Photoelectron angular distributions for Ar, Xe, N2, O2, CO, CO2, and NH3 were obtained at 584 A by observing the photoelectrons at a fixed angle and simply rotating the plane of polarization of a highly polarized photon source. The radiation from a helium dc glow discharge source was polarized (84%) using a reflection type polarizer

Hancock, W. H.

Samson, J. A. R.

English

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https://ntrs.nasa.gov/search.jsp?R=19760011845 2020-03-22T17:13:11+00:00Z
ANGULAR DISTRIBUTION OF PHOTOELECTRONS AT 584
USING POLARIZED RADIATION
b;
W. H. Hancock and James A. R. Samsun
Behlen Laboratory of Physics
University of Nebraska
Lincoln, Nebraska 68588
ABSTRACT
Photoele.tron angular distributions for Ar, Xe, N 2 , 02 , CO, CO2
1
and NH3 have been obtained at 584 a by observing the photoelectrons at
a fixed angle and simply rotating the plane of polarization of a highly
polarized photon source. The r, ,diation from a helium do glow discharge
source was polarized (84%) using a reflection type polarizer.
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(NASA-CE-146319)	 ANGULAR DISTRIBUTION OF
	 N76-18933
PHOTOELECTRONS AT 584 A USING POLARIZL•D
RADIATION (Nebraska Univ.)	 13 p HC $3.50
CSCL 20I	 Unclas
	
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Although the first measurements of the angular distribution of photo-
electrons ejected from free atoms date back to 1931 it is only during the
past ten years that interest has been revived in Uis area. l ' Z However,
the majority of thn published results have been concerned with the angular
distribution of electrons ejected from the rare gases. The motivation has
been to verify theoretical predictions that the angular distributions are
sensitive to the kinetic energy of the electrons and to the orbital from
which they are ejected. There are surprisingly few measurements of electron
angular distributions from molecules. The most extensive work on molecules
has been reported by Carlson and co-workers for discrete emission lines
at 584 9 (HeI) and at 736, and 744 9 (NeI).3-6
The purpose of the present work is two-fold; (a) it presents angular
0
distribution data at 584 A for comparison with previously published results
and (b) it presents a new technique that is simple and accurate to use.
In addition, results are presented for ammonia for the first time.
EXPERIMENTAL
In order to measure angular distributions of photoelectrons, the
electron flux is measured as a function of angle relative to the plane of
polarization of the radiation. Traditionally, this has been done by using
unpolarized radiation and rotating the electron spectrometer or light source
about an axis which is perpendicular to the beam axis. 2 ' 3 ' 7 ' 8 These methods
present several difficulties in that the interaction volume viewed varies
and must be corrected for in the data analysis. Also, motion of the
apparatus can cause shifting contact potentials and in the case of rotating
I
41.
W74	 ..R
-3-
the detector, the electrons have different trajectories relative to residual
magnetic fields.
Other workers have overcome the problem of varying interaction volume
by using plane polarized synchrotron radiation and rotating the electron
spectrometer about the beam in the plane perpendicular to the beam axis.9
This method still retains the problems of contact potentials and magnetic
field,
All these problems were alleviated in the present experiment by fixing
the position of the electron spectrometer at 90 0 from the beam direction
and then polarizing and rotating the plane of polarization of the beam.10
This methe ,	 c used in 1931 by Chaffee l	using a transmission
0
i
polarizer in the 2200 A spectral 	 region to determine the angular distribution
of the photoelectrons ejected from potassium. 	 However, no tratismission
0
polarizers exist for radiation below 1200 A. 	 Thus, a reflection type
F
polarizer similar to that described by Horton, et al. 11 was used to
0
polarize 584 A radiation from a helium d.c. discharge lamp. 	 This type
of polarizer, however, produces partially plane polarized light.
	
Provided
r
the degree of polarization is known it is possible to determine the angular
k
distribution as shown below.
It has been shown theoretically 12 ' 13 that for dipole transitions the
number of electrons N ejected per unit solid angle in a specific direction
by plane polarized radiation is given by
(1)
-4-
where ° is the photoionization cross sec , -,ion, $ is an asymnetr; parameter
that can take on values ranging from -1 to + 2, 0 is the angle between
the electric vector and the electron direction, and P2(cose)=3/2cos20-1/2.
Equation (1) has been generalized l4,15 to include the case of partially
polarized or elliptically polarized radiation and takes the form
N a cr { I + 1/20{9+1 (9 cos2O x + Cos 2 9y )-1}}	 (2)
where g is defined by the ratio I x/Iy ; I x is the intensity of the radiation
vibrating along the x axis and I  is the intensity vibrating along the
y axis. The degree of polarization P is related to g by the equation
P - (g-1)/(g+l). The angles Ox and a  refer to the direction of the
photoelectron with respect to the x and y axe!, respectively, as shown in
Fig. 1. Either the x or y axis must be oriented parallel to the direction
of maximum polarization. In the present ,experiment 0y = 90-Ox and ez=90
0
.
Thus, by measuring N at two different angles Ox and knowing g, the asymmetry
parameter 0 can be found. The angle e x was varied by rotating the
polarizer.
r
The polarizer consists of three mirrors arranged so that the photon
_	
beam enters and exits on the same axis as shown in Fig. 2. A computer
study, using known optical constants and	 Fresnel's equations, 16 was
0
made t.- calculate the degree of polarization, at 584 A, produced by
various reflective materials and as a function of angle of incidence.
The materials included silver, gold, osmium, glass, aluminum oxide, and
platinum. Gold provided the best compromise between transmission and
degree of polarization at the angles shown in Fig. 2.
.a
The polarizer was first constructed using glass microscope slides
coated with gold. However, because of scattering from the uneven sur-
faces of the slides, they were replaced by 1/4 wave optical flats with
a research grade optical polish. A transmission of 10% was measured
with a 1/2 M Seya monochromator. The degree of polarization was measured
with a vacuum uv polarization analyzer 17 based-on the 450 reflectance
technique of RaM novitch et al. 18 A value of 84% was obtained. These
values were each within a few percent of the values predicted using the
0
known optical constants for gold at 584 A, namely, n = 1.07 and k = 0.35.
The plane of polarization was changed by rotating the polarizer about
its axis. A laser was used in the extensive alignment procedure to
insure that there was no deviation of the photon beam during rotation.
A cylindrical mirror electron energy analyzer was used with the
photon beam incident at right angles to the axis of the analyzer. The
entrance slit to the analyzer was masked'so that only a 6 0 solid angle
was viewed at 900 from the beam direction. The detector was a "Spiraltron"
electron multiplier.
In order to eliminate errors caused by target gas pressure fluctuations
or lamp intensity variations, the data were taken using multiscan
techniques and with a multichannel analyzer. Data were taken for only
two orientations of the polarizer because this provided better statistical
results than curve fitting techniques. 19 However, initially data were
taken as a function of angle to test for symmetry. The two positions were
such that electrons were sampled in the direction of polarization and
perpendicular to it.
The polarizes was set in one position while the energy analyzer
scanned over one photoelectron energy peak in approximately two minutes.
-6-
This information was stored in one block of 123 channels in the multi-
scalar. The polarizer was then rotated by an electric motor to the other
position. The energy analyzer scanned the same energy peak. This
information was stored in the next 128 channels of the multiscalar.
The motor then rotated the polarizer to the first position and the
whole sequence was repeated. The motor drive was interfaced with the
multiscalar so that the entire procedure was automatic. The date were
transferred to punched paper tape and then directly to a computer.
The computer integrated the area under the electron energy peaks. The
ratio of these areas for the two polarizer orientations and the known
degree of polarization were used with Eq. (2) to determine the asymmetry
parameter.
The degree of polarization decreased slowly over a period of about
one week due to contamination of the mirrors by pump oil and exposure
to uv radiation. Consequently the degee of polarization was measured
for every data run.
RESULTS
The angular asymmetry parameters for Ar, Xe, N 2 , 02 , co " CO2 , and
NH3 are listed in table 1. The data were taken with an energy resolution
of 50 meV. In most cases the S-values are for specific vibrational
levels as indicated. Where more than one vibrational level is given,
the 6-value was found to be the same for each level indicated: Ii' no
vibrational level is given, the R-value represents the average value
taken over all the levels of the state.
Argon was used as a standard to insure the proper working of the
apparatus. A value of 5=0.95+0.02 was obtained. This compares favorably
FL^
a-7-F
with other literature values, some of which are given in table 1. No
r;
corrections are made to the data other than subtracting out any back-
ground of scattered electrons, which in the case of argon was negligible.
The fact that the system consistently reproduced the argon 0-value
through several realignments and for a variety of values of the degree
p
	
of polarization gives confidence that the system is working (P varied
from 65 to 84%). Where other data exist for comparison the overall
agreement is good. We verify the anomolous behavior between the v=0
and v=1 vibrational levels of the ground electronic states of N2 and
CO originally observed by Carlson. 3 For the 02 ground state Morgenstern
et al. and Carlson disagree. Our results support the negative i3--value
found by Carlson. However,there is a major disagreement between our
values and Carlson's for the B-state of CO +. This measurement has
been repeated several times, yielding the same results. No other
published values exist for comparison. In a similar vane we disagree
with the 13--value given by Morgenstern et al, for the B-state of 02.
In both cases the B-state electrons represent the lowest energy electrons
(0 to 1.5 eV) and thus the most difficult to measure by more conventional
methods. , The present technique is ideally suited for low energy electrons
where contact potentials or varying magnetic fields would tend to.give
false results. Thus, we have a high degree of confidence in our values. The
error bars given in table 1 represent standard•deviations.
ACKNOWLEDGEMENT
This work was supported by the National Aeronautics and Space
Administration under Grant NGR 28-004-021.
REFERENCES
1. M. A. Chaffee, Phys. Rev y 37 (1931) 1233.
2. J. Berkowitz and H. Ehrhardt, Phys. Lett., 21 (1966) 531.
3. T. A. Carlson and A. E. Jonas, J. Chem. Phys., 55 (1971) 4913.
4. T. A. Carlson, G. E. McGuire, A. E. Jonas, K. L. Cheng, C. P.
Anderson, C. C. Lu and B. P. Pullen, in D. A. Shirley (editor),
Electron Spectroscopy (Proc. Int. Conf. at Asilomar, Calif., 1971),
North-Holland Publ. Co., Amsterdam, 1972, p. 207.
S. T. A. Carlson and G. E. McGuire, J. Electron Spectrosc, 1 (1972/73)
209.
6. T. A. Carlson, Chem. Phys. Lett j 9 (1971) 23.
7. R. Morgenstern, A. Niehaus and M. W. Ruf, Chem. Phys. Lett., 4
(1970) 635.
8. D. A. Vroom, A. R. Comeaux and J. William McGowan, Chem. Phys.
Lett., 3 (1969) 476.
9. R. G. Houlgate, J. B. West, K. Codling and G. V. Marr, J. Phys. B:
Atom. Molec. Phys., 7 (1974) L470.
10. W. H. Hancock and J. A. R. Samson, Bull. Am. Phys. Soc., 19 (1974) 1203.
11. V. G. Horton, E. T. Arakawa, R. N. Hamm and M. W. Williams, Applied
Optics, 8 (1969) 667.
12. C. N. Yang, Phys. Rev., 74 (1948) 764.
13. J. Cooper and R. N. Zare, J. Chem. Phys., 48 (1968) 942.
14. J. A. R. Samson and J. L. Gardner, J.O.S.A., 62 (1972) 856.
15. J. A. R. Samson and A. F. Starace, J. Phys. B., 8 (1975) 1806.
16. J. A. R. Samson, Techniques of Vac. U. V. Spectrosc John Wiley
& Sons, Inc., 1967, pp. 296-319.
1e
17. J. A. R. Samson, to be published (1976).
18. K. Rabinovitch, L. R. Canfield and R. P. Madden, Applied Optics,
4 (1965) 1005.
19. Jan-Tsyu J. Huang, J. Wayne Rabalais acid Frank 0. Ellison, J.
Electron Spectrosc., 6 (1975) 85.
20. R. M)rgenstern, A. Niehaus and M. W. Ruf in L. M. Branscomb et al.
(editors), Electronic and Atomic Collisions (Abstracts VIIth Int.
Conf. at Amsterdam, The Netherlands, July 1971) North-Holland Publ.
I	 Co., Amsterdam, 1971, p. 167.
21. A. Niehaus and M. W. Ruf, Z. Physik, 252 (1972) 84-94.
22. J. L. Dehmer, W. A. Chupka, J. Berkowitz and W. T. Jivery, Phys.
Rev. A., 12 (1975) 1966.
a 
FIGURE CAPTIONS
Figure 1. Ejected electron direction in terms of ex, ey, e z relative to the
photon direction and direction of polarization. The % axis is the direction
of maximum polarization.
Figure 2. Schematic diagram of the rotatable reflection type polarizer.
i
TABLE 1
ANGULAR DISTRIBUTION ASYMMETRY PARAMETER B FOR PHOTOELECTRONS EJECTED FROM
VARIOUS GASES BY 584 A RADIATION
Gas
	
State	 Present Work	 Carlsona	 Morgenstern	 Dehmer°
Ar 2Pl2,2/1 0.951.02 0.85±.05 0.951.02	 0.89±.04
Xe 2 P 3/2 1.78±.04 1.451.05 1.71±.02	 1.77±.05
2ph 1.64±.04 1.351.05 1.641.06	 1.631.05
N 2 X2 E9 v=0 0168±.05 0.5 ±.1 0.691.03
V21 1.5 ±.1 1.4 1.4 ±.03
A2 IIU v=0,1 0.37±.05 0.3 ±.1 0.431.04
B2 
u
v=0 1.36±.1 1.251.1 1.251.04
02 X21& 1.2 -0.27±.05 -0.3 1.1 0.04±.05v --
9
a411
.39±.1 0.4 1.1 0.551.07(v=3)
u
b4E9 .57±.1 0.7 ±.1 0.73±.05(v=0)
B 2E9 1.041.2 _ _ _-• 1.35±.07(v=1)
CO X2E9 v=0 0.95t.05 0.8 ±.1
v-1 0.56±.07 0.4
A2 11 v=0,1 0.311.05 0.3 ±.1(v=•2)
VEY 0.28±.05 1.0 1.2
CO 2 X2 II9 v=0,1 -0.141.05 -0.1	 ±.l
A2 11 v=0,1,2 0.80±.05 0.8 ±.1
B 2 E+ -0.651.05 -0.4 ±.1
C2E9 1.15±.05 1.3 ±.2
NH 3 2A2 0.821.1
2E 0.211.1
aFrom ref. 3, 4, 5
6 From raf. 20, 21
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Angular distribution of photoelectrons at 584A using polarized radiation

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