Surface-state contribution to the optical anisotropy of Ag„110… surfaces:
A reflectance-anisotropy-spectroscopy and photoemission study
K. Stahrenberg,* T. Herrmann, N. Esser, J. Sahm, and W. Richter
Institut fu ¨r Festko ¨rperphysik der TU Berlin, Hardenbergstrasse 36, 10623 Berlin, Germany
S. V. Hoffmann and Ph. Hofmann
Institute for Storage Ring Facilities, University of A
˚
rhus, 8000 A
˚
rhus C, Denmark
Received 28 July 1998
The 110 surface of an Ag crystal was investigated by reflectance anisotropy spectroscopy and angle-
resolved photoemission spectroscopy. A strong resonance in the optical spectra of the clean surface is assigned
to a surface-state transition at the Y
¯
point of the surface Brillouin zone. This resonance is absent on the
oxygen-covered surface. The accompanying photoemission spectra show the corresponding occupied surface
state on the clean surface as well as its disappearance with oxygen coverage. S0163-18299852040-1
Reflectance anisotropy spectroscopy RAS is an optical
method which allows the sensitive investigation of surface
optical properties of semiconductors
1–3
and metals.
4,5
RAS
measures the difference of the complex reflectivity along two
perpendicular axes in the surface. In the case of optically
isotropic bulk materials, any RAS signal must be related to
anisotropies induced by the surface.
6
Several mechanisms may contribute to the surface-
induced optical anisotropy. i Electronic transitions between
localized surface states constitute one of the interesting cases
allowing for direct surface state spectroscopy.
7,8
ii Transi-
tions involving near surface bulk states whose symmetry is
reduced by the presence of an anisotropically reconstructed
surface surface-induced bulk states may be another origin
for optical surface ansiotropies.
9,10
They give rise to features
in the spectra close to the bulk critical points. Finally, apart
from these single electron contributions iii collective free-
carrier oscillations at the surface surface plasmons may
also affect the optical spectra.
11
The only example for surface-state contributions to the
reflectance anisotropy on a metal surface so far is the
Cu110 surface. Here a sharp peak in the spectrum at an
energy of 2.1 eV was assigned to electronic transitions in-
volving surface states at the Y
¯
point of the surface Brillouin
zone.
8,12
However, since in Cu the transition energies of bulk
d electrons to the Fermi level are also located in this energy
range, the observed feature in the Cu110 spectra might as
well also contain contributions arising from surface modified
bulk states. Indeed, the RAS spectra indicate such a contri-
bution from near surface bulk states because part of the an-
isotropy still remains after exposure of the surface to O or
CO.
8
Thus the 2.1 eV structure in the Cu110 spectra does
not constitute a pure surface-state transition.
Silver, on the other hand, has a surface electronic struc-
ture similar to Cu. However, contributions from the d -band
transitions to the optical spectra are expected at much higher
energies above 4 eV Ref. 13 than the surface-state tran-
sition energies 1.7 eV Ref. 7. An anisotropic contribution
from surface-state transitions to the RAS spectra of the
Ag110 surface thus would be expected in the near infrared
region, energetically separated from the d bands.
Measurements of the optical anisotropy of Ag110 under
ambient
14
and UHV conditions
15
have been already reported.
In these experiments no contributions from surface electronic
states to the RAS spectra were detected. In our paper we
report angle-resolved photoemission and RAS measurements
on clean and oxygen-covered Ag110 crystals. On the clean
surface we find a peak in the RAS spectra whose appearance
correlates with the occupied surface state observed simulta-
neously with photoemission spectroscopy. We consider the
appearance of the surface state in the photoemission spectra
as the most sensitive check of the surface preparation. The
peak in RAS and photoemission is immediately quenched
upon oxygen adsorption. We assign this peak therefore to a
transition involving surface states at the Y
¯
point of the sur-
face Brillouin zone.
Experiments were performed in an ultrahigh-vacuum
UHV chamber base pressure 5 10
-11
Torr at the SX700
beamline of the A
˚
rhus electron storage ring ASTRID. The
vacuum chamber was equipped with standard facilities for
sample preparation and characterization. The Ag110
sample of 10 mm diam was aligned with Laue x-ray back-
scattering to 0.1° and mechanically polished to a final rough-
ness better than 0.03 m. It was mounted in UHV by two
tungsten wires. In situ cleaning of the surface was done using
cycles of argon ion sputtering 8 A/cm
2
, 500 eV, 10 min at
300 K and subsequent annealing to 670 K until no contami-
nations could be detected anymore by XPS. Thereafter the
LEED pattern showed the typical (1 1) structure with
sharp spots and a low background intensity. After surface
preparation, oxygen was dosed. The Ag110 face shows a
series of oxygen-induced ( n 1) reconstructions with
n =7,6,5,4,3,2.
16
The Ag110-(3 1)-O and (4 1)-O re-
constructions were prepared by backfilling the chamber with
5 10
-6
Torr O
2
for 290 s and 5 10
-7
Torr O
2
for 500 s at
room temperature, respectively.
The RAS spectrometer is a custom built system which is
based on the standard design by Aspnes.
6
A spectral range
from 1.2 to 6 eV was accessible using a tungsten lamp for
the low-energy region 1.2 to 3.2 eV and a short arc xenon
lamp for the higher photon energies 1.5 to 6 eV together
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PHYSICAL REVIEW B 15 OCTOBER 1998-II VOLUME 58, NUMBER 16
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