96 Surface Science 162 (1985) 96-102 North-Holland, Amsterdam
INFLUENCE OF TEMPERATURE A N D OF P H Y S I S O R B E D X e / K r O N T H E SPIN RESOLVED P H O T O E M | S S I O N FROM Pt(l I I)
A. EYERS, G. S C H O N H E N S E , U. FRIESS, F. SCH)~FERS * and U. H E I N Z M A N N
l"rttz- Ilaher- Institut der M a x - Planck -Gesellschaft, karad~vweg 4 - 6. D- IO(X) Berlin 33, Germany and
kakulti:t fur Phw'ik, Unu,ersttat Bwlefeld, D-4800 Blelefeld. Fed. Rep. of Germany
Received 1 April 1985: accepted for publication 22 April 1985
Spin polarized photoemission from P t ( l l l } has been studied under normal incidence and normal emission as function of the temperature between 30 and 550 K using circularly polarized synchrotron radiation of BESSY. The photoelectron intensities and the electron spin polarizations are reduced with increasing temperature and also by increasing coverage of physisorbed rare gas atoms.
I. Introduction
Photoelectrons ejected from unpolarized targets by circularly polarized light are generally spin polarized due to the existence of the spin-orbit interaction and the corresponding dipole-selection rules [1-4]. In non-magnetic solids the electron spin polarization (ESP) depends on the symmetry properties of the electron bands [3,5]. Therefore measurements of this type allowed a symmetry resolved band mapping of Pt(111 ) [3].
in this paper we report on angle- and spin-resolved photoemission spec- troscopy of P t ( l l l ) in normal incidence and normal emission, which has been studied as function of the target temperature between 30 and 550 K and as function of rare-gas coverage using circularly polarized synchrotron radiation at the electron storage ring BESSY. Temperature effects in angle-resolved photoemission processes have been discussed in terms of thermal and other disorders by Williams et al. and Shevchik [6]. Heimann and Neddermeyer [7]
observed a pronounced broadening of the photoemission structures for direct transitions at Au(111) with increasing temperature. For metal halides an appreciable broadening of the energy-distribution curves of the photoelectrons with increasing temperature was observed by Bauer et al. [8].
* BF.SSY. I.entzealle 100, I)-1000 Berlin 33, Germany.
0039-6028/85/$03.30 '~:~ Elsevier Science Publishers B.V.
(North-Holland Physics Publishing Division)
A. E.vers et al. / Spin resolved photoernission from Pt( i l l ) 97
2. Experimental
The measurements have been performed using the synchrotron radiation emitted above or below the storage-ring plane and monochromatized by a 6.5 m Ni monochromator of the Monk-Gillieson type [9] and found to be elliptically polarized with a high degree of circular polarization [2]. The photoelectrons emitted are analyzed with respect to their kinetic energy and their spin polarization by a rotatable simulated hemispherical spectrometer system and by an UHV Mott detector, respectively [3]. The adsorbatc was introduced via a doser nozzle in front of a liquid He-cooled targct manipulator.
The P t ( l l l ) crystal was cleaned by a combination of ion bombardment, heating in oxygen and flashing and was characterized by AES and LEED.
3. Experimental results and discussion 3.1. Influence of the target temperature
One example of a spin-resolved photoelectron spectrum of Pt(111) at 300 K is presented in fig. 1. The spin-polarization values shown are normalized to a complete circular polarization with positive helicity; each error bar contains
aCI a
t / ) Z
z_
0
60
-~ 4o z 20
.~ o
N ~:-20
~
-t.0-60
I • I
Pt ( 111 ) hv = lO.6eV
o
% ~ - : v * ' , , , , ,.j.. ' " Q
I I I
l
I
|1tlltllt li I II 111"111 111 II I I
i I
I I / I
-4 -~ 0
ENERGY BELOW E F ( e V ) b
|
I1' I
I-
Fig. 1. Spin-resolved photoelectron spectrum of Pt(111) for normal incidence and normal emission at 300 K. U p p e r part: Intensities measured with the two counters in the Mott detector, .solid and open circles. Lower part: Corresponding electron-spin polarization obtained from the data in the upper part and normalized to a complete circular polarization of the light. The experimental energy resolution A E is 150 meV F W H M (improved with respect to ref. [3]).
98 A. L)'ers et al. / Spin resolt, ed photoemtssion from I't( l 11)
o
> -
Z
z
, , , , ,
Pt ( I11 I
hv = 10.6eV 2
i i i I i
-/. -2 0
ENERGY BELOW E F (eV)
8 0 , ,
*~ 60 f IIk'llllll d i I
'
40 ' '
i 80t1~,~ ~111 Ill
Peak I
I I I I
i i
' Peo.k'2 t
100 200 300 /,00 500 TEMPERATURE (K)
Fig. 2. Upper part: Highly resolved photoelectron intensity spectra for normal incidence and normal emission at a temperature of 300 K (dashed line) and 40 K (solid line). Lower part: The corresponding spin polarization of peaks 1 and 2 (upper part) at hu = 10.6 eV (close to the theoretical calculation [11] as a function of the temperature. A E as in fig. 1.
the uncertainty of photon and electron polarization analysis. For normal emission the ESP has been found to be aligned along the surface normal, which defines the z-axis. The experimental spectrum at a photon energy of 10.6 eV clearly shows two peaks with positive and negative ESP, which arises due to direct interband transitions of the symmetry type A 4 . 5 - - ' A~ (positive ESP) and A 6 --* A 6 (negative ESP) [3].
The measured ESP, which never exceeded + 55% at room temperature is, however, lower than the values of +100% predicted by group theory for transitions between non hybridized bands [5]. Fig. 2 shows the ESP measured of the first two peaks in the intensity spectrum at hu = 10.6 eV as function of the temperature between 30 and 550 K. With decreasing temperature the ESP increases from + 50% to more than + 70%. Opposite to ferromagnetic materi- als, where the depolarization of 4f photoelectrons increases on approaching the Curie point T,. [101, this effect might be explained by a decreased emission of unpolarized electrons from non-k±-conserving transitions and other phonon assisted non-direct transitions which enhance the inelastic contributions to the direct transitions. With increasing temperature the non-direct unpolarizcd transitions become stronger. This effect may reduce the ESP measured in the peaks of the photoelectron spectrum at 10.6 eV. In addition, going from 300 to
A. Eyers et al. / Spin resolved photoemission from Ptfl 11) 99 40 K, the peak widths become smaller by about 50 meV and hence a deeper minimum between the intensity peaks is observed. It is worth noting that the ESP of peak 2 in fig. 2 at room temperature is a little bit higher than shown in fig. 1, due to the overall smaller contribution of the inelastic background compared with the peak height. Taking into account that electrons in the inelastic background are unpolarized the ESP analysis of both direct peaks alone yields data close to + 80% for a temperature of 30 K. Each value shown in fig. 2 has been obtained just after the crystal has been cooled down to the corresponding temperature. This procedure has been done to work with a clean crystal, because we observe a depolarization (of about 10%) as function of the time (about 1 h) due to the rest gas adsorption on the crystal. This high positive polarization measured in the first peak is in rather good agreement with a new relativistic one-step photoemission calculation for P t ( l l l ) by Ackermann and Feder [11], who predicted an ESP of about 90% for the first peak at a photon energy of 11 eV.
3.2. Influence o f rare-gas adsorption
The photoelectron intensities of the direct transitions of Pt(111) are strongly reduced with increasing coverage of Xe or Kr, as shown for Xe at two different photon energies in fig. 3. A similar decay of d-band intensities depending on coverage of different adatoms was observed also for N i ( l l 0 ) at 21.2 eV by Jacobi and Rotermund [12]. The absolute photoelectron intensity and the ESP of the peaks of the adatoms are very high near threshold and vary strongly with the photon energy, as discussed in ref. [4], while at hv = 15.5 eV, however, the intensities from the physisorbed Xe atoms are already strongly reduced.
Fig. 4 shows the ESP in dependence of the photon energy for clean Pt and for Pt covered by a complete ¢~ layer of Xe atoms. The ESP in the first two peaks of Pt(ll 1) is more reduced in the threshold region of the rare-gas photoemis- sion than towards higher photon energies. This effect can be explained by a stronger interaction between the outgoing electrons and the physisorbed atoms for electrons of smaller kinetic energies. The adsorption induced depolarization effect is more. pronounced in the reality than shown in fig. 4, because it is partly masked by the polarization enhancement due to the decrease of the temperature as discussed in section 3.1. Such depolarization effects for the bulk electrons, which have also been observed for different surface layers of Ni, Gd, Ce, Au and Cs at Ge and GaAs [13], may be due to spin-flip scattering at the Xe overlayer. In accordance with this, the ESP is reduced with increasing coverage and for a complete Xe monolayer we only found an ESP less than
+15%.
Another effect of the increasing coverage of Xe or Kr atoms is a new peak at 0.25 eV below E v (fig. 3), which does not show any dispersion for different coverages of Xe and Kr atoms. This is demonstrated in fig. 5 (peak marked by
100 A. Eyers et al. / Spin resolved photoemissmn from Pt(111)
c
> - z z
• j , , , , , , , , , 1 , , , ,
i n
',, I'l Xe/Pt (111)
i
• ~ 11 h v = 1 1 0 e V i tl
r / ! / I h v = 1 5 . 5 e V
~ . . / \ "- . . . ~ / - - \ ...
I h ~ \
F '~--.. ~ , ( \ \ - . ... . p ~ J ~
! ' \ - " " A \ e # " ~ . . . . " _ A ^~
:P, \ - - . #AX °'I " . . . A , % ,
-Ji
' - - . . ?},} :> ...l ... ' ~a,' /\~ \',.
-t, -2 0 -8 -6 -t, -2 0 ENERGY BELOW E F (eV]
6O
- - 40
Z O
-20
-t.0
- 6 0
l t P e a k I
l I [ I 1 I I
I' I l
+ +
÷÷ ÷
P e a k 2
÷ +
' J I J
I I I
t I I I
I I i i
9 10 II 12
hv (eV)
Fig. 3. Photoelectron spectra of Pt(111) for normal incidence and normal emission taken at various coverages of xenon: (a) clean substrate, (b)-(d) growth of commensurate ~3 x J3 (R30 °) islands of Xe, (d) complete ~3 layer, (e) compression phase, (f) complete incommensurate hop monolayer.
Fig. 4. Photoelectron-spin polarization of the peaks l and 2 of Pt(l l 1) (see fig. 2) as function of the photon energy for clean Pt (T=300 K, open circles) and for Pt covered by a complete 7e3 x v/3(R30 °) layer of Xe (T= 50 K. full circles).
an a r r o w ) for p h o t o n energies b e t w e e n 12.0 a n d 16.5 eV. A t hv < 12.0 eV it is o v e r l a p p e d b y the first p e a k c o r r e s p o n d i n g to the d i r e c t t r a n s i t i o n of Pt(111), which m a y be p a r t l y r e s p o n s i b l e for the d e p o l a r i z a t i o n (fig 4): W i t h a v/3 - layer o f Xe a n d a m o n o l a y e r K r we m e a s u r e d an ESP ol ~ 2.7 + 3.7% a n d 7.8 _+ 3.7%, respectively, in this p e a k at a p h o t o n e n e r g y of 15.5 eV. T h i s new p e a k which is clearly e n h a n c e d b y a d s o r b a t e c o v e r a g e (cf. fig. 3) might be a l r e a d y p r e s e n t for c l e a n Pt, as i n d i c a t e d in a recent c a l c u l a t i o n [11 ].
4 . C o n c l u s i o n
Spin p o l a r i z e d p h o t o e l e c t r o n s c o r r e s p o n d i n g to d i r e c t i n t e r b a n d t r a n s i t i o n s o f Pt in the A d i r e c t i o n have been m e a s u r e d as f u n c t i o n o f the t a r g e t t e m p e r a t u r e a n d o f the rare gas c o v e r a g e using spin- a n d a n g l e - r e s o l v e d p h o t o e l e c t r o n s p e c t r o s c o p y . A t low t e m p e r a t u r e s (30 K) spin p o l a r i z a t i o n s o f close to + 80% have been o b s e r v e d in the d i r e c t t r a n s i t i o n s , m u c h h i g h e r t h a n at r o o m t e m p e r a t u r e . F u r t h e r m o r e , it has been seen that o n l y a c o v e r a g e of less
A. Eyers et al. / Spin resolved photoemisszon from Pt(l l I) '^ . . . >(e'/Pl" (i l l i I
~ • hv
165 162 158 [
~s6 /
15z, I 152 / 1so I
. 1 ~ /
z 1.~6 /
1L, ,~
I~2
/
I~0 l
~ 3 s I 136 / 13~ | 132 / 130 [ 129 / 126 / 1ZZ"
122 /
101
-10 -8 -6 -t, -2 0 ENERGY BELOW E F
Fig. 5. Photoelectron spectra of Pt(111) with a complete Xe v/3 layer for various photon energies (normal incidence and normal emission).
than a monolayer of physisorbed rare gas atoms strongly reduced the ESP of photoelectrons from Pt(111).
Acknowledgement
We thank (3. Borstel, J. Kirschner, F. Meier, and N. MOiler for discussions.
The co-operation with BESSY and the support by the BMFT are gratefully acknowledged.
References
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