PHOTOELECTRIC EMISSION OF POTASSIUM AND RUBIDIUM HALIDES IN THE EXTREME ULTRAVIOLET

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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 RUBID-

IUM HALIDES IN THE EXTREME ULTRAVIOLET. Journal de Physique Colloques, 1971, 32 (C4),

pp.C4-290-C4-294. �10.1051/jphyscol:1971453�. �jpa-00214654�

JOURNAL DE PHYSIQUE

Colloque C4, suppMment

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 halo- genures de potassium et de rubidium a tempkrature normale dans le domaine de

eV. Nous montrons que la variation spectrale du rendement des isolants est determinke par le spectre d'ab- sorption et la diffusion electron-Clectron. Les valeurs de l'affinite electronique ont ete dkterminees en supposant que la diminution ou l'augmentation du rendement quantique dues a la diffusion ink- lastique des photoClectrons se produit en dessous et en dessus de

+ ^x). Les rendements quantiques Cgaux

1 ou supQieurs sont observes dans un domaine d'bnergie supkrieur

20eV.

Abstract. - Quantum yields of photoelectric emission of potassium and rubidium halides at room temperature in the region from 10 to

eV are reported.

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

or more are observed in a region above 20 eV.

1.

In the present report, the pho- toelectric emission of potassium and rubidium halides in the region between 10 and 40 eV at the room tem- perature 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 impor- tant critical points in the Brillouin zone relative to the vacuum level, and the dynamic behaviors of photo- electrons after excitation. They are brought about through the energy analysis of the emitted photoelec- trons.

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, trans- port 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 impor- tant 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 deter- mines the quantum efficiency of emission. Therefore, in general, the structure of absorption spectrum is reproduced in the spectral yield of photoelectric emis- sion, because the depth of generating photoelectrons is smaller for the higher absorption. Otherwise there will be no spectral dependence of photoelectric emis- sion 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 elec- trons exchange at each collision is allowed conti- nuously 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 C4-291

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 mecha- nisms 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 t o 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

and Ziw,, where Ziw,

E,

and Ko,

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 momen- tum 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

will naturally incrase collisions of the emerging type, but on the other hand, collisions of the preventing type will still keep increasing if the scat- tering 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 photo- electric emission takes place in this second region of photon energies higher than Ziw,.

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 mono- chromator with 200 or 100

wide entrance and exit slit was determined by means of a double ioniza- tion chamber with helium or argon [I] below their ionization limit, and a photomultiplier coated with

sodium salicylate above argon limit. Another photo- multiplier 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 simulta- neously [2]. Once calibrated, the monitoring multi- plier was used throughout as a secondary standard in determining the quantum yield. Ion chamber was then replaced by a chamber for measuring photo- current 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

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,

O L , b . " . ' . ' ^{" 8 . .} . . * . I

20 30 4 0

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 illus- trated 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 dot- ted curve is the earlier result obtained by Metzger

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 separa-

ted by the two well defined minima

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' 4 p , 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 mini- mum p also leads to the consistent result if one assu- mes 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 chlo- ride. General behavior is quite similar while a group

RbCL ^Ec+Eg ~1

>

k

V)

z

-14w - 1 2 ~

-102

- 0 8 0 - 0 . 6 ~ I-

0

I

10 20 30 4 0

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 scatter- ing in this region, it may be interpreted by some other mechanism, such as the scattering due to the imperfec- tion. Our model also suggests that the energy distri-

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 mini- mum a at various retarding potentials. It is clearly

RbCl \k:

PHOTON ENERGY

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 excita- tion 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 expe-

rience shows that it is very hygroscopic and the dete-

rioration of the evaporated film took place for each

run repeated on the same specimen every

minutes

even at the pressure 3 x l o F 7 torr. Therefore it is

conceivable that the present result does not repre-

sent the true character of the bulk material. Never-

theless, absence of well defined minima seemed to

be quite reproducible, and electron affinity of rubi-

dium fluoride is possibly negative.

PHOTOELECTRIC EMISSION OF POTASSIUM AND RUBIDIUM HALIDES ^(3-293

PHOTON ENERGY ( eV)

FIG. 4.

-

In figure 5, we show the results of the four potas- sium 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 corres- ponding 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

KF

KBr KI RbF RbCl RbBr RbI

O(KEr1

, . # . . , . . . . I . . . . -

10 20 30

PHOTON ENERGY ( eV)

FIG. 5.

-

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 +

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

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 2 E , where photoelectric yield gives a minimum. We studied carefully the region in which this effect will be expect- ed, but no definite evidence was obtained.

Acknowledgment. -

The work reported in the pre- sent 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

R.),

METZGER

[21

SASAKI

and SUGAWARA

Annual Report

SKIBOWSKI

and STEINMAN

Institute of Plasma Physics.

[3]

SAITO

to

published.

DISCUSSION

Mr. BLECHSCHMIDT. -The decrease in the yield sion due to scattering but also by the rapid decrease spectrum at

Eg

ing is too drastic as to be understood by a mecha- enhance each other.

nism 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 negli- gibly small. Have you any explanation for this contra- dictory behaviour

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 photoelec- trons with kinetic energies higher than expected for Auger electrons.

Mr. BLECHSCHMIDT. - There are two competing Mr. SKIBOWSKI. - It is in a sense the matter of scattering mechanisms at this photon energy, one of definition. In what I mean by a term

Augerprocess

which is the scattering by valence electrons, while the effect following to a recombination is also included.

the other is due to core electrons. Among them, I agree to your argument that this particular process the former will be more important than the latter. should have a larger cross section than otherprocesses.

But the rapid decrease of the yield in this energy region, The experimental results are favorable to this argu-

for instance in RbI, is caused not only by the suppres- ment.