Optical techniques for actinide research

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Optical techniques for actinide research

B. Veal

To cite this version:

B. Veal. Optical techniques for actinide research. Journal de Physique Colloques, 1979, 40 (C4),

pp.C4-163-C4-170. �10.1051/jphyscol:1979452�. �jpa-00218848�

Optical techniques for actinide research (*)

B. W. Veal

Argonne National Laboratory, Argonne, IL 60439, U.S.A.

Résumé. — Des progrès substantiels ont été réalisés ces dernières années dans le développement des techniques spectroscopiques d'étude des propriétés électroniques. Les applications de ces techniques dans le domaine de la recherche sur les actinides sont relativement mineures mais d'importance croissante. Des études de spectroscopie de photoémission, d'absorption et de réflexion et des techniques de rayons X sont présentées et illustrées à l'aide de quelques exemples pris dans le domaine des actinides.

Abstract. — In recent years, substantial gains have been made in the development of spectroscopic techniques for electronic properties studies. These techniques have seen relatively small, but growing, application in the field of actinide research. Photoemission spectroscopies, reflectivity and absorption studies, and X-ray techniques will be discussed and illustrative examples of studies on actinide materials will be presented.

1. Introduction. — In recent years, significant

advances have been made in the development of spectroscopic techniques and their exploitation for materials research. However, for the actinide mate- rials, this field of research remains virtually unex- plored. Because of the obvious value that these techniques have for investigation of electronic pro- perties, and because of the rich variety of electronic structure phenomena that can be found in actinide materials, it is certain that the spectroscopic techni- ques will become more vigorously exploited as the specialized capabilities required for handling actini- de materials become available.

Those experiments which might properly be called

optical or spectroscopic comprise an inappropriate-

ly long list for this presentation. In general, nearly any photon experiment might qualify as an optical

spectroscopy. For this presentation we will simply

choose to discuss a few optical spectroscopic tech- niques which are useful for elucidating electronic structure in actinide materials. Other spectroscopic techniques not included in this brief list may nicely complement those discussed here. Related spectro- scopies such as Auger electron spectroscopy, elec- tron energy loss spectroscopy and ion neutralization spectroscopy might also be appropriate for this discussion but will not be considered. We shall limit the discussion to reflectivity, X-ray and ultra-violet photoemission, and X-ray absorption and emission spectroscopies [1].

Due to space limitations, this discussion of spec- troscopic techniques will necessarily be very incom- plete and somewhat qualitative to emphasize the kind of information that can be obtained. Only illustrative examples will be presented. Subtleties in

(*) Work supported by the U.S. Department of Energy.

the techniques and analyses cannot be adequately discussed.

The valence band spectrum obtained by X-ray photoemission spectroscopy (XPS) usually provides a reasonably accurate direct measure of the occu- pied density of states. Because of this apparent interpretative simplicity, XPS is a very good first experiment for studying electronic structure of the occupied states in metals. The technique also lends itself to the study of both itinerant and local valence states. Chemical bonding information can be ob- tained from XPS core level studies. (Due to the wide variety of information available from XPS studies, somewhat more attention will be devoted to the XPS studies than to the complementary spectroscopies.)

With the knowledge of valence band states ob- tained from XPS, excellent follow-up spectroscopies are UPS and optical reflectivity both of which can provide information about the empty conduction states as well as the occupied valence states. The techniques often contain too much information for reasonably straightforward analysis and data inter- pretation may demand extensive theoretical support.

The X-ray absorption and emission techniques also nicely complement the above spectroscopies. Both conduction and valence electron structure informa- tion can be obtained. Synchrotron radiation from electron storage rings can provide a very high inten- sity radiation source over a spectral range from the infrared to X-ray region. The high intensities and continuous scanning capabilities afforded by syn- chrotron radiation can contribute substantially to- ward improving all of the above spectroscopies.

2. Electron emission spectroscopies. — The pho- toemission spectroscopies depend upon the energy analysis of electrons emitted from a sample surface

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979452

C4-164 B. W. VEAL as the result of photons of known energy impinging on the sample. Measurement of the emitted electron flux as a function of kinetic energy gives a mapping of the occupied density of states n(E). For valence band studies using XPS, the photoemitted electrons are excited to final states far above the Fermi level E, where the energy bands are expected to have essentially plane wave character. Thus, the measu- red spectrum should be sensitive to the occupied states with little distortion from k dependence in the empty conduction bands. The observed XPS spec- trum for a given metal will contain distortions from the actual density of states because of matrix ele- ments effects. For itinerant states, this distortion is usually small.

With ultra-violet photoemission spectroscopy (UPS), valence and conduction band structure infor- mation is convoluted and crystal momentum k beco- mes an important parameter so that one does not measure a simple n ( E ) but rather a joint density of states that is dependent on k. Contributions to the observed spectrum depend upon the location within the Brillouin zone where the transition occurs. Thus, UPS is more dependent on details of the band structure than is XPS. These details can be inter- preted with the aid of suitably accurate and detailed energy band calculations. Additional electronic structure information can be obtained from angular resolved photoemission (ARP) spectroscopy. It ap- pears that one can continuously map out energy dispersion relations throughout the Brillouin zone.

For the itinerant ferromagnet iron, an ARP study was able to follow dispersion relations for both the

majority and minority spin bands [2]. The demons- tration of this application of UPS is very recent and has not yet been exploited for any actinide systems.

The cross-section for photoemission of electrons of a given angular momentum character depends upon the energy of the exciting photons. Thus, from photon energy dependence studies, one can obtain information about the angular momentum character of the occupied states. Figure

1

shows photoemis- sion spectra for UO, taken with 21.2 eV (HeI), 40.8 eV (HeII), and 1 253 eV (MgKa) radiation [3].

Peak B consists primarily of

0

2p derived electrons while peak A corresponds to localized

U

5f electrons in the 5f2 ground state configuration [4].

As discussed above, the valence band XPS spec- trum is often a very accurate representation of the itinerant valence electron states in a solid. For local states occupying a partially filled shell, as are often encountered in actinide compounds and the heavy actinide elements, however, the interpretation of XPS spectra is different. One no longer measures a ground state property of the material but rather a spectrum of the possible (or accessible) excited states of the localized electrons. Since the localized electrons in a partially filled shell are strongly corre- lated, the excitation of one of those electrons is felt by the remaining members of the shell. Excitations (representing the possible angular momentum coupling schemes and possible configuration changes), of the local electron system provide a channel for absorption of some of the energy of the incident photon. The observed spectra are thus a measure of accessible multiplets which can be exci- ted by the incident photon during the process of photoemission. XPS multiplet spectra have been observed and analysed in numerous studies of lanthanide systems [5]. The simplest systems are probably the elemental lanthanide metals where a relatively small portion of the total XPS spectrum (near ,E,) is contributed by itinerant or band elec- trons. Essentially all observed features can then be analysed with multiplet theory. Except for U and Th, no XPS (or UPS) measurements have been

BINDING ENERGY (eV)

Fig. 1.

-

Photoemission spectra for UO, taken at photon excita- tion energies of 21.2 eV (He I), 40.8 eV (He 11), and 1 253 eV (Mg Ka). The U 5f electrons (peak A) have a very different dependence on photon energy than the 0 2p's (peak B) (see Ref. [3]).

BINDING ENERGY i e V )

Fig. 2. -XPS spectra of localized Sf states in light actinide oxides compared to the calculated final state multiplet spectra (see Ref. [63.

reported for elemental actinide metals. Only data for actinide oxides have been analysed using multiplet theory. Figure 2 shows XPS spectra for dioxides of N p , Pu, and Am compared with the theoretically computed free ion multiplet spectra broadened to simulate experiment [6]. There is substantial corres- pondence between the calculated and observed spectra. However, a potentially more fruitful experi- ment would provide similar measurements for the metals. Experimentally, better resolution should be obtained so that more spectral detail would be available for analysis. The O 2p orbitals, which overlap the multiplet spectra in the oxides, would be absent. And most importantly, the multiplets should provide direct input into assessing the extent to which the 5f electrons are localized.

Chemical bonding information in compounds can be obtained from the measurement of XPS core level binding energies. The binding energies of the elec- tron core levels of an atom are dependent upon the local charge environment at the atomic site. That local charge is determined by the distribution of molecular orbitals in the compound. Figure 3 illus- trates the effect for the U 4f levels of U

³

O

^g

where chemically shifted peaks corresponding to U

⁴⁺

and U

⁶⁺

ionization states are observed [7].

3. Reflectivity and absorption. — Reflectivity experiments, typically spanning the spectral region from the near infrared (1 eV or less) to a few tens of electron volts, have been extensively applied to the study of the electronic structure of a wide range of materials. Modulation techniques contribute a large subset to the reflectivity and absorption spectrosco- pies. Reflectance studies have seen limited applica- tion in the field of actinide research. Studies on oxides of light actinides (up to PuO

z

) and on some elemental metals have been reported [8].

The measured parameters of interest [usually ob- tained by Kramers-Kronig inversion of the reflecti-

Fig. 3. — XPS spectra for the spin-orbit split U 4f levels in U³0⁸. Chemically shifted peaks corresponding to U⁴* and U⁶⁺ ions are observed (see Ref. [7]).

vity R{a>y] are the components of the complex die- lectric function e(a>) = S^CD) + is

2(<o). e2

relates directly to the electron band structure. Photons at energy ha> can excite electrons from all occupied valence states between E

F

and E

F — hco into conduc-

tion states between JS

F

and E

F + hu>. s2(<a) is thus a

convolution of the state densities of the valence and conduction bands within energy hu> modulated by electric dipole selection rules and momentum conservation (k dependence) considerations. A quantitative first-principles calculation of e

²

(w) is difficult and costly requiring, as a starting point, a determination of the energy bands E(k) throughout the Brillouin zone and the wave functions *p

k(E).

However, electronic structure information can often be inferred from positions and intensities of spectral features even when available theoretical input is severely limited. Figure 4, for example, shows the

Fig. 4. — The real (solid line) and imaginary (dashed line) parts of the dielectric function e — s, + fe for U 0² displayed vs. photon energy. The band model shown is proposed to account for the observed spectral features (see Ref. [9]).

C4-166 B. W. VEAL components of E(W) for UO, as measured by

J.

Shoenes [9] and the energy level scheme which he proposes to account for the observed features.

The positions and intensities of spectral features, selection rules and sum rules are discussed to sup- port the proposed model. It is argued that both localized 5f states and itinerant band states contribu- te to the observed spectrum. In support of the model, Gubanov, Rosen and Ellis

[lo]

(GRE) have examined UO, within the framework of theoretical molecular cluster models and confirm several featu- res of the proposed electron energy level scheme.

They obtain the expected band gap, d-electron crys- tal field splittings, and energies of major features.

The contribution which the localized 5f electrons make to the spectrum of e,(w) probably remains most uncertain. Shoenes argues that, like XPS, multiplet excitations in the Sfn-' system provides a channel for absorption of incident radiation when a localized 5fn electron is excited to a higher band.

Shoenes argues that, for the f1 final state, only two (spin-orbit split) levels are expected. The proposed spin-orbit splitting of 1.1 eV exceeds, by over a factor of two, the spin-orbit splitting obtained by GRE. GRE argue that excitations within crystal field (or ligand field) split 5f levels (concurrent with the f + d transitions) can account for the observed spectral structure. Naegele et

al.

[1 l] have presented a somewhat different electronic structure model to explain their UO, reflectivity data. A critical compa- rison of the two models cannot be made here. The above discussion is intended only to provide an example of the way band structure information might be obtained from reflectance data.

Another technique applicable to the study of insu- lators containing localized 5f electrons (such as UF, or CaF, containing dissolved actinide ions), is trans- mission optical absorption or fluorescence spectro- scopy where transitions within a given f" multiplet are excited [12]. This is an electric dipole forbidden process (for Russell-Saunders states) so that absorp- tion coefficients are weak and cannot be observed when interband absorption processes occur. For suitable systems, however, this technique probably provides the most direct information about the 5f electron excitation spectrum. The large number of observable transitions provides redundant informa- tion from which the 5f ground state can be specified as well as the appropriate crystal field parameters.

4. X-ray techniques.

-

X-ray emission and ab- sorprion spectroscopies provide alternative means for examining the valence and conduction bands of solids. When electrons are excited from a core state to the conduction band by X-rays, the absorption spectrum provides a measure of conduction band states that are accessible for electric dipole excita- tions. Similarly, de-excitation from valence band states to an empty core level produces an emission

spectrum that is dependent on the band structure of the occupied valence states. The combination of absorption and emission spectra, then, should provi- de valuable information about the electronic states in the vicinity of E,. Additional information about the valence (or conduction) bands can be gained by examining spectra associated with transitions to (or from) different core states thus exploiting the angu- lar momentum selectivity afforded by electric dipole selection rules.

Absorption and emission studies have been re- ported for uranium and plutonium metals and for Tho, and UO, [8]. For the metals, rather feature- less, single peaks in both the absorption and emis- sion spectra (4d,,, core levels) are attributed to 5f electron states. Results are shown in figure 5 for

a-Pu [l3]. For localized 5f valence states, more complexity may be encountered in the X-ray spectra because of the possibility of exciting transitions within the local 5f system.

Fig. 5. -X-ray absorption and emission spectra for a-PU metal (see Ref. [13]).

Appearance potential spectroscopy [I41 (APS) is a tool similar to the X-ray techniques that is also useful for investigating conduction electron structu- re. Electrons are excited from core levels to the conduction band by an incident electron beam. The electron energy near excitation threshold is plotted against X-ray emission intensity. The emission mo- nitors the rate of core hole production and hence the excitation probability of the incident electron beam.

This excitation probability is related to the conduc- tion band density of states. Figure 6 shows conduc- tion electron APS results of Park and Houston for a -uranium metal [IS] plotted with XPS valence band data [4]. These experimental results are compared with the f-electron contribution to the total density

of states as computed by Freeman

et

al. [16]. This comparison was made [rather than total n

(E)]

since the f electrons probably dominate the XPS spectrum [3, 41, and in the conduction band region, the calculated f-electron n (E) is similar to the total n(E). The relative scaling of the XPS and APS spectra is arbitrary. Furthermore, as we have seen in other spectroscopic experiments involving 5f states, the tendency toward localization may introduce in- terpretation complexities. Nonetheless, the agree- ment between theory and experiment shown in figure 6 is encouraging.

ENERGY (eV)

Fig. 6. - The calculated f-electron density of states [16]

(histogram) for a - U compared with XPS measurements [4] below E, and with APS measurements [IS] above E F .

5. Bonding of metal oxides in silicate glass. - Recently, work has been undertaken at Argonne National Laboratory to investigate the bonding of metal oxides in sodium silicate glasses. Some results on uranium oxides and Fe,O, dissolved in Na,0 . 2 SiO, will be presented to illustrate an ap- proach for obtaining bonding and structural informa- tion for these amorphous materials.

The silicate glasses are important for the actinide community because they have remarkably high solu- bilities for numerous metal oxides from all regions of the periodic table and thus are attractive candidate materials for long term storage of radioactive was- tes. Many active nuclear species with great variation in atomic number must be stored. Fundamental information about the bonding properties of metal oxides in the silicate glasses can provide useful input

for evaluating the confinement integrity of glass storage systems.

When UO, is dissolved into sodium silicate glass, with the sample melting done in air environment, we find that essentially all uranium appears in the

+

6 ionization state. If the preparation is conducted in vacuum, then uranium predominately retains the

+

4 ionization state. These results are illustrated in figure 7. For the vacuum melt (curve a), the peak near the valence band edge is a measure of the occupancy of localized 5f electrons. No Sf occupan- cy appears in the air melt (curve b, nor in a glass sample prepared with UO,) indicating that the ura- nium is in the

+

6 oxidation state [4]. Similar conclu- sions can be drawn from examining the 4f core levels whe;; the

+

6 ionization state appears at the higher binding energy (Fig. 7, insert). [Probably only

+

4 or

+

6 ionization states would be expected, consis- tent with observations of Verbist

et al.

[7] for oxides of uranium .]

RINClING ENERGY (eV)

Fig. 7. -XPS valence band and U 4f (insert) spectra for (NazO. 2 SiOz)o., . ( U 0 3 0 2 when (a) the melt was done in va- cuum and (b) the melt was done in air. For the vacuum melt, the U" ionization state dominates ; for the air melt, the U6' state dominates.

The oxidation state of uranium in the amorphous silicate matrix can be readily discerned. It also appears, from a detailed study of the 0 1 s line, that additional information pertaining to the bonding of the oxide in the matrix can be obtained. In essence, the procedure is to monitor the rate of oxygen bond alteration in the matrix silicate caused by incorpo- rating a metal oxide.

The behaviour of UO, (or UO,) in the silicate glass appears to be dramatically different from the beha- viour of Fe,O, dissolved in the glass. The XPS results indicate that no apparent disruption of the oxygen bonds in the glass matrix is observed when UO, (or UO,) is added. Rather the XPS data are consistent with the conclusion that the uranium oxides enter the glass as a simple mixture. In contrast, for Fe,O, glass, matrix oxygen bonds are

C4-168 B. W. VEAL clearly disrupted to permit the Fe" ion to bond to three oxygens of the glass matrix [17].

The arguments are best illustrated for Fe,O, where matrix bond alteration occurs. Figure 8 b illustrates 0 1s levels for Na,0

. 2

SiO, [18]. Figure 8 a shows the

0

1s line for the iron glass. Subtracting the Na,0 . 2 SiO, contribution from the iron glass spec- trum, we obtain the difference spectrum shown in figure 8c. Had the Fe,03 entered the glass matrix without chemical rearrangement (e.g., as a simple mixture), one would expect the difference spectrum to exhibit a simple quasi-Gaussian line shape.

Rather, the shape of figure 8c indicates that oxygen I s bonds of the N a , 0 . 2 SiO, are destroyed on addition of Fe,O, to produce a new bonding site, apparently associated with Fe-0 bonds. To obtain a simple (quasi-Gaussian) line shape (curve d) by subtraction of curve b from curve a, curve b must be scaled by a factor whose magnitude implies that for each added molecule of Fe,O,, three oxygen bonds of the disilicate matrix are broken and re- placed by bonds t o the iron atoms. That is, the original oxygen bonds in the disilicate [cor- responding to Fig. 8 b ] are consumed to produce Fe- 0-Si bonds in the iron glass.

For UO,, simply subtracting the Na,0. 2 SiO, contribution from the (Na,O . 2 Si03,,(U03,, spec- trum produces a simple difference lineshape which cannot be improved by relative scaling. Thus, no bond alteration in the matrix glass can be observed.

Probably, molecular units of UO, (or UO,) are contained in open regions of the amorphous matrix without formation of strong molecular orbitals between the matrix and oxide.

OXYGEN Is

(a)

DIFFERENCE

8 .

. . . .

SCALED DIFFERENCE

.

C_- C 1

B I N D I N G ENERGY (eV1

Fig. 8.

-

XPS 0 1s lines for (a) sodium disilicate containing Fe,O, and (b) for sodium disilicate. The simple difference spec- trum (c) indicates that 0 1s bonds of the disilicate matrix are broken by the added Fe,O,. The scaled difference (d) indicates that 3 bonds of the matrix are broken for each added molecule of Fe203.

References

[I] For reviews on the various techniques, see references in Chap. I11 of The Actinides : Electronic Structure and Related Properties, Vol. 11, edited by A. J. Freeman and J. B. Darby, Jr. (Academic Press) 1974.

[2] KEVAN, S. D., WEHNER, P. S. and SHIRLEY, D. A., submit- ted to Solid State Commun.

131 EVANS, S . , J.C.S. Faraday, I1 73 (1977) 1341.

[4] VEAL, B. W. and LAM, D. J., Phys. Lett. 49A (1974) 466 ; Phys. Rev. B 10 (1974) 4902.

[5] CAMPAGNA, M., WERTHEIM, G. K. and BUCHER, E., Struct.

Bonding 30 (1977) 100 and references therein.

[6] VEAL, B. W., LAM, D. J., DIAMOND, H. and HOEKSTRA, H. R., Phys. Rev. 15 (1977) 2929.

[7] VERBIST, J., RIGA, J., PIREAUX, J. J. and CAUDANO, R., J.

Electron Spectrosc. Relat. Phenom. 5 (1974) 193.

[8] Proc. 2nd International Conference on the Electronic Struc - ture of the Actinides, Wroclaw, Poland, 1976, edited by J . Mulak, W. Suski and R. Troc ; Plutonium 1975 and Other Actinides, edited by H. Blank and R. Lindner (North-Holland, Amsterdam) 1976, and references the- rein.

[9] SHOENES, J., J. Appl. Phys. 49 (1978) 1463.

[lo]

GUBANOV, V. A., ROSEN, A. and ELLIS, D. E., Solid State Commun. 22 (1977) 219 ; also J. Nucl. Znorg. Chem., to be published.

[1 l] NAEGELE, J., MANES, L., BIRKHOLZ, U., Plutonium 1975and other Actinides, edited by H. Blank and R. Lindner (North-Holland, Amsterdam) 1976,- p. 393.

[12] See, for example, CARNALL, W. T. et al., Proc. 2nd Inter- national Conference on the Electronic Structure o f the Actinides, Wroclaw, Poland, 1976, edited by J. Mulak, W. Suski and R. Troc, p. 105, and references therein ; DELAMOYE, P., HUBERT, S., HUSSONNOIS, M., KRUPA, J. C., GENET, M., GUILLAUMONT, R., EDELSTEIN, M., CONWAY, J . , this conference and references therein.

[I31 BONNELLE, C. and LACHERE, G., Plutonium 1975 and Other Actinides, edited by H . Blank and R. Lindner (North- Holland, Amsterdam) 1976, p. 337.

[14] See, for example, PARK, R. L. and HOUSTON, J. E., Phys.

Rev. B 6 (1972) 1073.

[I51 PARK, R. L. and HOUSTON, J. E., Phys. Rev. A 7 (1973) 1447.

[16] FREEMAN, A. J., KOELLING, D. D. and WATSONYANG, T. J., J. Physique Colloq. 40 (1979) C4-134.

[I71 These results are consistent with structural information obtained from EXAFS studies on the iron glasses : KNAPP, G. S., CHEN, H., LAM, D. J. and VEAL, B. W., to be published.

[18] VEAL, B. W. and LAM, D. J., Proc. of the International Conference on SiO, and its Interfaces (Pergamon h e s s ) 1978, to be published.

DISCUSSION COMMENT BY Dr.

J.

NAEGELE.

-

I would like add

to two comments.

The figure

1

shows the recent reflectivity data for UO, by Schoenes

[I]

(curve b) together with data we published in the past [2, 31 (curve a). Despite a difference in the absolute reflectivity and some additional weak structures around 3 eV, the posi- tioning of the main structures in energy is in good agreement.

Fig. 1. - Optical reflectivity for UO, (curve a [2], curve b [I]) and Tho, (curve c [3]).

In the same figure, the reflectivity of Tho, [3]

(curve c) is shown. This compound has no occupied 5f states and its reflectivity presents no structures below about

7

eV. Hence we argued (in analogy with the interpretational method used by Veal and Lam for the identification of 5f states in XPS data) that all structures up to about

7

eV in UO, should be due to transitions from occupied 5f levels. We think that the comparison of the spectra of the actinide oxides [3] is of the greatest importance for interpre- tational purposes.

On the other hand, no detailed interpretation of the spectra is certain before calculations of the joint density of states have been made. Fortunately, these can be derived now from recent calculations of the band structure of these oxides and of the CaF, structure. The availability of these theoretical re- sults as well as the energy schemes derived from cluster theories would set now the moment for a detailed analysis of the spectroscopic data for the actinide oxides.

The second comment wishes to point out the

I I I I I I I I I , I ~

?2 UI c d

L

uoz

-

>

k

U) Z W I-

Z

Z

z

U)

-

In

I

W

1 1 1 1 1 1 1 1 , , 1

-10

-

5 Ef'O

BINDING ENERGY ^{( e v )}

Fig. 2 . - Photoemission results (UPS) for U 0 2 (curves a, b and c for excitation energies of 21.2, 40.8 and 48.4 eV, respectively).

extreme care which is needed for UPS. The figure

2

shows UPS data in the valence band range for carefully argon ion sputtered UO, single crystal (no carbon contamination observable in XPS) from

21.2

to 48.4 eV. These data are to be compared with the data reported in this paper. The 5f nature of the peak close to the Fermi level is clearly shown here as in the data reported. However, a much larger Sf emis- sion is found in our spectra. We believe that this is due to the absence of carbon contamination in our case, whereas in the reported UPS data a residual carbon contamination could not be avoided. The presence of even a small carbon contamination (as recorded by much less surface sensitive XPS) dis- torts substantially the UPS photoemission distribu- tion. This is also of particular relevance for the valence band. Notice, in fact, the dramatic change in shape with varying excitation energy and the additio- nal weak peak on the high binding energy side as shown in the figure.

When compared with recent band structure calcu- lations from Brooks

et

al. [4], a better understanding of the electronic structure can be obtained. For low excitation energies the transitions from occupied slates will be also determined by the availability of suitable final band states (with regard to optical selection rules). Since the valence band is composed of two bands with different symmetry, the strong asymmetry for the photoemission band at low exci- tation energies can be induced by transitions for which the selection rules cannot be fulfilled.

B. W. VEAL

References

[l] SHOENES, J . , J. AppL Phys. 49 (1978) 1463. [3] NAEGELE, J., MANES, L. and WINKELMANN, H., Proc. 2nd 121 NAEGELE, J., MANES, L. and BIRKHOLZ, U., in Plutonium Int. Conf. on the Electronic Structure o f the Actinides,

1975and other Actinides, eds. : H . Blank and R. Lindner 1976, Wroclaw, Poland, p. 163.

(North-Holland Publishing Co., Amsterdam) 1976, C41 KELLY, P. J . , BROOKS, M. S. S. and ALLEN, R., J. Physique

p. 393. Colloq. 40 (1979) C4-184.

COMMENT

BY B. H. BRANDOW, L O S

ALAMOS. -

On XPS +UPS data shown by Dr. Naegele, in discussion following talk by B. Veal :

In the photoemission spectra which Dr. Naegele has just shown, it is noteworthy that, with increasing photon energy, the high-binding-energy side of the oxygen 2p band signal seems to be increasing at about the same rate as the relative intensity of the 5f peak: This is just what one would expect from the band-theoretical description of the oxygen-uranium band, where the bottom of the 2p band has the Bloch states with strongest 2p-Sf hybridization. This inter- pretation is nicely supported by the cluster calcula- tions of Ellis, Gubanov, and Rosen, reported at this conference.

COMMENT

BY

Dr.

BROOKS.

-

Since magnetic studies show that there are two f-electrons in UO, we don't believe that a partial density of states analysis of a band structure will tell the whole story (or may even give a false impression) of the f-p band.

COMMENT

BY Dr. D.

E.

ELLIS. - 5f partial densi- ties of states have been calculated for UO, by the cluster technique. They are available at poster ses- sion.

COMMENT BY Dr. J . SCHOENES TO Dr. NAEGELE.

- I

am very happy about your proposal to compare

your and my interpretation with theory, because there is an excellent agreement between my data and the calculation of both Ellis, Gubanov and Rosen and Brooks et al. In addition the sum rule leading to n,,, has been applied incorrectly by

J.

Naegele and has led to the wrong assignments.

COMMENT BY Dr.

FOURNIER. -

XPS- Photoemission appears as a very interesting techni- que for the crucial problem of narrow band versus localized states at the Fermi level for 5f electrons. In the second case which corresponds to a mixed valence state, one can see different core lines corresponding to the different ionizations. This has been nicely demonstrated for mixed valence lantha-

nide compounds on the 3d doublet. It could be observed on the 4f doublet of actinide compounds (for example it was looked for and not found for UAl, in which therefore Sf state form a narrow band).

B. W. VEAL.

-

I assume that you are referring to valence or configuration fluctuations in localized electron systems.

A

crucial question regarding the observability of valence fluctuations is whether the excited (final) states of the two configurations that are stimulated by the XPS measurement are suffi- ciently well separated to be observed.