PHOTOELECTRIC EMISSION OF POTASSIUM AND
RUBIDIUM HALIDES IN THE EXTREME
ULTRAVIOLET
T. Sasaki, H. Sugawara, Y. Iguchi
To cite this version:
T. Sasaki, H. Sugawara, Y. Iguchi. PHOTOELECTRIC EMISSION OF POTASSIUM AND
RUBIDIUM HALIDES IN THE EXTREME ULTRAVIOLET. Journal de Physique Colloques,
1971, 32 (C4), pp.C4-290-C4-294. <10.1051/jphyscol:1971453>. <jpa-00214654>
HAL Id: jpa-00214654
https://hal.archives-ouvertes.fr/jpa-00214654
Submitted on 1 Jan 1971
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JOURNAL DE PHYSIQUE
Colloque C4, suppMment au no 10, Tome 32, Octobre 1971, page C4-290
PHOTOELECTRIC EMISSION OF POTASSIUM
AND RUBIDIUM HALIDES IN THE EXTREME ULTRAVIOLET
T. SASAKI and H. SUGAWARA
Institute of Plasma Physics, Nagoya, University, Furocho, Nagoya 464, Japan
Y. IGUCHl
Institute for Optical Research, Kyoiku University, Hyakunincho, Tokyo 160, Japan
Rksum6. - Nous donnons les rendements quantiques de l'emission photoklectrique des halogenures de potassium et de rubidium a tempkrature normale dans le domaine de 10 a 40 eV. Nous
montrons que la variation spectrale du rendement des isolants est determinke par le spectre d'absorption et la diffusionelectron-Clectron. Les valeurs de l'affinite electroniqueont ete dkterminees en
supposant que la diminution ou l'augmentation du rendement quantique dues a la diffusion inklastique des photoClectrons se produit en dessous et en dessus de fiw = 2(Eg x). Les rendements
quantiques Cgaux a 1 ou supQieurs sont observes dans un domaine d'bnergie supkrieur a 20eV.
+
Abstract. - Quantum yields of photoelectric emission of potassium and rubidium halides at
room temperature in the region from 10 to 40 eV are reported. It is shown that the spectral yield of
insulator is determined by the absorption spectrum and the electron-electron scattering. The values
of electron affinity have been determined by assuming that suppression and enhancement of the
quantum yield due to the inelastic scattering of photoelectrons take place below and above
fiw = 2(Eg + x). The quantum yields as high as 1 or more are observed in a region above 20 eV.
1. Introduction. - In the present report, the photoelectric emission of potassium and rubidium halides
in the region between 10 and 40 eV at the room temperature will be described.
Photoelectric emission from solids has been studied
extensively in a recent decade as a powerful tool for
investigating the electronic structure of solids, because
it provides additional informations to the absorption
spectrum, namely, the absolute location of the important critical points in the Brillouin zone relative to the
vacuum level, and the dynamic behaviors of photoelectrons after excitation. They are brought about
through the energy analysis of the emitted photoelectrons.
Photoelectric emission from solid, however, is a
more complicated process than ionization of atoms and
molecules, because it involves three different processes,
namely, the excitation of electrons by photons, transport of excited electrons, and their escape from the
surface. Photoelectrons are subject to a number of
scattering before escaping from the solid surface, and
sometimes informations of the band structure they
bore initially may be obscured or lost. Therefore,
understanding the scattering mechanism is essential
in the study of the photoelectric emission. Among
the various scattering mechanisms, the most important in this respect is the intrinsic electron-electron
interaction. A photoelectron excited well inside the
bulk solid material loses its energy at each collision
with another electron and if its final energy after
collisions is below the vacuum level, it will no longer
be able to escape from the solid surface. The mean
free path for these collisions determines the mean
escape length of photoelectrons, which in turn determines the quantum efficiency of emission. Therefore,
in general, the structure of absorption spectrum is
reproduced in the spectral yield of photoelectric emission, because the depth of generating photoelectrons is
smaller for the higher absorption. Otherwise there
will be no spectral dependence of photoelectric emission at all, because nearly all the photons incident
upon the solid are eventually absorbed in the XUV
region, provided the specimen is thick enough.
However, there is another important factor which
determines the general shape of the spectral yield of
insulators. I t is the enhancement or suppression of the
photoelectric emission due to the inelastic scattering.
In metals, the amount of energy which colliding electrons exchange at each collision is allowed continuously from zero. Accordingly, a photoelectron can
interact with any electron below the Fermi surface.
As a result, energy loss of electrons is allowed for any
amount of energy and multiple scattering is quite
plausible. This effect limits the quantum efficiency of
metals normally below 10 %.
In the case of insulators, on the other hand, the large
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971453
PHOTOELECTRIC EMISSION OF POTASSIUM AND RUBIDIUM HALIDES
energy gap E, prevents the inelastic scattering if the
kinetic energy of electrons in the conduction band is
lower than Eg,or more exactly, Ex, the energy required
for exciton formation. These electrons can escape
without losing energy if it is higher than the electron
affinity X, and their final momenta are directed toward
a surface. This picture may seem too simple, because
it dozs not take into account other possible mechanisms for inelastic scattering, such as the scattering
due to the color centers, impurity states and other
imperfections of the crystal. However, to the first
approximation, the extrinsic scattering as these may be
neglected and the general feature of photoelectron
spectra can be described according to above picture.
When the energy of the photoelectron is high enough
to excite an electron to the exciton states or to the
continuum states, the photoelectron may be scattered
t o the lowest part of the conduction band and conse.quently no longer be emitted as far as the electron
affinity is positive. The suppression takes place in this
region.
If the photon energy is increased further, there will
be two critical photon energies where the scattered
electron is allowed to emerge again out of the solid.
Let them be Zio, and Ziw,, where Ziw, = 2 E, i- x,
and Ko, = 2(Eg f x). Zio, is an energy in which an
electron would have a sufficient energy to emerge
only when a photoelectron exchanges an exact amount
of energy (1) Eg or (2) Eg + with an electron at the
top of the valence band. Since both energy and momentum must be conserved in the collision, the cross
section for such a collision will be in general quite
limited, or rather accidental even if it happens, while
there are other types of collisions exchanging energies
between above two values, and they will prevent both
.electrons from escaping. Increasing photon energy
above Zio, will naturally incrase collisions of the
emerging type, but on the other hand, collisions of the
preventing type will still keep increasing if the scattering matrix element is assumed to be constant,
because the available volume of the momentum space
for scattering will increase. Therefore, the photo.electric yield will not increase appreciably, or keep
decreasing until Ziw, is reached. At fiw,, both the
scattered electrons or at least one of them are able to
.escape for any collision, and the yield will increase
very rapidly. In this way, the enhancement of photoelectric emission takes place in this second region of
photon energies higher than Ziw,.
C4-291
sodium salicylate above argon limit. Another photomultiplier monitoring the instantaneous intensity of
the synchrotron radiation in the visible region was
heterochromatically calibrated by the ion chamber
which measures the extreme ultraviolet light simultaneously [2]. Once calibrated, the monitoring multiplier was used throughout as a secondary standard
in determining the quantum yield. Ion chamber was
then replaced by a chamber for measuring photocurrent which was normalized against the monitor
signal obtained simultaneously. A plate of stainless
steel was mounted as an emitter at the center of a
cylindrical collector of the same material and the
sample was evaporated onto the emitter plate outside
the collector at the pressure of about 3 x
torr.
Spectral width of the slit was about 3 A when the slit
opening was 200 p.
3. Results and Discussion. - Figure 1 shows the
spectral yield of rubidium iodide as a typical result.
The upper curve is the absolute quantum yield, defined
as the number of photoelectrons per incident photon,
+
2. Experimentals. - Our experiments were carried
out using the synchrotron radiation emitted by an
electron synchrotron of the Institute for Nuclear
'Study, Tokyo, operated at 0.9 GeV as the light source.
Photon flux coming out of the 0.5 meter Seya monochromator with 200 or 100 p wide entrance and
exit slit was determined by means of a double ionization chamber with helium or argon [I] below their
ionization limit, and a photomultiplier coated with
O L , b . " . ' . '
"
20
8
.
.
30
. .
*
.I
40
PHOTON ENERGY ( eV )
FIG. 1. - Photoelectric yield and absorption spectrum of RbI.
A : The present result ; B : Metzger [4] ; C : Absorption
spectrum [3].
and the lower curve is the absorption spectrum illustrated for comparison. The absorption spectra as this
have been obtained for all Rb and K halides at the
Tokyo synchrotron by Saito and others 131. The dotted curve is the earlier result obtained by Metzger [4].
The agreement is good both in general behavior
and the absolute value, but some important structures
as A' and B' are absent in his result. It is remarkable
that most of the structures observed in the absorption
spectrum are reproduced in the yield spectrum as
indicated in the figure. In this spectral region, the
transitions from the core states Rbf 4 p , which are
denoted with prime, beginning at about 16 eV, are
included. However, it is found that these are rather
fine structures modifying three broad bands separated by the two well defined minima a and 8, which
are absent in the absorption spectrum. The position
C4-292
T. SASAKI, H. SUGAWARA AND Y. IGUCHI
of twice the energy gap 2 Eg, and the sum of the two
energy gaps Eg Ec, where Ec is the gap between the
bottom of the conduction band and the highest core
level for Rb' 4p, are indicated by arrows in the
figure. In both cases, the rapid decrease begins slightly
below the indicated energies, and it suggests that
excitons take part in the scattering process. This
effect will be more conspicuously observed in the cases
of potassium halides. The prominent peak of yield
at L, 11.8 eV and the following decrease down to a,
14.4 eV, have no counterpart in absorption spectrum
and are formed by suppression of emission due to
scattering. The same effect is observed from 23 to
25 eV, where photoelectrons excited from the core
states play an important role. The regions following
minima a and p are formed by enhancement of yield
due to the scattering, and as a result the absolute
yield rises even higher than 1 in a range between 20
and 24 eV. Under the assumption that %a,= 2(Eg x)
at a, we have estimated the value of the electron affinity
of rubidium iodide to be 1.1 eV. The second minimum p also leads to the consistent result if one assumes the peak A' is the lowest exciton formed at r
point and the binding energy of the core exciton is the
same as that of the valence band.
Figure 2 shows a similar picture for rubidium chloride. General behavior is quite similar while a group
+
bution of photoelectrons in a region above and below
a will be quite different.
In order to demonstrate that it is the case, we show
in figure 3 the spectral yield of RbCl around the minimum a at various retarding potentials. It is clearly
RbCl
\k:
+
PHOTON ENERGY ( e ~ ) .
Ec+Eg
RbCL
~1
FIG.3. - Retarded photoelectric emission of RbCl.
>
k
z
-14w
V)
- 1 2 ~
-102
-080
I
10
20
30
I-0.6~
0
40
FIG. 2. -Photoelectric yield and absorption spectrum of
RbCl.
A : The present result ; B : Metzger [4] ; C : Absorption
spectrum [3].
of core excitons from A' to E' seen in the absorption
is divided into two opposite sides of the minimum a,
because its energy gap is a little larger than iodide.
Also in this case, the drop begins earlier than twice
the gap, 16.4 eV. The estimated electron affinity is
0.5 eV. In a region between 12 and 16 eV, it is observed
that thereis a certain enhancement of the yield. Because
our simple model would indicate no inelastic scattering in this region, it may be interpreted by some other
mechanism, such as the scattering due to the imperfection. Our model also suggests that the energy distri-
shown that photoelectrons are retarded at a potential
as low as 2 volts for the higher photon energy than a,
while they remain comparatively unretarded below a.
It is also interesting to note that the peaks of excitons,
A' to E', remain unretarded even in the region above
a. This will be considered as an evidence that excitation of core exciton is immediately followed by an
Auger transition which transforms it into a core hole
and an electron at high conduction state. According
to their sharp absorption peaks, photoelectrons will
be generated in the vicinity of the surface and likely
to be emitted without scattering.
In figure 4, we will summarize the results of the four
rubidium halides. Rubidium fluoride is quite different
from other three halides. Apparently it shows no well
defined minima and the yield remains below unity
throughout the region studied. However, our experience shows that it is very hygroscopic and the deterioration of the evaporated film took place for each
run repeated on the same specimen every 30 minutes
even at the pressure 3 x loF7 torr. Therefore it is
conceivable that the present result does not represent the true character of the bulk material. Nevertheless, absence of well defined minima seemed to
be quite reproducible, and electron affinity of rubidium fluoride is possibly negative.
(3-293
PHOTOELECTRIC EMISSION OF POTASSIUM AND RUBIDIUM HALIDES
O(KEr1
,
10
PHOTON ENERGY ( eV)
FIG.4. - Photoelectric yields of rubidium halides.
In figure 5, we show the results of the four potassium halides altogether. The previous results published
by a DESY group [5] are in excellent agreement with
the present result, although they did not determined
the quantum yield in absolute values. They suggested
that the yield will be higher than unity above 20 eV
and it has been established by the present result.
In potassium halides, the absorption peaks corresponding to core states K + 3 p are reproduced in the
yield spectra more conspicuously than in rubidium
halides. It suggests that the oscillator strength for
FIG. 5.
.
#
.
.
,
.
.
.
.
I
.
.
.
.
-
20
30
PHOTON ENERGY ( eV)
- Photoelectric yields of potassium halides.
valence electrons in the core excitation region are more
exhausted in potassium halides than in rubidium halides
partly due to the fact that the core states K + 3 p
lies deeper than Rb' 4 p.
In table I we summarize several important numerical
values including the electron affinities,evaluated in the
present work.
Extraordinarily high quantum yield of photoelectric
emission of alkali halides in the extreme ultraviolet
as determined in the present work is explained on the
one hand by the low electron affinities as such, and
Electron afinities of potassium
and rubidium halides as estimated from the photoelectric emission data
and optical energy gaps at room temperature
KF
KC1
KBr
KI
RbF
RbCl
RbBr
RbI
Ex : Energy of lowest exciton peak.
Eg : Energy gap between valence and conduction band.
Ex, : Energy of lowest core exciton peak.
E, : Energy gap between highest core level and conduction band.
(3-294
T. SASAKI, H. SUGAWARA AND Y. IGUCHI
on the other hand by the large escape length. Large
energy gaps play an important role.
Metzger had once proposed, based upon his results,
that there are some evidences that the two excitons
are formed by a single photon. He observed in some
cases strong absorption band at about 2E, where
photoelectric yield gives a minimum. We studied
carefully the region in which this effect will be expected, but no definite evidence was obtained.
Acknowledgment. - The work reported in the present paper has been carried out under cooperation of
Dr. T. Nasu, Mr. S. Sato, Mr. A. Ejiri, Mr. S. Onari,
Mr. K. Kojima and Mr. T. Oya.
The authors are grateful also to the other members
of INS-SOR who took care of experimental facilities
including Seya monochromator, and to the machine
group of the Tokyo synchrotron headed by Prof.
S. Yamaguchi for their assistance and encouragement.
References
[I] SAMSON
(J. A. R.), J. Opt. SOC.Am., 1964, 54, 6 .
[21 SASAKI
(T.) and SUGAWARA
(H.), Annual Report 19691
1970, Institute of Plasma Physics.
(H.) et al., to be published.
[3] SAITO
[4] METZGER
(P. H.), J. Phys. Chem. Solids, 1965,26,1879.
[5] BLECHSCHMIDT
(D.), SKIBOWSKI
(M.) and STEINMAN
(W.),Opt. Commun., 1970, 1 , 275.
DISCUSSION
Mr. BLECHSCHMIDT.
-The decrease in the yield
spectrum at ho = Eg $ E, due to core level scattering is too drastic as to be understood by a mechanism where a highly excited electron from the valence
band is scattered in the core level, because we know
from our energy distribution measurements that the
amount of electrons at an energy greater than Eg
above the bottom of the conduction band is negligibly small. Have you any explanation for this contradictory behaviour ?
Mr. BLECHSCHMIDT.
- There are two competing
scattering mechanisms at this photon energy, one of
which is the scattering by valence electrons, while
the other is due to core electrons. Among them,
the former will be more important than the latter.
But the rapid decrease of the yield in this energy region,
for instance in RbI, is caused not only by the suppres-
sion due to scattering but also by the rapid decrease
of absorption coefficient. The both effects apparently
enhance each other.
Mr. SKIBOWSKI.
- YOU have mentioned an Auger
decay process being responsible for the structure
found in the photoyield spectra of the Rb-halides.
Can't it be possible that in addition a recombination
process has to be considered giving rise to photoelectrons with kinetic energies higher than expected for
Auger electrons.
Mr. SKIBOWSKI.
- It is in a sense the matter of
definition. In what I mean by a term <{ Augerprocess D,
the effect following to a recombination is also included.
I agree to your argument that this particular process
should have a larger cross section than otherprocesses.
The experimental results are favorable to this argument.