TDS and XPS studies of the adsorption of O2 on electrolytic silver

A692 different sets of analytical expressions. directly applicable to modulated molecular beam experiments, are presented. The first one gives the amplitude attenuation and the phase-shift of the first harmonic of the desorbed beam. The second one gives an effective relaxation time, obtained by Integration of the temporal waveform of the desorbed pulses. By suitable variation of the modulation frequency and the substrate temperature, one can separately measure the desorptlon energies from the homogeneous substrate and from the sinks, and the activation energy for surface diffusion. This model is apphed to modulated molecular beam measurements onto two different systems: new results on the adsorption of Cd on NaCl(100) and results of Lin and SomorJai [Surface Sci. 107 (1981) 573) on the chemisorption of NO on Pt(557) and Pt(l11). In both cases. the kinetic parameters of the elementary processes are obtatned.

Surface Science 163 (1985) 435-443 North-Holland, Amsterdam

43s

QUASIDYNAMICAL LEED STRUCTURE h-(100)1 x 5 SURFACE RECONSTRUCTION N. BICKEL and K. HEINZ

DETERMINATION

OF THE

Institut fiir Angewandte Physrk, Lehrstuhl f6rFestkiirperphyslk der Umversrtiir Erlangen - .Ytirnherg, Erwrn - Rommel- Str. I, D - 8520 Erlangen, Fed. Rep. of German? Received

20 May 1985; accepted

for publication

11 July 1985

A quasidynamical LEED analysis is performed for the Ir(100)l X 5 surface in the energy range from 50 to 440 eV. On the whole 9 geometries of the reconstructed 1x5 surface unit cell are calculated with 12 first to second layer relaxations for each. Comparison with experimental I(E) curves through the Pendry r-factor prefers clearly the hexagonal bridge model with a buckling of 0.5 A and first to second layer distance of 1.98 A in agreement with recent full dynamical treatments. The variation of the r-factor with different surface geometries is also similar to the full dynamical calculation and parallels the more exact curve only with an upward shift. So the quasidynamical method proves to be successful even for very complex surface structures saving much computer efforts. The total of 9 X 12 geometry variations needs only about 5 h CPU time on a Cyber 845.

Surface Science 163 (1985) 444-456 North-Holland. Amsterdam

444

TDS AND XI’S STUDIES OF THE ADSORPTION ELECTROLYTIC SILVER BAO Xinhe and DENG Jingfa Department

and DONG Institute Received

of Chemist?,

OF 0, ON

Fudan LJmversr!v, Shanghai, People? Rep. of Chrnu

Shuzhong of Modern Physics, Fudan Linwer.uty. Shanghai, 14 February

1985; accepted

for publication

PeopleYr Rep, of Chrno

9 July 1985

Thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS) have been employed to study the adsorption of oxygen on electrolytic silver. In the pressure range of l-lo-” Torr. several kinds of oxygen species have been observed on the surface: (A) Adsorbed

A693 molecular oxygen (Ed = 101 kJ/mol, Y = 4 X 10” s-l), (B) surface bound atomic oxygen (Ed = 134 kJ/mol, v = 4.7 X lOI s-‘) and (C) sub-surface atomic oxygen. Results of XPS show that the binding energies of Is orbitals of these species of oxygen on silver are 528.3, 529.7, 530.2 and 532.2 eV. Two bands centered at 2.8-3.0 and 9.0 eV below the Fermi level appear on the difference spectrum of the valence band which again indicates that both atomic and molecular oxygen exist on the surface of silver.

457

Surface Science 163 (1985) 457-477 North-Holland, Amsterdam

CHEMISORPTION OF BROMINE ON CLEAVED SILICON (111) SURFACES: AN X-RAY STANDING WAVE INTERFERENCE SPECTROMETRIC ANALYSIS *

B.N. DEV, V. ARISTOV **, N. HERTEL ***, T. THUNDAT W.M. GIBSON

and

Department of Physics, State University of New York at Albany, 1400 Washingion Avenue, Albany, New York 12222, USA Received

12 February

1985; accepted

for publication

1 July 1985

The coverage and location of bromine adsorbed from dilute methanol solution on cleaved silicon (111) surfaces have been determined by X-ray standing wave interference spectrometry. Two different locations of bromine atoms have been observed. One position, similar to that observed previously for chemically cleaned silicon (111) crystals, is directly over the first monolayer silicon atoms (atop position). The second position is much more loosely bound and may be in the open surface interstitial hollow over-the-fourth-layer atoms. In contrast to the chemically cleaned surface, where surface silicon atoms are bulk-like, we conclude that for a cleaved crystal, in the presence of bonded bromine atoms, the surface silicon atoms are relaxed outward by 0.13 f 0.06 A. It is suggested that this relaxation may open access to the interstitial site. Adsorption on chemically cleaned surfaces gave correlated coverage in the atop position of 0.3 monolayer (ML) or less. For cleaved surfaces, coverage of bromine in the strongly bound atop position never exceeded 0.24 ML which implies possible steric hindrance to bromine adsorption on adjacent sites. The weak bonding site was observed with coverage up to 1 ML and could be obtained even for samples with saturation oxygen coverage before bromine adsorption. The cases of coadsorption of bromine were analyzed with a general model which allows difference of population at individual sites.

478

Surface Science 163 (1985) 478-488 North-Holland, Amsterdam

ADSORPTION

OF Sn ON CLEAVED

A. TALEB-IBRAHIMI,

Si(ll1)

SURFACES

C.A. SEBENNE, F. PROIX and P. MAIGNE

Laboratoire de Physique des Solides gssocit? au Centre National de la Recherche Scientifique Universite Pierre et Marie Curie, F- 75230 Paris Gdex 05, France Received

28 March

1985; accepted

for publication

No. 154,

6 July 1985

The initial steps of ultrahigh vacuum deposition of Sn on clean cleaved Si(ll1) surfaces kept at room temperature were studied using low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS). AES shows the formation of a