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Optical reflectivity measurements on thorium metal samples
C. Alvani, J. Naegele
To cite this version:
C. Alvani, J. Naegele. Optical reflectivity measurements on thorium metal samples. Journal de
Physique Colloques, 1979, 40 (C4), pp.C4-131-C4-132. �10.1051/jphyscol:1979440�. �jpa-00218836�
JOURNAL DE PHYSIQUE Colloque C4, supplément au n° 4, Tome 40, avril 1979, page C4-131
Optical reflectivity measurements on thorium metal samples
C . Alvani a n d J. N a e g e l e
Commission of the European Communities, Joint Research Centre, European Institute for Transuranium Elements, Postfach 2266, D-7500 Karlsruhe 1, F.R.G.
Résumé. — L'étude systématique de la réflectivité optique a été entreprise sur des échantillons de thorium dont la surface a été préparée par polissage mécanique et électrolytique ou qui ont été utilisés tels quels après obtention par le procédé van Arkel. Les résultats des mesures de réflectivité optique sont comparés avec les résultats publiés. Une augmentation de la rugosité de la surface provoque une diminution de la réflectivité et induit probablement des plasmons de surface mis en évidence par le minimum de réflectivité mesuré à 4 eV.
Les meilleurs résultats n'ont été obtenus qu'avec des surfaces électropolies, cependant, dans des conditions bien déterminées, le procédé van Arkel conduit à de bons résultats.
Abstract. — Thorium samples, the surfaces of which were mechanically polished, electropolished and as grown from the van Arkel process, were systematically studied by optical reflectivity measurements. These optical data were compared with published measurements showing significant discrepancies. Increasing surface roughness was found to lower the reflectivity considerably and probably to induce surface plasmons indicated by a deep minimum in reflectivity at 4 eV. Best reflectivity data were obtained only for electropolished surfaces. Well chosen conditions for the van Arkel process give also good results.
1. Introduction. — T o d e t e r m i n e t h e c h a r a c t e r of t h e 5f-electrons in actinide m e t a l s , several optical investigations h a v e b e e n p e r f o r m e d o n t h o r i u m [ 1 , 2] a n d a m e r i c i u m [3]. F o r t h o r i u m , t h e surfaces w e r e p r e p a r e d in different w a y s (Ref. [1] : electro- polished single crystal ; Ref. [2] : b u l k material elec- tropolished a n d s p u t t e r e t c h e d in a glow discharge).
T h e results s h o w significant d i s c r e p a n c i e s . C o n s e - quently t h e s e d a t a m u s t b e i n t e r p r e t e d differently.
It m u s t b e p o i n t e d out as h a s b e e n stated earlier [1] t h a t serious difficulties arise f r o m attri- buting features in t h e s p e c t r o s c o p i c d a t a t o specific transitions in t h e calculated b a n d s c h e m e since a large n u m b e r of possible s t a t e s might b e involved.
T h e r e f o r e , a b e t t e r , n o t only qualitative [ 1 , 2 ] b u t q u a n t i t a t i v e c o m p a r i s o n n e e d s detailed m a t r i x ele- m e n t calculations w h i c h a r e n o t available at p r e s e n t . O n t h e o t h e r h a n d a c o m p a r i s o n of t h e reflectivity of t h o r i u m with t h a t of p r o t a c t i n i u m , t h e first m e t a l in t h e actinide series w h i c h is a s s u m e d t o h a v e o c c u - pied 5f-levels, could h e l p greatly t o identify t r a n s i - tions involving 5f-states.
T h e aim of this s t u d y w a s t o find a p r e p a r a t i o n p r o c e d u r e t h a t reliably avoids surface i n d u c e d r e - flectivity s t r u c t u r e s . T h o r i u m w a s t a k e n for c o m p a - r i s o n with a n d a s stand-in for p r o t a c t i n i u m t h e m e t a l t o b e m e a s u r e d n e x t . F o r this p u r p o s e t h e reflectivity of differently p r e p a r e d t h o r i u m s a m p l e s h a s b e e n m e a s u r e d f r o m 2 e V t o 5 e V a n d c o m p a r e d w i t h published d a t a .
2. Sample preparation. — T h e s u r f a c e s w e r e p r e - p a r e d using t h r e e different m e t h o d s :
— M e c h a n i c a l polishing d o w n t o 0.05 urn grain size.
— V a n A r k e l p r o c e s s . I n a sealed e v a c u a t e d q u a r t z t u b e , t h o r i u m is t r a n s p o r t e d as iodide in t h e v a p o u r p h a s e t o a h o t s u b s t r a t e (electropolished tungsten) w h e r e t h e iodide dissociates a n d a poly- crystalline Th-layer of high p u r i t y is d e p o s i t e d . I n o r d e r t o get t h e Th-layer as c o m p a c t a n d flat a s possible t h e t e m p e r a t u r e of t h e s u b s t r a t e a n d t h e partial p r e s s u r e of t h e iodide h a v e b e e n varied systematically.
— E l e c t r o c h e m i c a l polishing. T h e a d v a n t a g e of this p r o c e d u r e o v e r o t h e r m e t h o d s , specifically m e - chanical a b r a s i o n , is principally in obtaining an u n w o r k e d , strain free a n d microscopically flat surface [4].
A solution of p e r c h l o r i c acid (1.5 %) in m e t h a n o l [ 5 , 6, 7] h a s b e e n u s e d in a stainless steel b e a k e r as c a t h o d e . A t e m p e r a t u r e of — 80 °C w a s m a i n t a i n e d b y a flow of cooled nitrogen g a s . This p r o c e d u r e h a s b e e n applied t o single crystal a n d t o polycrystalline v a n A r k e l d e p o s i t e d T h s a m p l e s .
After p r e p a r a t i o n t h e s a m p l e s w e r e e n c l o s e d in a w i n d o w - s e a l e d s a m p l e holder u n d e r an argon a t m o - s p h e r e .
3. Results and discussion. — F i g u r e 1 s h o w s t h e reflectivity o b t a i n e d for mechanically (curve a) a n d electrochemically ( c u r v e b) polished T h single c r y s - tals t o g e t h e r with t h e published d a t a . C u r v e a s h o w s a r e m a r k a b l e reflectivity m i n i m u m a t 4 eV as r e p o r t - e d b y Veal et al. [2], w h e r e a s c u r v e b s h o w s m u c h higher reflectivity w i t h o u t t h e m i n i m u m as published
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979440
C. ALVANI AND J. NAEGELE
- PHOTON ENERGY E ( e V I
Fig. 1. - Reflectivity of thorium single crystal : a) mechanically polished ; b) electrochemically polished.
by Weaver and Olson [I]. Figure 2 shows the data for van Arkel deposited polycrystalline Th together with the published data : curves a (high porosity ; porosity was examined by electron microscopy) and b (low porosity) were obtained for samples prepared at substrate temperatures of 1 200 "C and 1 740 "C, respectively. Curve c was recorded on the same sample as for curve b but now electropolished.
The Kramers-Kronig conversion technique has been used to calculate the optical conductivity from the reflectivity data of curves b and c (Fig. 2). The results are shown together with the published data [I, 23 in figure 3. The trends seen in the reflecti- vity curves are also well reproduced in the optical conductivity. Those curves, derived from the reflec- tivity data without the minimum at 4 eV, agree and
I I I I I I
I
0 1 2 3 1 5 6
PHOTON ENERGY E ( e V 1
0
- 0 1 2 3 4 5 6 7 8
PHOTON ENERGY ( e V )
Fig. 3. - Optical conductivity of thorium derived from Kramers- Kronig conversion of reflectivity data : a) related to curve b (Fig. 2) ; b) related to curve c (Fig. 2).
show very similar well defined structures. The conductivity curve of Veal et al. [2] shows only very rough agreement.
The fact that curves b and c (Fig. 2) show substan- tially the same reflectivity structure although the surfaces were quite differently prepared (van Arkel as -grown and electropolished) supports an earlier assumption [I] that the minimum at about 4 eV is in error. Furthermore, the electropolished samples (single crystal and van Arkel Th) have given the highest reflectivity values. The electropolishing pro- cedure therefore is assumed to be the most reliable one.
Moreover, the oxygen contamination has proved to lower the absolute reflectivity values but not to change the character of the curve. The appearance of the deep reflectivity minimum at about 4 eV is therefore caused by surface roughness introduced by the preparation (mechanical treatment, van Arkel process at low temperatures, and glow discharge cleaning) and can be explained by losses due to surface plasmons the excitation probability of which is increased by surface roughness [I].
Fig. 2. - Reflectivity of polycrystalline van Arkel deposited
-
The authors are grateful tothorium : a) low substrate temperature ; b) substrate temperature Mr. H. Winkelmann for his expert technical assis-
close to the melting point ; c) electrochemically polished. tance
.
References
[I] WEAVER, J. H. and OLSON, C. G., Phys. Rev. 15B (1977) 4602. [4] WAGNER, C., J. Electrochem. Soc. 101 (1954) 225.
[2] VEAL, B. W., KOELLING, D. D. and FREEMAN, A. J . , Phys. 1.51 PETERSON, D. T. and HOPKINS, E. N., (UC-25) TID-4.500
Rev. Lett. 30 (1973) 1061. (1964).
[3] NAEGELE, J., SPIRLET, J. C. and WINKELMANN, H., Proc. 2nd [6] GABE, D. R., Metallography 5 (1972) 415.
Int. Conference Electronic Structure of Actinides, Wro- [7] WEAVER, J. H., OLSON, C. G. and LYNCH, D. W., Phys. Rev.
claw, Mulak, J., Suski, W., Troc., R. Eds. 1976, p. 275. 12B (1975) 1293.
