (3)
does not produce the function shown in fig. 4 for the relative collect-
ing efficiency when the voltage across the lens is zero. It is thought
that losses in the analyzer, responsible for both the reduced efficiency
at low electron energies compared with that predicted by the inverse of
the pass curve and the roll-off in the collecting efficiency for zero
volts across the lens, are different for the two cases because of
field penetration through the lens into the ionization region. The
results shown in fig. 3 for the different pass energies are also
indicative of different field penetration into the source.
The disagreement above 20 eV is probably caused by poor collection
of the diverging electron beam at the channeltron. In addition to the
difficulties noted above when measuring the pass curve, the relative
collecting efficiency between 5.5 and 25.0 eV was found to be a function
of the channeltron cone potential, even though the electrons were being
analyzed at a fixed pass energy of 3 eV. The relative efficiency varied
by 20% as the cone potential was raised from 100 to 1400 V. Although
the electrons originally formed with different energies have the same
energy at the exit slit on the inner cylinder, the beam divergence will
be different because of the action of the lens. Thus the beam will
cover different regions of the cone; the.gain of these detectors has
been shown to vary greatly across the face of the cone.9 Since
12
the analyzer was designed for a first order focus, the changing divergence
could be expected to have little effect on the resolution; no change
in the FWHM of the detected peak was noticed for the two different
energies.
CONCLUSION
The collecting efficiency properties of a cylindrical mirror
analyzer incorporating a pre-retarding lens have been described in
detail. The simplified method presented for the determination of the
collecting efficiency has been shown to give good agreement with the more
basic cross section method, with the exceptions noted. The agreement
at the higher energies could possibly be improved by redesigning the
analyzer to focus the electrons on the axis, close to the cone of the
detector. This simplified method should be applicable to the measure-
ment of the relative collecting efficiency of any lens/analyzer combina-
tion, provided precautions are taken to minize differential detection
effects.
ACKNOWLEDGEMENT
This research was supported in part by the National Aeronautics
and Space Administration and the Atmospheric Sciences Section, National
Science Foundation.
REFERENCES
1. J. L. Gardner and J. A. R. Samson, J. Electron Spectrosc. 2 (1973)
267.
2. R. T. Poole, R. C. G. Leckey, J. Liesegang and J. G. Jenkin, J.
Phys. E (Scientific Instruments) 6 (1973) 226.
3. J. A. R. Samson and J. L. Gardner, J. Opt. Soc. Amer. 62 (1972)
856.
4. J. A. R. Samson, Advan. Atom. Mol. Phys. 2 (1966) 177.
5. J. A. R. Samson and R. B. Cairns, Phys. Rev. 173 (1968) 80.
6. P. C. Kemeny, A. D. McLachlan, F. L. Battye, R. T. Poole, R. C.
G. Leckey, J. Liesegang and J. G. Jenkin, Rev. Sci. Instr. 44
(1973) 1197.
7. J. C. Helmer, Amer. J. Phys. 34 (1966) 222.
8. J. C. Helmer and N. H. Weichert, Appl. Phys. Letts. 13 (1968)
266.
9. R. L. Arnoldy, P. 0. Issacson, D. F. Gats and L. W. Choy, Rev.
Sci. Instr. 44 (1973) 1197.
FIGURE CAPTIONS
1. Schematic cross section of the analyzer. A-inner cylinder;
B-collimating slit; C, D-retarding slits; E-boron nitride
insulator.; F-photocathode; G-collimating slit; H-detector;
I-outer cylinder.
2. Collecting efficiency for a pass energy of 3 eV.
3. Collecting efficiency for the pass energies indicated,
relative to that for a pass energy of 3 eV.
4. Collecting efficiency for zero retardation in the lens;
spectra are recorded by scanning the voltage across
the cylinders.
5. Count rate of Xe 2P3/2 electrons at 584 S. The dashed
curve represents an E1 / 2 dependence.
6. Comparison of the collecting efficiency obtained by the
two methods. - cross section method; - - - inverted
pass curve method; e normalization point.
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Collecting efficiency of a cylindrical mirror electron energy analyzer with preretarding lens

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