Spin-resolved photoemission from Ir(111): Transitions into a secondary band and energetic position of the final state bands

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~ Solid State Communications, VoI.61,No.3, pp.187-192, 1987.

Printed in Great Britain.

0038-I098/87 $3.00 + .00 Pergamon Journals Ltd.

SPIN-RESOLVED PHOTOEMISSION FROM Ir(lll): TRANSITIONS INTO A SECONDARY BAND AND ENERGETIC POSITION OF THE FINAL STATE BANDS

N. M~ller, B. Kessler, B. Schmiedeskamp, G. Sch~nhense and U. Heinzmann Universit~t Bielefeld, Fakult~t f~r Physik, D-4800 Bielefeld

and

Fritz-Haber-Institut der MPG, D-1000 Berlin 33 (Received October Ist, 1986 by P. Wachter)

Spin resolved photoelectron spectroscopy with Ir(lll) was performed in normal incidence of circularly polarized VUV radiation and normal electron emission. Using the spin information the spectra were separated with regard to the symmetry of the initial states. Besides the dominant transitions into the free electron like parts of the unoccupied bands, transitions into a secondary unoccupied band were unambiguously identified. Transitions into this secondary band cannot be evidenced without spin analysis. T h e data are in excellent agreement with a fully relativistic first-principles band structure calculation of Noffke and Fritsche, except for an overall shift of AE = 0.8 eV ±0.3 eV for the energy of the unoccupied final bands.

I. Introduction

In electronic transitions caused by circularly polarized photons the electron spins are commonly aligned. This is also true for unpolarized targets (1-7). This alignment arises from the spin-orbit interaction. For direct transitions the resulting electron spin polarization (ESP) is determined by the symmetry of the states involved, and is de- scribed by the corresponding (relativistic) selection rules for optical dipole transitions.

The detectability of the spin effects depends on the magnitude of the resulting spin-orbit energy splitting in comparison to both the lifetime broadening of the excited electronic states and the experimental resolu- tion. For medium- and high-Z materials the spin-orbit coupling is strong, and compared to the detectability limits the electronic states are split and/or modified considerably.

Correspondingly, the nonrelativistic selection rules, which only account for the spatial symmetry, break down while the relativistic ones (which include spin) persist ( 8 ) . With solids, a spin resolved photoemission experiment thus yields information not obtainable without accounting for spin effects, namely about the symmetry and the hybridization of the states involved [5,6,9-12).

In photoemission from solids via non hybridized final states with normally incident circularly polarized light and normal emission along a high symmetry line (i.e. in cubic crystals along A or ~), electrons arising from one distinct direct transition are totally polarized parallel or antiparallel to the photon polarization. The sign of the spin polarization (13) depends only on the symmetry of the initial state involved and the helicity of the incident light ( 9 , 1 0 ) , as the final

187

states must be totally symmetric for the case of normal emission (141 .

The resulting photoelectron intensity spectra should thus consist of two partial spectra correlated to transitions characterized by P = +i or P = -i. These partial spectra I + and I are related to the measured total intensity I and the measured spin polarization P by

I+ = ~I(l+P) and I = I(I-P) (i).

Structures which are superimposed in the total intensity I may then be separated in the partial intensities I + and I thus giving an improved identification and- localization of peaks. This procedure separates the spin-orbit split initial states. The separation is not attainable by using linearly polarized light, since the spin dependent effects occur only due to the definite phase relations in the coupling of the x- and y-component of circularly polarized light (9,10). Using linearly polarized light it is only possible to separate initial states of different spatial symmetry as long as they are not mixed by spin-orbit coupling (14,8). The intensity separation given in eq. (I) has been successfully applied before in photoemission from magnetized materials to distinguish transitions originating from majority and minority spin bands (15,16), and in a previous spin resolved photoemission study of Pt(lll) (ll) as well as in spin polarized LEED (17).

In this paper we use partial intensities obtained in a spin-resolved photoemission experiment to investigate details of the Ir band structure along FAL, and to perform a cross comparison with the relativistic band structure calculation of Ir given by Noffke and Fritsche (18) (NF, see Fig. i).

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188

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Fig. I. B a n d s t r u c t u r e of Ir a l o n g FL c a l c u l a t e d by N o f f k e and F r i t s c h e (NF) I18). In t h e o c c u p i e d b a n d s the line types, c h a r a c t e r i z e the s y m m e t r y of the states: A~ "'" , A~ - - , A3 + A 3 _ _ _ The s o l i d a r r o w s are t r a n s i t i o n s

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a s s i g n e d b y NF ( A , B , C , D , F , G , H ) ; t r a n s i t i o n E is d r a w n for c o m p l e t i o n . T h e s i g n s i n s e r t e d into the a r r o w s i n d i c a t e the sign of t h e s p i n p o l a r i z a t i o n of e l e c t r o n s e x c i t e d by O + light.

T h e y f o l l o w f r o m the r e l a t i v i s t i c d i p o l e s e l e c - tion r u l e s g i v e n by W 6 h l e c k e a n d B o r s t e l [91 . T h e p a r t s of b a n d s 7 a n d 8 m a r k e d by h e a v y lines are "free e l e c t r o n like".

T o i n t e r p r e t the s t r u c t u r e s a p p a r e n t in p h o t o e m i s s i o n s p e c t r a m e a s u r e d b y v a n d e r V e e n et al. [191 (VHE) NF h a v e c o n s i d e r e d not o n l y the t r a n s i t i o n s into the free e l e c t r o n like s e g m e n t s of the u n o c c u p i e d b a n d s 7 a n d 8 b e t w e e n L 6 + a n d F 6- (see Fig. i, t r a n s i t i o n s A, B, C, a n d D) but a l s o t r a n s i t i o n s to the r e m a i n i n g p a r t s of the b a n d s 7 a n d 8 (Fig. I, t r a n s i t i o n s (E), F a n d G/H). T h e s e t r a n s i t i o n s to s e c o n d a r y b a n d s ( s e c o n d a r y c o n e s (201) s h o u l d be n e g l i g i b l e d u e to the f i n i t e l i f e t i m e of the e x c i t e d e l e c t r o n i c s t a t e s [211. F u r t h e r - more, the c o r r e s p o n d i n g s t r u c t u r e m e a s u r e d by V H E is o n l y a s h o u l d e r b e t w e e n E a n d the p e a k a r i s i n g f r o m t r a n s i t i o n A. A S V H E h a v e u s e d p a r t l y p - p o l a r i z e d l i g h t t h i s s h o u l d e r c o u l d a l s o b e e x p l a i n e d b y s u r f a c e s t a t e e m i s s i o n .

O n e p u r p o s e of this p a p e r is to i d e n t i f y the t r a n s i t i o n s E, F a n d G / H b y m e a n s of s p i n - d e p e n d e n t p a r t i a l i n t e n s i t i e s . A s e c o n d is to p r o v e the e n e r g e t i c p o s i t i o n of t h e u n - o c c u p i e d b a n d s 7 a n d 8. W h i l e the c a l c u l a t i o n of N F r e p r o d u c e d m o s t v a l u e s for the p o s i t i o n s of c r i t i c a l p o i n t s m e a s u r e d b y V H E w i t h i n the e x p e r i m e n t a l u n c e r t a i n t i e s , t h e L - p o i n t of b a n d 8 w a s c a l c u l a t e d to b e 0 . 5 e V l o w e r 6 in energy. F u r t h e r d i s c r e p a n c i e s in c r i t i c a l p o i n t e n e r g i e s w e r e o b s e r v e d b y M a c k et al.

(22). T h e y h a v e f o u n d s e c o n d a r y e l e c t r o n e m i s s i o n s t r u c t u r e s c o r r e l a t e d w i t h L 6 - a n d a r e s o n a n t p h o t o e m i s s i o n p e a k r e l a t e d t o t h e flat b a n d r e g i o n n e a r F 6- l y i n g a b o u t 0.7 e V h i g h e r in e n e r g y than c a l c u l a t e d by NF.

2. E x p e r i m e n t a l

The m e a s u r e m e n t s w e r e p e r f o r m e d at the e l e c t r o n s t o r a g e r i n g B E S S Y u s i n g the c i r c u l a r l y p o l a r - ized o f f - p l a n e r a d i a t i o n m o n o c h r o m a t i z e d b y a 6.5 m n o r m a l i n c i d e n c e m o n o c h r o m a t o r (23).

T h e b a n d w i d t h of the l i g h t w a s 0.5 nm, the d e g r e e of c i r c u l a r p o l a r i z a t i o n (88 ±3)%

I24). The d i r e c t i o n of light i n c i d e n c e co- i n c i d e d w i t h the Ir(lll) s u r f a c e n o r m a l w i t h i n 0.3 ° . P h o t o e l e c t r o n s e m i t t e d in an a c c e p t a n c e half a n g l e of 3 ° a r o u n d the s u r f a c e n o r m a l w e r e c o l l e c t e d b y a s i m u l a t e d h e m i s p h e r i c a l e n e r g y a n a l y z e r [25) f o l l o w e d b y an U H V M o t t - d e t e c t o r for E S P a n a l y s i s . T h e e n e r g y a n a l y z e r was o p e r a t e d w i t h c o n s t a n t p a s s e n e r g y r e s u l t i n g in an e n e r g y r e s o l u t i o n < 150 m e V i n d e p e n d e n t of the i n i t i a l e n e r g y . A p p a r a t u s a s y m m e t r i e s of the M o t t - d e t e c t o r are e l i m i n a t e d by c h a n g i n g the h e l i c i t y of the light.

T h e Ir(lll) c r y s t a l s u r f a c e w a s a l i g n e d w i t h i n 0.i ° u s i n g an x - r a y d i f f r a c t o m e t e r , g r o u n d on SiC (mesh 600 - 800) a n d p o l i s h e d w i t h d i a m o n d p a s t e in s u c c e s s i v e s t e p s f r o m 50 U to 0.25 U- T h e c r y s t a l w a s h e l d b y Ir w i r e s l y i n g in s p a r k - c u t g r o o v e s a n d w a s m o u n t e d on t o p of a l i q u i d He c o o l e d t a r g e t m a n i p u l a t o r . H e a t i n g w a s p e r f o r m e d b y e l e c t r o n b o m b a r d m e n t u s i n g an I r / T h O l o w t e m p e r a t u r e filament. The h e a t i n g t e m p e r a t u r e w a s c o n - t r o l l e d by a W / I r t h e r m o c o u p l e [26).

C h a r a c t e r i z a t i o n of the Ir(lll) s u r f a c e was d o n e in s i t u b y A u g e r s p e c t r o s c o p y a n d LEED. The c l e a n s u r f a c e w a s p r e p a r e d b y N e + b o m b a r d m e n t and by r e p e a t e d c y c l e s of h e a t i n g in o x y g e n at a b o u t Ii00 K a n d b y f l a s h i n g to a b o u t 1400 K. T h e o x y g e n w a s a d m i t t e d u s i n g a d o s e r g i v i n g a p a r t i a l p r e s s u r e of a b o u t 10 -6 m b a r in f r o n t of t h e c r y s t a l . T o m i n i m i z e p h o n o n e f f e c t s (271 d u r i n g the m e a s u r e m e n t s the c r y s t a l t e m p e r a t u r e w a s h e l d at a b o u t 60 K w h i c h is a b o u t 14% of t h e Ir D e b y e - t e m - p e r a t u r e G D = 420 K [28).

3. R e s u l t s a n d D i s c u s s i o n

A t y p i c a l set of d a t a o b t a i n e d by s p i n r e s o l v e d p h o t o e m i s s i o n is p r e s e n t e d in Fig. 2. F r o m the t o t a l i n t e n s i t y I (upper p a n e l ) a n d t h e c o r r e s p o n d i n g p o l a r i z a t i o n P ( m i d d l e p a n e l ) m e a s u r e d for an e x c i t a t i o n e n e r g y h ~ = 16 e V the p a r t i a l i n t e n s i t i e s I+ a n d I w e r e o b t a i n e d u s i n g e q u a t i o n (i) (lower p a n e l ) . I+ a n d I i l l u s t r a t e the i m p r o v e d d i s t i n c t i o n of t h e

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Vol. 61, No. 3

p e a k s in t h e s p i n - r e s o l v e d p h o t o e m i s s i o n s p e c t r a c o m p a r e d t o t h e t o t a l i n t e n s i t y I.

F o u r d o m i n a n t p e a k s l a b e l l e d A, B, C, a n d D a r e r e s o l v e d in t h e t o t a l i n t e n s i t y I as w e l l as in t h e p a r t i a l i n t e n s i t i e s I + a n d I . I + a n d I d e m o n s t r a t e t h a t as in t h e c a s e of P t ( l l l ) A, B, C, a n d D are c o r r e l a t e d t o t r a n s i t i o n s y i e l d i n g t o t a l l y p o l a r i z e d e l e c t r o n s ; the s i g n of P s e r v e s to a s s i g n t h e s e t r a n s i t i o n s d o u b t l e s s l y to t h e o c c u p i e d b a n d s 6, 5, 4, a n d 2 (see Fig. i). It is w o r t h n o t i n g t h a t t h e d e g r e e of p o l a r i z a t i o n JPJ o f u p t o 80% (as s h o w n in t h e m i d d l e p a n e l o f Fig. 2) e x c e e d s all d a t a p r e v i o u s l y m e a s u r e d for n o n m a g n e t i c 3D c r y s t a l s .

B e s i d e s the p e a k s A, B, C, a n d D in t h e t o t a l i n t e n s i t y a w e a k s h o u l d e r e x i s t s n e a r E F. T h i s s h o u l d e r is c o r r e l a t e d w i t h a p r o - n o u n c e d s t r u c t u r e in the s p i n p o l a r i z a t i o n a n d a p p e a r s as a c l e a r l y r e s o l v e d p e a k in t h e I _ - s p e c t r u m . T h i s p e a k (shoulder) o r i g i n a t e s f r o m a t r a n s i t i o n F b e t w e e n b a n d s 5 a n d 8, as p o s t u l a t e d b y NF. T o v e r i f y the a s s i g n m e n t of t r a n s i t i o n F m o r e e x t e n s i v e l y , a n d to l o o k for a t r a n s i t i o n E f r o m b a n d 6 t o b a n d 8 as w e l l as for t h e t r a n s i t i o n s G / H f r o m 5/6 t o 7 n e a r F, w e h a v e t a k e n s p e c t r a at s e v e r a l p h o t o n e n e r g i e s h V b e t w e e n 14.8 e V a n d 17.8 e V a n d o b t a i n e d the s p i n - r e s o l v e d r e s u l t s p r e s e n t e d in Fig. 3.

W h i l e the t o t a l i n t e n s i t i e s , w h i c h a r e a g a i n d o m i n a t e d b y t h e p e a k s A, B, C, a n d D, d o n o t r e v e a l w e l l d e f i n e d t r a n s i t i o n s E, F a n d t h e i r e x p e c t e d d i s p e r s i o n , t h e p a r t i a l i n t e n s i t i e s I+ a n d I_ d o so. T h u s t h e e x i s t e n c e o f t h e t r a n s i t i o n s E a n d F g o i n g f r o m b a n d 6 a n d 5 ( s y m m e t r y A~ + A~ a n d s y m m e t r y A~) to b a n d 8 ( s y m m e t r y A~) is ~ e m o n s t r a t e d b y ~ d e n t - i f y i n g t h e c o r r e s p o n d i n g p e a k s in t h e p a r t i a l i n t e n s i t i e s I+ a n d I_, r e s p e c t i v e l y . T h e t h r e - s h o l d s for t h e a p p e a r a n c e of t h e p e a k s E a n d F a r e h V = 1 6 . 4 e V ±0.i e V a n d h V = 15.1 e V

±0.i eV, r e s p e c t i v e l y . T h e y a r e d e d u c e d f r o m the d a t a u s i n g t h e c o n s t a n t i n i t i a l s t a t e m e t h o d g i v e n b y K n a p p e t al. 129). W i t h t h e s e t h r e s h o l d s f o u n d f o r E a n d F t w o p o i n t s o f t h e f i n a l s t a t e b a n d 8 c a n b e f i x e d in the b a n d s t r u c t u r e E(k) u s i n g t h e f e r m i l e v e l c r o s s i n g m e t h o d 129). A s t h e r e a r e n o d e H a a s - v a n A l p h e n d a t a for t h e f e r m i l e v e l c r o s s i n g p o i n t s of b a n d s 5, 6 a l o n g F A L in Ir, w e u s e t h e l e v e l c r o s s i n g s g i v e n b y N F a n d p e r f o r m t h u s a c h e c k of t h e c a l c u l a t e d b a n d s t r u c t u r e . T h e r e s u l t is p r e s e n t e d in Fig. 4: b a n d 8 h a s t o b e s h i f t e d b y 0 . 8 e V ±0.3 e V t o w a r d s h i g h e r e n e r g i e s . T h e u n c e r t a i n t y of ±0.3 e V

is e s t i m a t e d t a k i n g i n t o a c c o u n t t h e u n c e r t a i n - t y of t h e m o n o c h r o m a t o r c a l i b r a t i o n (~0.15 eV) a n d t h e e r r o r of t h e t h r e s h o l d d e t e r m i n a t i o n . T h i s e n e r g y s h i f t m a y b e d u e to an e s s e n t i a l d i f f e r e n c e b e t w e e n m e a s u r e m e n t a n d c a l c u l a t i o n : T h e m e a s u r e d e n e r g i e s are e x c i t a t i o n e n e r g i e s (real p a r t s of t h e q u a s i - p a r t i c l e e n e r g i e s ) , the c a l c u l a t e d e n e r g i e s are g r o u n d - s t a t e o n e g (30). T h e d i f f e r e n c e ~ - g p a r t i c l e e n e r g i e s

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is r e a s o n a b l e for m e t a l s a t E - E = = 1 5 e V (30,32).

T h e s h i f t of b a n d 8 t o ~ i g h e r e n e r g i e s is s u p p o r t e d b y s t r u c t u r e J (see Fig. 3):

S t a r t i n g at a b o u t h ~ = 17 e V at the l o w e n e r g y side of p e a k C a s h o u l d e r J is g r o w i n g . A t h 9 = 1 7 . 6 e V it f o r m s a d o u b l e p e a k t o g e t h e r

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Fig. 2. T o t a l i n t e n s i t y I (upper p a n e l ) , e l e c t r o n s p i n p o l a r i z a t i o n P ( m i d d l e p a n e l ) a n d p a r t i a l i n t e n s i t i e s I+, I (lower p a n e l ) m e a s u r e d at a p h o t o n e n e r g y h V = ( 1 6 . 0 ±0.15) eV.

For P, the e r r o r b a r s g i v e the s t a t i s t i c a l u n c e r t a i n t y (i x O d u e t o p a r t i c l e c o u n t i n g ) . T h e r e is an a d d i t i o n a l s c a l i n g e r r o r of ±10% d u e t o c a l i b r a t i o n u n c e r t a i n t i e s of t h e l i g h t p o l a r - i z a t i o n a n d of t h e M o t t - d e t e c t o r . F o r I+, I the h i g h t of t h e s y m b o l s g i v e s the s t a t i s t i c a l error. T h e a d d i t i o n a l u n c e r t a i n t y d u e to the s c a l i n g of P is o m i t t e d , it h a s n o e f f e c t on t h e r e s u l t s . T h e t o t a l i n t e n s i t y h a s b e e n m e a s u r e d i n d e p e n d e n t l y of t h e p o l a r i z a t i o n .

w i t h p e a k C, t h e s e p a r a t i o n of C a n d J b e i n g a b o u t 0.5 eV. T h e p e a k J is p r e s e n t o n l y in the p a r t i a l i n t e n s i t y I_, as for p e a k C. Thus b o t h J a n d C a r e d u e t o a d i r e c t t r a n s i t i o n A 3 + A ~ w i t h P = -I. H e n c e J m u s t b e a s s i g n e d t~ b e the o n s e t of a t r a n s i t i o n f r o m b a n d 3 t o b a n d 8 n e a r L (see Fig. 4). It c a n n o t be a s s i g n e d to a s e c o n d a r y e l e c t r o n s t r u c t u r e

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I90 SPIN-RESOLVED PHOTOEMISSION F R O M Ir(]]]) Vol. 61, No. 3

-6 -4 -2 E F -6 .z, -2 E F -6 -4 -2 E F

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I N I T I A L E N E R G Y

Fig. 3. Total intensity (first panel), partial intensity I+ (second panel), and partial intensity I (third panel) m e a s u r e d at various

p h o t o n energies from 14.8 eV to 17.8 eV. The

given points are larger than the statistical errors (see. Fig. 2). The structures d i s c u s s e d in the text and the thresholds for transitions E and F are marked.

correlated with the critical point L 6- of band 8, as a secondary electron structure would show P ~ 0. T r a n s i t i o n J results in an energetic p o s i t i o n of L 6- at 14.2 eV ±0.3 eV, compared to the value of 13.54 eV calculated by NF. This d i f f e r e n c e is in accordance with the energetic shift of 0.8 eV ±0.3 eV given above. It is remarkable that the peaks J and C are of comparable intensity. On the basis of intensity calculations for normal photoemission from Ag(lll) ( 3 3 ) and Au(lll) [34), the transition J should not be m e a s u r a b l e due to a vanishing surface transmission factor.

The transitions G and H from bands 5 and 6 to band 7 near F (see Fig. i) cannot be identified in the total intensity spectra. The I_-spectrum shows up a structure K close to the expected p o s i t i o n for G/H, but K also exists below the threshold for G/H (15.6 eV). The dis- persion of K is e q u i v a l e n t to the d i s p e r s i o n of peak A w h i c h in addition is inconsistent with the existence of G/H. The I+-spectrum also shows up a peak L, w h i c h disperses like peak B.

Both peaks K and L are significant and cannot be removed by considering systematic uncertainties of the measurements. These peaks may be due to the weak h y b r i d i z a t i o n resulting from a mixing of band 7 with bands of the same double group symmetry A 6 lying higher in energy, e.g.

band 9 (see Fig. I). Besides the spatial parts of the spinors transforming like A 1 the wave- functions of band 7 (and 8) contain a few per- cent of spatial parts t r a n s f o r m i n g like A 3 (35 I.

Assuming appropriate matrix elements the structures K and L then must exist, and be strongly related to the structures A and B.

Our measurements given in Fig. 3 allow

[r (111)

I ........

A

>

O

)--

n-- W Z W

.J,

7-

8*

7 +

8*

... .], ...

- 8 ... "...

.

L A r

Fig. 4. B a n d s t r u c t u r e of Ir along FL and results of our measurements. The solid and d o t t e d lines represent the band structure c a l c u l a t e d by Noffke and Fritsche (NF) [18). The broken lines give the u n o c c u p i e d bands shifted to higher energies by ~E = (0.8 ±0.3) eV. The m a p p i n g points m a r k e d by circles (Q,O) c o r r e s p o n d to transitions into band 8, the points m a r k e d by rhombs (0, ~ ) to transitions into band 7.

The m a p p i n g points near F m a r k e d by rectangles ( i , ~ ) are taken from m e a s u r e m e n t s at h~=19 eV and h~=20 eV not p r e s e n t e d in Fig. 3. Filled symbols represent P > 0, open symbols P < 0.

Transitions starting at the dotted bands (symmetry A~) are not allowed for the geometry used in this work.

a further test of the band structure calculation by NF. AS the calculation gives small energy differences b e t w e e n n e i g h b o u r i n g bands much more accurately than large bandgaps (the q u a s i - p a r t i c l e c o r r e c t i o n varies slowly with energy), it is reasonable to shift band 7 also towards higher energies by 0.8 eV as experimentally found for band 8. Taking into account this shift the use of the final state band structure calculation in connection w i t h the spin-resolved partial intensity spectra allows a symmetry resolved mapping of the occupied bands. The mapping points resulting from the transitions A, B, C, and D to band

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Vol. 61, No. 3 SPIN-PESOLVED PHOTOEMISSION FROM Ir(lll) 191 7 are included in Fig. 4 together with those

from the transitions E and F to band 8. They agree with the calculated bands within the experimental uncertainties. (The systematic calibration error of the photon energy is cancelled in this procedure). Some systematic shifts of 0.i to 0.2 eV to lower energies are, however, found for bands 2 and 4. These shifts may also be due to quasi-particle corrections. But ground state one particle calculations are exact only within the limits given by the observed shift (30).

4. Conclusions

Transitions into a secondary unoccupied band have been identified and symmetry characterized by spin-polarization measurements. This is not easily possible by use of the photoelectron intensity alone (36). The spin resolved band mapping procedure yields excellent agreement with the NF bandstructure calculation in the occupied d-bands as well as in the unoccupied final bands if the latter are shifted in energy by 0.8 eV compared with theory.

Several properties of Ir(lll) have facili- tated the derivation of the results presented.

The high Debye temperature of 420 K (28) yields a small broadening of photoemission peaks due to thermal vibrations. Also, the (occupied) d-bands energetically lie close to E F resulting in a weak hole lifetime broadening. This is especially true for the two upper d-bands (5 and 6), see Figs. 1,4) which cross the

Fermi level. Compared to other crystals (e.g.

Pt(lll) (5,37)) it is a further advantage of Ir(lll) that the emission of these bands cannot be superimposed by surface emission from other directions with high density of states (i.e. the Q- or Z-direction) because the corresponding flat bands lie above E

(18). It should also be noted that Ir(lll~

constitutes one of the rare examples of a non-reconstructed surface and that the preparation of the clean surface is possible without complications (partly a consequence of the high melting point T M = 2410°C). Ir(lll) can thus serve as a m o d e ~ target for photoemission from high-Z materials, especially under the aspect of spin resolved photoelectron spectroscopy.

A c k n o w l e d E e m e n t s - The authors wish to thank

Mrs. Eichele, G. Neff, E. Umbach (TU Munchen) and Mrs. Tangermann (LMU M[~nchen) for help in the preparation of the Ir(lll) crystal.

The crystal was kindly provided by T. Rhodin (Cornell University) and by B. Addiss (Ma- terials Preparation Lab., Materials Sciences Center, Cornell University). We are grateful to F. Noffke and L. Fritsche (TU Clausthal) for providing details of the Ir band structure.

We thank M. W6hlecke, M. Neumann, G. Borstel, and K. Snowdown (Universit~t Osnabr~ck) for a critical reading of the manuscript and for helpful discussions. We appreciate the engage- ment and the technical assistance of U. FrieB (FHI), C. Westphal, G. Hilgers, and V.

Schimmang.

Support of the BMFT (F6rderkennzeichen 0 5 3 3 1 A X I / O ) i s gratefully acknowledged.

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