Photoemission studies of electron states in silver chloride and alkali halide

Spicer, W. E.

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https://ntrs.nasa.gov/search.jsp?R=19690009212 2020-03-12T07:49:44+00:00Z
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NASA QUARTERLY STATUS REPORT #2
for Period Ending 31 October 1968
CONTRACT: NGR-05-020-066
Principal Investigator
W. E. Spicer
Stanford University
Stanford, California
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NASA QUARTERLY STATUS REPORT #2
for Period Ending 31 October 1968
I. PHOTOEMISSION STUDIES OF ELECTRON STATES IN SILVER CHLORIDE
Robert S. Bauer
A. Introduction
A good knowledge of the energy bands of silver chloride would be most
valuable in understanding the physics of the photographic process. Using
photoemission techniques, we are experimentally determining such informa-
tion as the position of the Ag (4d) band, regions of high densities of
states in the valence band, and details of tt- photoexcitation process.
Both the yield and energy distribution of photoemitted electrons have been
measured in a range of photon energies extending through 11.8 eV on epi-
taxial thin films of AgCl prepared by evaporation in ultrahigh vacuum.
Our current understanding of the photoemission results is summarized below.
B. Energy Distributions
The energy distribution curves (EDCs) normalized to absolute quantum
yield are presented in Fig. 1. The EDCs.are characterized by four pieces
of structure whose dependences on photon energy are summarized in Fig. 2.
The peak which emerges from below the threshold in the curves between
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Page 2
0.01--
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7.0
	
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9.0	 10.0	 11.0
ELECTRON ENERGY, E, ABOVE VALENCE BAND MAXIMUM (eV)
Fig. 1(a) ENERGY DISTRIBUTIONS NORMALIZED Ta ABSOLUTE QUANTUM YIELD FOR
ELECTRONS PHOTOEMITTED FROM AgC1 FOR PHOTON ENERGIES BETWEEN 8.4 eV AND 10.2 eV.
Nr.►
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(Aa-UO104d/su0iloala) 3P
Fig. 1(b) E.NERGY DISTRIBUTIONS NORMALIZED TO ABSOLUTE QUANTUM YI.ELD FOR
ELECTRONS PHOTOEMITTED FR0k AgCl FOR PROTON ENERGIES BETWEEN 10.2 eV AND 11.0 eV.  
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Fig . 1(c)	 ENERGY DISTRIBUTIONS NORMALIZED M ABSOLi1TE QUANTUM YIELD FOR
ELECTRONS PHOTOEMITTED FROM AgCl FOR PHOTON ENERGIES BETWEEN 11.0 eV AND 11.6 eV.
r
Page 5
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PHOTON ENERGY, b y (eV)
Fig. 2 STRUCTME PLOT FOR THE AgCl ENERGY DISTRIBUTION CURVES OF FIG. 1
11.0
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Page 6
10.0 eV and 10.8 eV (Figs. 1(a) and (b)) has a behavior which can be
described by the nondirect constant matrix element model for photo-
emission. 1 Beyond 10.8 eV, this peak changes position by the same amount
as the change in photon energy (&E = phv) as can be seen in the structure
plot (Fig. 2). A straight line fit to these points intersects the E-axis
at about -2.7 eV. Thus, this peak is due to a high density of states
region 2.7 eV below the top of the valence band. High resolution measure-
ments of the optical absorption edge of AgCl at low temperatures by F. C.
Brown and his coworkers 2,,3 agree quite well with the theory of phonon-
assisted indirect transitions with excitonic effects. 2 It has been pro-
posed for quite some time that this indirect behavior is due to the Cl
I
(3p) band being the highest occupied band and having its maximum at a
point other than the center of the Brillouin zone, r.4 Published tight-
bindings and APW6.energy band calculations and recent piezo-optical
measurements  have further supported this interpretation. If we assume
that the halogen p band is the highest valence band and that the ap-
proximately 3 eV width calculated by Bassani, Knox, and Fowler s (BKF) for
this band is a reasonable value, the density of states region 2.7 eV below
the valence band maximum is associated with the Cl (3p) band. It should
be noted that the location of the p band at the L point by BKF 5 agrees
quite well with this energy.
The structure which emerges beyond 11.0 eV (see Fig. 1(c)) is un-
affected by the threshold function in only the last few EDCs. From these
few structure positions, a high valence band density of states would be
predicted at -3.3 eV. Based on the assumptions enumerated above, this
density of states is associated with the,Ag(4d) electron states. The
location of the d band in the BKF calculation s
 is in good agreement with
this energy.
Page 7
At low photon energies (see Fig. 1(a)) there is a single peak whose
position does not change by amounts equal to the increment in photon
energy (AE ^Ahv). Since part of the final state energy region over
which this occurs is identical with the region over which the previously
discussed Cl (3p) peak moves with AE = phv, the behavior of the low-energy
peak must be due to reasons other than structure in the final density of
states or threshold function modulation. The behavior may be due to the
importance of conserva y inn of crystal momentum k as a selection rule for
this transition. This peak is not prominent beyond 10.6 eV; however, two
shoulders are present in the EDCs (Figs. 1 (b) and (c)). The higher energy
shoulder moves as'AE = phv while the lower energy one moves in the same
manner as the peak seen for photon energies below 10 eV. As noted in
Fig. 2, the higher energy shoulder electrons appear to originate at a
region of high density of states about 0.9 eV below the valence band
maximum. Work is continuing to understand the physical origin of these
pieces of structure in the energy distributions.
C. Yield
Atypical plot of the absolute yield of photoemitted electrons is
presented in Fig. 3 for a film that was about 130 R thick. The quantum
yield is seen to rise sharply with by < 8.8 eV from a threshold of ap-
proximately 7.4 eV. The yield at photon energies below this threshold
is due to photons which are not absorbed by the AgCl film and excite
electrons in the Ag substrate. The resulting electrons which escape
through the AgCl thin film cause the observed low-energy structure seen
in Fig. 3. This is similar to the subthreshold yield measured for thin
alkali halide films on Cu substrates,8
7
Page •8
10
6.0	 7.0	 8.0	 9.0	 10.0	 11.0	 12.0
PHOTON ENERGY, hv(eY)
Fig. 3 SPECTRAL DISTRIBUTION OF ABSOLUTE YIELD OF ELECTRONS PHOTOEIQTTED FROM
,.	 AN EPITAXIAL FILM OF AgC1 ON A Ag SUBSTRATE (FILM THICKNESS W 130 a).
The yield for photon energies below about 7.5 eV is due
to photoemission from the Ag substrate.
1
1	
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7
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Page 9
Beyond the threshold region, the yield rises slowly to about 10.0 eV
where there is a change in slope. Since the energy distributions of
electrons photoemitted at low photon energies (.i.g. 1(a)) show that the
low-energy electron peak has fully emerged from below the vacuum level
by hN. = 9.0 eV, the quantum yield could be expected to have reachPA
nearly a constant value at this photon energy as is the case for ti.e
yield for 10.4 eV < by
 C 11.0 eV and by > 11.4 eV. The observed re-
duction in the yield from this constant level could be due to the reduced
absorption in the region around the dip in of at 9.4 eV (see Fig. 4).
The relatively low absorption in this region means that on the average
the electrons are excited deeper in the film thus increasing the probability
for aft-electron to be scattered before reaching the surface and being un-
Roable to escape. As is observed in otherlids, ll this reduced electron
transport probability could lead to the reduced yield. The increases
in ;Meld from 10.0 eV through 10.4 eV and 11.0 eV through ll.- 42' eV could
be dv.e to the "uncovering" of new regions of high densities of states in
the valence band. As seen in the EDCs (Fig. 1) in these photon energy
regions, the structure associated with the halogen p band begins to emerge
at 10.0 eV; similarly, the structure associated with the Ag (4d) band
emergea starting at 11.0 eV. This behavior could produce the changes in
slopes of the yie'.d curve at these photon energies.
* The absorption coefficient presented in Fig. 4 is the result of a
Kramers-Kronig analysis of the reflectivity of White and Straley 9 which
was smoothed over a 1 eV range in accord;.nce w: tL earlier measurements. The
reflectivity data values were generously supplied by J. J. Whitey III. Some
details of the calculation were presented in an earlier report. 10
Page 10
ON
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Fig. 4 ABSORPTION COEFFICIENT FOR AgCl. (See footnote on p. 8). I
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Page 11
D. Conclusion
Some of the significant differences between the 'results of this study
and previously reported measurements have been discussed in an earlier
report. 
10 
It should be added that the structure seen in the yield (Fig. 3)
beyond the threshold was seen for the first time due to the higher re-
solution of the experiments and better sample surface conditions. It
should be kept in mind that the association of regions in the density of
states seen in the photoemission data with the electronic energy band
structure are based on the widely accepted `-7.,12 view that the highest
occupied band is the Cl (3p) band. The foundations of this assignment
are being reviewed in light of the results of this photoemission in-
vestigation and new "first-principles' band calculations by Herman.13
Future studies will be directed toward elucidating characteristics of
electron states of the silver halides AgBr and AgI.
Page 12
1 R. C. Eden, Ph.D. Dissertation, Stanford University, 1967 (unpublished);
Rept. SEL-67-038 (TR No. 5221-1) Stanford Electronics Laboratories, Stan-
ford, California.
2 B. L. Joesten and F. C. Brown, Phys. Rev. 148, 919 (1966).
3 B. L. Joesten and F. C. Brown, Phys. Letters 17, 202 (1965);
F. C. Brown, T. Masumi, and H. H. Tippins, J. Phys. Chem. Solids 22,
101 (1961).
4 F. Seitz, Rev. Mod. Phys. 23, 328 (1951); J. A. Krumhansl, in Photo-
conductivity Conference edited by R. G. Breckenridge, B. R. Russell,
and E. E. Hahn (John Wiley and Sons ) Inc., New York, 1956), p. 450;
F. C. Brown, J. Phys. Chem. 66, 2368 (1962).
5 F. Bassani, R. S. Knox, and W. Beall Fowler, Phys. Rev. '137, Al217 (1965).
6 P. M. Scop, Phys. Rev. 139, A934 (1965).
7 G. Ascarelli, Phys. Letters 26A, 269 (1968) and Phys. Rev. Letters 20;
44 (1968).
8 T H. DiS efano, NASA Grant NGR-05-020-066 Annual Report, May 1, 1967
to May 1, 1968, p. 17.
9 J. J. White III and J. W. Straley, J. Opt. Soc. Am. 58, 759 (1968).
10 R. S. Bauer, NASA Grant NGR-05-020-066 Quarterly Statu g Report #1,
May 1, 1968, through July 31, 1968, p. 1.
11_B. H. Schechtman, Ph.D. ' Dissertation, Stanford University, 1969.
12 R. B. Aust, Phys. Rev. 170, 784 (1968).
13 F. Herman, private communication, 1968.
C
Page 13
II. ALKALI HALIDES
T. DiStefano and W. E. Spicer
Studies have been made of the alkali iodides (LiI, NaI, KI, RbI, and
CsI). Energy distribution curves (EDCs) as well as quantum yield has been
measured. Particular attention was paid to the doublet structure observed
in the valence states of these materials. This structure has been as-
sociated with the halogen spin orbit splittin;. A splitting of 1.1 eV is
observed for CsI in good agreement with the atomic splitting. However, the
splitting is found to increase as one goes to the lighter alkali compounds.
It is approximately 1.6 eV for LiI. The data is now being analyzed and
will be reported on more completely in the next quarterly report.
Apparatus has been built so that optical reflectivity measurements
can be taken from the photoemissive surfaces without breaking vacuum.
This has been successfully used for CsI. The results have strong Simi-
*
larity with the absorption spectrum of Eby and Teegarden, except that a
strong reflection peak was observed above 10 eV which would not have been
anticipated from any of the earlier optical studies. A strong piece of
structure also appears in the EDCs near the maximum energy in this photon
energy range. These effects will be examined more closely.
Preliminary work has been done on the coevaporation of two alkali
halides to form mixed crystallites. Experimental difficulties have been
encountered in the coevaporation. However, it is felt that these are under-
stood and that they will be eliminated shortly.
* J. E. Eby and K. J. Teegarden, Phys. Rev. 116; 1099 (1959).


Photoemission studies of solids Quarterly status report, period ending 31 Oct. 1968

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