Lanthanide and Actinide Chemistry and Spectroscopy - American

20

P h o t o e l e c t r o n S p e c t r a of

Actinide Compounds

Lanthanide and Actinide Chemistry and Spectroscopy Downloaded from pubs.acs.org by UNIV OF PITTSBURGH on 02/12/16. For personal use only.

B. W. VEAL and D. J. LAM Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439

Photoemission spectroscopy applied to chemistry and electronic properties studies i s a f a i r l y recent development. The x-ray photoemission spectroscopy (XPS) technique was developed, p r i marily to be a chemical analysis tool (1). In particular it was observed that the absolute binding energies of the atomic-like electron core levels are dependent on the chemical state of the atom under study. This observation led to the widespread use of XPS for basic and applied chemistry studies. Many studies were also undertaken to better understand the physics of the various excitation processes involved. Consequently, XPS has become a powerful tool for studying electronic structure of the outer electron states in solids. Proceeding in parallel with XPS was the development of ultra violet photoemission spectroscopy (UPS) (2). This technique exploits low energy photons and must be confined to studies of electron states rather near the Fermi level (EF). For investigating occupied electron states in the v i c i n i t y of EF, UPS and XPS can serve as excellent complementary spectroscopies. The actinide element series, l i k e the lanthanide series, i s characterized by the f i l l i n g of an f-electron s h e l l . The chemical and physical properties, however, are quite different between these two series of f-electron elements, especially in the f i r s t half of the series. The differences are mainly due to the d i f ferent radial extension of the 4f- and 5f-electron wavefunctions. For the rare-earth ions, even in metallic systems, the 4f electrons are spatially well localized near the ion sites. Photoemission spectra of the f electrons in lanthanide elements and compounds always show " f i n a l state multiplet" structure (3), spectra that result from partially f i l l e d shells of localized electrons. In contrast, the 5f electrons are not so well l o c a l ized. They experience a smaller coulomb correlation interaction than the 4f electrons in the rare earths and stronger hybridization with the 6d- and 7s-derived conduction bands. The 5f's thus

0-8412-0568-X/80/47-131-427$05.00/0 © 1980 American Chemical Society

428

LANTHANIDE

AND ACTINIDE CHEMISTRY

AND

SPECTROSCOPY

have a g r e a t e r tendency toward i t i n e r a n c y than do the 4 f s . The r e s u l t i s that 5 f e l e c t r o n s i n a c t i n i d e elements and compounds may r e v e a l i t i n e r a n t , l o c a l , or intermediate behavior. Furthermore, the a c t i n i d e ions can adopt a v a r i a b l e valence s t a t e i n chemical compounds and the 5 f e l e c t r o n s may p a r t i c i p a t e i n bonding. In t h i s paper, we s h a l l present a b r i e f overview of the a p p l i c a t i o n of p h o t o e l e c t r o n spectroscopy to the study of a c t i n i d e materials. Some phenomenology w i l l be d i s c u s s e d as w i l l s t u d i e s of s p e c i f i c m a t e r i a l s . Only i l l u s t r a t i v e examples w i l l be presented ( 4 ) .


f

T h e o r e t i c a l Background Photoemission i s viewed as a process wherein an absorbed photon e x c i t e s an e l e c t r o n w i t h i n the s o l i d to a f i n a l energy s t a t e g r e a t e r than the work f u n c t i o n . The e l e c t r o n then migrates to the s u r f a c e and escapes. F i g u r e 1 shows a schematic represent a t i o n of the photoemission process. A photon with energy K u) may e x c i t e an e l e c t r o n from some i n i t i a l s t a t e below the Fermi l e v e l to a f i n a l s t a t e E. A f t e r moving through the s o l i d to the sample s u r f a c e , the e l e c t r o n escapes i n t o the vacuum with k i n e t i c energy E^ a f t e r having l o s t energy equal to the work f u n c t i o n i n overcoming the s u r f a c e p o t e n t i a l b a r r i e r . Since the e l e c t r o n k i n e t i c energy E^. i s monitored, one has a measure of E and E^. Thus, w i t h monochromatic e x c i t i n g r a d i a t i o n , one i s able to measure t r a n s i t i o n p r o b a b i l i t i e s between s t a t e s which are at known energies r e l a t i v e to Ep. The energy of the e x c i t i n g r a d i a t i o n f o r these experiments v a r i e s from s e v e r a l e l e c t r o n v o l t s to k i l o e l e c t r o n v o l t s . The low energy extreme i s r e f e r r e d to as UV photoemission spectroscopy (UPS) and the high energy extreme as x-ray photoemission spectroscopy (XPS). However, the spectrum can be continuously scanned using synchrotron r a d i a t i o n . Core L e v e l and L o c a l i z e d Valence

States

One of the most important c a p a b i l i t i e s of the XPS technique i s the measurement of absolute core l e v e l b i n d i n g energies as a means of probing the l o c a l charge s t a t e of the i o n under study. The o x i d a t i o n s t a t e of the i o n can sometimes be c l e a r l y d i s c e r n e d . An example i s the XPS measurement f o r the intermediate oxides of uranium, U 3 O 3 and U 2 O 5 , reported by V e r b i s t et a l . ( 5 ) . Doubl e t s t r u c t u r e i n the U 4 f l i n e s was a t t r i b u t e d to U^ and U^ " ions s i n c e the 4 f peaks appeared w i t h n e a r l y the same b i n d i n g energies as the 4 f l i n e s i n U 0 and U O 3 . I t should be cautioned that photoemission s p e c t r o s c o p i c measurements look at energy d i f f e r e n c e s between an n - e l e c t r o n ground s t a t e and an n-1 e l e c t r o n e x c i t e d s t a t e . Therefore, photoemission provides a good approximation to ground-state p r o p e r t i e s +

2

4


20.

VEAL AND L A M

Photoelectwn

Spectra of Actinide

Compounds

429

Figure 1. Schematic of photoemission process for a metal. A monochromatic photon h oo excites an electron from initial state E to final (vacuum) state E. The electron escapes with kinetic energy E after passing through sample surface having work function cf>. For fixed h w, monitoring photoemission intensity I vs. E measures transition probabilities between states Ei and E. Valence states as well as core states can be observed. h

k

k


430

LANTHANIDE AND

ACTINIDE CHEMISTRY AND

SPECTROSCOPY

only i n the l i m i t when Koopman's theorem i s v a l i d (6), i . e . , f o r l a r g e e l e c t r o n i c systems having extended one-electron wavefunctions l i k e the valence and conduction band s t a t e s i n m e t a l l i c and semiconducting s o l i d s . Koopman's theorem i s not v a l i d f o r core e l e c trons s i n c e t h e i r wavefunctions are w e l l - l o c a l i z e d about the atomic site. Observed core l e v e l p o s i t i o n s are u n c e r t a i n , r e l a t i v e to the ground-state c o r e - l e v e l energies, by the g e n e r a l l y unknown f i n a l s t a t e r e l a x a t i o n energy. S i m i l a r l y i f a ( l o c a l i z e d ) e l e c t r o n i s removed from an incompletely f i l l e d outer s h e l l of an i o n , d i f f e r e n t f i n a l - s t a t e c o n f i g u r a t i o n s of the ion can lead to e x p e r i mentally observed m u l t i p l e t s t r u c t u r e ( 7 ) . These d i f f e r e n t f i n a l s t a t e s of the ion may be viewed as a form of r e l a x a t i o n which r e s u l t s from intra-atomic c o r r e l a t i o n e f f e c t s . F i g u r e 2 shows XPS data f o r d i o x i d e s of neptunium, plutonium, and americum compared to the appropriate f m u l t i p l e t c a l c u l a t i o n s (8). These m u l t i p l e t s p e c t r a do not represent the m u l t i p l e t s t r u c ture of e i t h e r the f or the fn-1 c o n f i g u r a t i o n s . They are, i n s t e a d , the f i n a l s t a t e m u l t i p l e t s t r u c t u r e of the f ~ l c o n f i g u r a t i o n modulated by the t r a n s i t i o n p r o b a b i l i t y from the f ground s t a t e to the f l m u l t i p l e t s . n

n

n

n

n

_

Intensities For q u a n t i t a t i v e a n a l y s i s of surface chemical compositions using the XPS technique, measurements of s u b s h e l l p h o t o i o n i z a t i o n c r o s s - s e c t i o n s (SPC's) are needed. Problems involved i n the determination of r e l a t i v e SPC's from the measurement of r e l a t i v e l i n e i n t e n s i t i e s i n an XPS spectrometer have been discussed by Cardona and Ley (2). The c r o s s - s e c t i o n measurements r e q u i r e an appropriate c o n s i d e r a t i o n of the sample concentration, energy dependence of e l e c t r o n analyzer transmission, the 'angle of the incoming x-ray beam r e l a t i v e to the outgoing e l e c t r o n s and the energy dependence of e l e c t r o n escape depths. Using appropriate compounds, most reported l i n e i n t e n s i t i e s have been measured r e l a t i v e to the f l u o r i n e Is c r o s s - s e c t i o n . Reference 2 contains a comprehensive t a b u l a t i o n of peak i n t e n s i t i e s and includes repr e s e n t a t i v e l e v e l s f o r most of the elements. Evans et a l . (9) have reported the most recent i n t e n s i t y measurements f o r uranium compounds. These are i n t e g r a t e d i n t e n s i t y measurements and i n c l u d e c o r r e c t i o n s f o r the experimental c o n s i d e r a t i o n s c i t e d above. The m u l t i p l e t c a l c u l a t i o n s discussed above (see F i g . 2) determine r e l a t i v e i n t e n s i t i e s w i t h i n e x c i t e d m u l t i p l e t s . However, the f e l e c t r o n i n t e n s i t i e s are not r e l a t e d to s, p, or d i n t e n sities. C a l c u l a t i o n s of the expected XPS spectra f o r the a c t i n i d e dioxides uranium through berkelium were reported by Gubanov et a l . (10). Results f o r U0 are shown i n F i g . 3 along with experimental spectra. These c a l c u l a t i o n s , extending about 30 eV below the Fermi l e v e l , are based on a one-electron m o l e c u l a r - c l u s t e r approach. 2

20.

VEAL AND L A M

Np0


3

Pu0

2

f -*f

Photoelectron

Spectra of Actinide

Am 0

2

Compounds

431

2

2

10 BINDING ENERGY (eV) Figure 2. XPS spectra of localized 5i states in three actinide oxides compared with calculated final-state multiplet spectra. The calculated multiplets are broadened to simulate experiment.

UO,

Journal of the Chemical Society, Faraday Trans.

Figure 3. Measured (a) and calculated (b) U0 XPS spectra. The solid line in (b) has Lorentzian broadening of 0.4 eV, the dashed line, 0.9 eV to simulate experiment. 2

432

LANTHANIDE AND


SPECTROSCOPY

I n t e n s i t i e s were determined using t r a n s i t i o n p r o b a b i l i t i e s taken from the c a l c u l a t i o n s of S c o f i e l d (11). In general, the (energy) p o s i t i o n s of f e a t u r e s i n the c a l c u l a t e d s p e c t r a correspond w e l l w i t h the experimentally observed f e a t u r e s but some c a l c u l a t e d i n t e n s i t i e s do not show good q u a n t i t a t i v e agreement with e x p e r i ment . 5 f - e l e c t r o n s and


A.

Bonding

Oxides.

Due to the photon energy dependence of the photoemission cross s e c t i o n f o r e l e c t r o n s w i t h d i f f e r e n t o r b i t a l angular moments, a c a p a b i l i t y i s a v a i l a b l e f o r i n v e s t i g a t i n g the r o l e of 5f e l e c t r o n s i n bonding. A systematic dependence of the l o c a l i z e d 5f e l e c t r o n peak i n t e n s i t y on the degree of o x i d a t i o n of the uranium atom was found i n a s e r i e s of uranium b i n a r y and ternary oxides (8). As the o x i d a t i o n s t a t e of uranium i n the oxides i s increased, e l e c t r o n s are t r a n s f e r r e d from the l o c a l i z e d 5f s t a t e s i n t o the "bonding" molecular o r b i t a l s which are predominately 0 2p i n c h a r a c t e r . Results are shown i n F i g . 4 f o r s e v e r a l b i n a r y oxides. The 5f i n t e n s i t i e s (per e l e c t r o n ) are s u b s t a n t i a l l y greater than the 0 2 p s . With increased o x i d a t i o n , the uranium v a l e n c e s t a t e increases and more 5f p a r t i c i p a t i o n might be expected i n the "bonding o r b i t a l s " . However, Veal et a l . (8) concluded, from a q u a n t i t a t i v e study of the valence band XPS i n t e n s i t i e s , that 5f e l e c t r o n s do not appear to s i g n i f i c a n t l y c o n t r i b u t e to the bonding molecular o r b i t a l s . For the hexavalent uranium compounds, i t appears that the 5f l e v e l s are pushed above Ep. However, molecular c l u s t e r c a l c u l a t i o n s f o r a c t i n i d e oxides (10) i n d i c a t e that 5f s t a t e s do show a c t i v e p a r t i c i p a t i o n i n the metall i g a n d bond. f

Complementary s t u d i e s of the 5f s t r u c t u r e i n UO2 were o b t a i n ed using UPS. F i g u r e 5 shows photoemission s p e c t r a of Evans r e corded at 21.2, 40.8, and 1253.6 eV (12). The 5f peak near E i s dominant at 1253.6 eV but i s b a r e l y d i s c e r n a b l e at 21.2 eV. A d d i t i o n a l s p e c t r a of U0 at 21.2, 40.8, and 48.4 eV were obtained by Naegele et a l . (13). A c h a r a c t e r i s t i c of the UO2 s p e c t r a [noted by B. Brandow i n Ref. (13)] i s that the high binding energy s i d e of the "0 2p" band grows i n i n t e n s i t y along w i t h the 5f peak as photon energy i s i n c r e a s e d . Since the bottom of the 2p band has the Bloch s t a t e s w i t h the strongest 2p-5f h y b r i d i z a t i o n , these r e s u l t s support the view that 5f e l e c t r o n s h y b r i d i z e w i t h the 0 sp's i n forming the m e t a l - l i g a n d bond. F

2

B.

I n t e r m e t a l l i c Compounds.

Binary i n t e r m e t a l l i c compounds of the l i g h t a c t i n i d e s d i s p l a y a wide v a r i e t y of magnetic and e l e c t r o n i c p r o p e r t i e s that are not w e l l understood. P h y s i c a l phenomena a s s o c i a t e d w i t h

VEAL AND L A M

Photoelectwn

Spectra of Actinide

Compounds


~i—i—i—i—I—i—i—i—i—I—i—i—i—i—I—i—i—r

URANIUM

uo

s.

,

2

U 0 (U0

\

4

OXIDES

9

2 > 2 5

)

J

\

i

Figure 4.

U 0 (U0 . 3

8

I i i

2

I

6 7

)

I I I I

I l I I I I -5 E ENERGY (eV)

i i

XPS valence spectra of several uranium oxides. The uranium 5f peak near E is attenuated with increasing uranium oxidation. F

433


434

LANTHANIDE AND


SPECTROSCOPY

i t i n e r a n t e l e c t r o n behavior, resonance 5 f - e l e c t r o n s t a t e s , s p i n f l u c t u a t i o n s and l o c a l i z e d e l e c t r o n behavior can a l l be found. No s i n g l e t h e o r e t i c a l framework can s u i t a b l y account f o r a l l phenomena observed. Several NaCl-type binary compounds, i n c l u d i n g UN, have been analyzed (14) using both t h e o r e t i c a l band s t r u c t u r e ( i t i n e r a n t ) and c r y s t a l f i e l d ( l o c a l i z e d ) approaches (although the a p p l i c a b i l i t y of one of these approaches g e n e r a l l y means that the other i s i n a p p r o p r i a t e ) . UN may be an intermediate case where n e i t h e r approach w i l l y i e l d very s a t i s f a c t o r y r e s u l t s . Photoemission spectroscopy can y i e l d v a l u a b l e i n s i g h t s i n t o t h i s problem. Figure 6 shows UPS spectra f o r UN and ThN (15). The strong peak near Ep seen i n UN i s t o t a l l y missing i n ThN. Since ThN has no 5f occupation but i s c r y s t a l l o g r a p h i c a l l y s i m i l a r to UN, the 5f nature of the peak near Ep i s confirmed. Figure 7 shows UPS data at 21.2 and 40.8 eV, again work of Norton et a l . (15). The 5f peak near Ep shows the same c h a r a c t e r i s t i c a l l y strong photon energy dependence that was observed i n U O 2 . (The remaining s p e c t r a l features are a s s o c i a t e d with s-p e l e c t r o n s . ) The very narrow 5f peak seen i n F i g . 7 implies that the 5f's occupy a narrow, s t e e p l y r i s i n g band or that the l e v e l s are e s s e n t i a l l y l o c a l i z e d i n which case XPS m u l t i p l e t theory (see above) should be a p p l i c a b l e . Taking the l a t t e r view i n F i g . 7, the c a l c u l a t e d n u l t i p l e t spectrum appropriate f o r a 5 f ground s t a t e i s compared :o experiment. There i s a remarkably good correspondence between :heory and experiment, p a r t i c u l a r l y at 40.8 eV where the 5f peak .s dominant. For f u r t h e r d i s c u s s i o n of these r e s u l t s , see Ref. 4. 3

S a t e l l i t e Structure S a t e l l i t e s appear as peaks on the high binding energy s i d e of the main peak i n an XPS spectrum. The l i n e s are g e n e r a l l y a s s o c i a t e d with d i s c r e t e energy l o s s e s c a l l e d "shake-up" or "shake-off" processes that are a t t r i b u t e d to sudden changes i n the l o c a l atomic charge that accompany e l e c t r o n e j e c t i o n . These processes i n v o l v e e x c i t a t i o n s i n the n-1 e l e c t r o n system and are g e n e r a l l y described as the low energy e x c i t a t i o n of a second e l e c t r o n "concurrent" with primary e l e c t r o n emission (16). The "shake-up" core l e v e l s a t e l l i t e spectra are g e n e r a l l y s e n s i t i v e to chemical bonding. S a t e l l i t e s p e c t r a f o r binary uranium oxides and f l u o r i d e s are discussed by Pireaux et a l . (17). They a t t r i b u t e the dominant s a t e l l i t e s to an e x c i t a t i o n from an occupied l i g a n d (predominantly 0 2p) o r b i t a l to an empty or p a r t i a l l y f i l l e d metal e l e c t r o n i c l e v e l . This i s a "charge t r a n s f e r " e x c i t a t i o n that accompanies the primary photoejection process. S a t e l l i t e s may a l s o be observable i n photoelectron spectra i f a 2-hole f i n a l s t a t e , w i t h s i m i l a r t o t a l energy and the same t o t a l angular momentum and p a r i t y as the o r i g i n a l core hole s t a t e , can occur. Bancroft et a l . (18) pointed out that there are s e v e r a l e n e r g e t i c a l l y f a v o r a b l e examples f o r such ( c o n f i g u r a t i o n i n t e r a c t i o n ) s a t e l l i t e s i n l i g h t a c t i n i d e s . T h e i r computed s a t e l l i t e


20.

VEAL AND L A M

Photoelectwn Spectra of Actinide Compounds

435

Figure 5. Photoemission spectra for U0 taken at photon excitation energies of 21.2 eV (He I), 40.8 eV (He II), and 1253.6 eV (MgKa). The uranium 5f electrons (Peak A) have a very different dependence on photon energy than the O 2ps (Peak B). 2

15

10

5

BINDING ENERGY (eV)

Figure 6.

UPS spectra for UN and ThN. The strong peak in UN which appears near E corresponds to U 5i electrons. F

LANTHANIDE AND ACTINIDE CHEMISTRY AND SPECTROSCOPY


436

BINDING ENERGY (eV) Figure 7.

UPS spectra for UN at 21.2 and 40.8 eV. The lower curve is a calculation of the final-state multiplet structure for the 5F final state.

20.

VEAL AND L A M

Photoelectron

Spectra of Actinide

Compounds

437

i n t e n s i t y r e s u l t s , based on the sudden approximation, shows that the 5 s , 5p and 6 s l e v e l s should produce the most prominent s a t e l lites. Depending on which low l y i n g f i n a l s t a t e s are involved i n the e x c i t a t i o n , the process may or may not show chemical s e n s i t i v ity. Kowalczyk (19) argues that when 5 f f i n a l s t a t e s are involved, the e f f e c t may be u s e f u l f o r i n v e s t i g a t i n g the degree of l o c a l i z a t i o n of 5 f e l e c t r o n s .


Ligand F i e l d E f f e c t s In a systematic study i n v o l v i n g more than twenty u r a n y l compounds, i t was e s t a b l i s h e d that the a x i a l l i g a n d f i e l d w i t h i n the uranyl u n i t can produce s u b s t a n t i a l s p l i t t i n g i n the XPS spectra of the a c t i n i d e 6 p 3 / level ( 8 ) . Figure 8 shows examples of the 6 P 3 / 2 l e v e l s p l i t t i n g s observed i n a sequence of uranyl samples with d i f f e r e n t primary U-0 separations (U-Oj). The experimental spectra of the u r a n y l compounds w i t h the smallest U-Oj separation, i n c l u d i n g the U 6 p 3 / s p l i t t i n g , i s w e l l r e p r e sented by the c h a r a c t e r i s t i c energies obtained from a r e l a t i v i s t i c molecular c l u s t e r c a l c u l a t i o n (20). An example i s shown i n F i g . 9. However, to obtain good agreement between theory and experiment f o r a l l the uranyls s t u d i e d , the e f f e c t of the uranium second-near neighbors had to be included. c

o

r

e

2

2

Spin-polarized

Photoemission

For magnetically ordered m a t e r i a l s , photoemitted e l e c t r o n s have a c h a r a c t e r i s t i c s p i n p o l a r i z a t i o n that r e f l e c t s the e l e c t r o n s p i n o r i e n t a t i o n o c c u r r i n g i n the sample before the photoemission process. Recently, techniques have been developed to measure t h i s photoelectron s p i n p o l a r i z a t i o n (photo ESP) (21). When the measured ESP moment i s a l i g n e d p a r a l l e l to the t o t a l magnetization, the spin p o l a r i z a t i o n i s designated as p o s i t i v e . Because the ESP technique s u f f e r s from low measurable i n t e n s i t i e s of p o l a r i z e d photoelectrons, the u s u a l e l e c t r o n energy d i s t r i b u t i o n (EDC) curves are not measured f o r p o l a r i z e d e l e c t r o n s . Rather, i n t e g r a t e d e l e c t r o n y i e l d s f o r a given p h o t o e x c i t a t i o n energy are measured and the percentage of p o l a r i z a t i o n of the i n t e g r a t e d y i e l d i s determined. The photon energy dependence of the p o l a r i z a t i o n gives information on the net s p i n of the e l e c t r o n s w i t h i n h o>-c|> ( i s the work function) of the Fermi l e v e l . Measurements are u s u a l l y reported f o r photon energies between 4 and 11 eV. Photo ESP measurements have been reported f o r the s e r i e s of i n t e r m e t a l l i c compounds US, USe and UTe (21). The ESP f o r these compounds i s negative f o r a l l ik w. The magnetic moment of uranium compounds i s predominately determined by the occupied 5f e l e c t r o n s . However, the p h o t o y i e l d of the f - e l e c t r o n s at photon energies l e s s that 11 eV i s very small r e l a t i v e to s, p or d e l e c t r o n y i e l d s . Thus, the observed photoelectron s p i n

438


i—i—i—i—r

1


t NQ2P

1 — i

r

Uronyls

"Bond" Na U0 8*I.90A 2

4

02s

i

\

0\

U6P„

Li U 0,o 2

* % *>K/

U6P,

3

8*1.76 A

F2s

-40

J

I -30

i

L -20

-10

ENERGY (eV)

Figure 8.

XPS spectra of three uranyl compounds taken within 40 eV of E . The ^uranium 6p splitting" varies with U — O separation 8. F

3/2

r


20.

VEAL AND LAM

Photoelectron

i U0 C0 2

Spectra of Actinide

r

Compounds

n

439

r

3

A

8=1.7

02s U6P,'1/2

/ \

U6R,

3/2

U6P U6

J

Figure

L. -40

9.

02 s P |

3/2

^ "BOND"*

1/2

-30

-20 ENERGY(eV)

-10

E

F

Comparison between experimental XPS spectrum and calculated energy levels for the uranyl compound U0 C0 2

3


440

polarization must result from conduction electrons that are polarized by the electrons in the partially filled f shell. These polarization results are consistent with results derived from magnetization, nuclear magnetic resonance and neutron scattering measurements (4,22).


Acknowledgment This work was supported by the United States Department of Energy. Literature Cited 1. Siegbahn, K.; Nordling, C.; Fahlman, A.; Nordberg, R.; Hamrin, K.; Hedman, J.; Johansson, G.; Bergmark, T.; Karlsson, S.; Lindgren, I.; Lindberg, B. "ESCA Atomic, Molecular, and Solid State Structure Studied by Means of Electron Spectroscopy Nova Acta Regiae Societates Scientiarm Upsaliensis, Ser. IV, Vol. 20, 1967. 2. Cardona, M.; Ley, L. in "Photoemission in Solids I", Cardona, M. and Ley, L . , Springer-Verlag 1978, 1-104. 3. XPS studies of lanthanide multiplet structure and fluctuations are discussed in Campagna, M.; Wertheim, G. K.; Structure and Bonding, 1976, 30, 99. 4. For a more comprehensive discussion of XPS studies of uranium compounds, see Veal, B. W.; Lam, D. J.; Vol. "Uran", Gmelin Handbuch der Anorganischen Chemie, to be published. 5. Verbist, J.; Rega, J . ; Tenret-Noel, C.; Pireaux, J. J.; d'Ursel, G.; Caudano, R., and Derouane, E. G. in "Plutonium 1975 and Other Actinides", ed. H. Blank and R. Lindner, North Holland 1976, p. 409. 6. Koopman, T. Physica 1933, 1, 104. 7. Cox, P. A.; Evans, S.; Orchard, A. F. Chem. Phys. Letters, 1972, 13, 386. 8. Veal, B. W.; Lam, D. J.; Hoekstra, H. R.; Diamond, H.; Carnall, W. T. in "Proc. 2nd Int'l. Conf. Elec. Struc. of Actinides", ed. J. Mulak, W. Suski and R. Tróc, Wroclaw, Poland 1976, p. 145 and references therein. 9. Evans, S.; Pritchard, R.; Thomas, J. J. of Elec. Spectr. and Rel. Phenom. 1978, 14, 341. 10. Gubanov, V. A.; Rosén, A.; Ellis, D. E. J. Phys. Chem. Solids, 1979, 40, 17. 11. Scofield, J. H. J. Electron Spectr., 1976, 8, 129. 12. Evans, S. JCS Faraday II, 1977, 73, 1341. 13. Veal, B. W. in the discussion section of J. de Physique, Colloq. C4, Suppl. No. 4, 1979, 40, C4-163. 14. "The Actinides: Electronic Structure and Related Properties", ed. A. J. Freeman and J. B. Darby, Academic Press, New York (1974), Vols. I and II. 15. Norton, P. R.; Tapping, R. L.; Creber, D. K.; Beyers, W. J. L.; private communication. 16. Shirley, D.A. in "Photoemission in Solids I", ed. M. Cardona and L. Ley, Springer-Verlag, 1978, pp. 165-195. "

20.

17. 18. 19. 20.


21. 22.

VEAL AND LAM

Photoelectron Spectra of Actinide Compounds

Pireaux, J. J.; Riga, J . ; Thibaut, E.; Tenret-Noel, C.; Caudano, R.; Verbist, J. J. Chem. Phys., 1977, 22, 113. Bancroft, G. M.; Sham, T. K.; Larson, S. Chem. Phys. Lett. 1977, 46, 551. Kowalczyk, S. J. de Physique, Colloq. C4, Suppl. No. 4, 1979, 40, C4-224. Ellis, D. E.; Rósen, A.; Walch, P. F. Int. J. Quantum Chem. Symp., 1975, 9, 351. Eib, W.; Erbudak, M.; Greuter, F.; Reihl, B. J. Phys. C: Solid St. Phys., 1979, 12, 1195; J. de Physique, Colloq. C4 Suppl. No. 4, 1979, 40, C4-72, and references therein. Jena, P.; Emmons, R.; Lam, D. J.; Ray, D. K. Phys. Rev. B, 1978, 18, 3562.

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March 10, 1980.

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Lanthanide and Actinide Chemistry and Spectroscopy - American

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