Vibrational relaxation rates for H on a Si1 0 0):2  1) surface: a two-dimensional model I. Andrianov a,b , P. Saalfrank a,b, * a Department of Chemistry and Centre for Theoretical and Computational Chemistry CTCC), Christopher Ingold Laboratories, University College London, 20 Gordon Street, London WC1H OAJ, UK b Universit at Regensburg, Institut f ur Physikalische und Theoretische Chemie, Universit atsstrasse 31, D-93053 Regensburg, Germany Received 24 September 2001 Abstract The results of calculations based on perturbation theory of vibrational relaxation rates due to coupling to substrate phonons for hydrogen atoms adsorbed on a Si1 0 0):2  1) surface are presented. For this purpose a two-dimensional model is adopted in which both the H±Si stretching and bending motions are included. It is shown that within this model the multiphonon emission and absorption processes play a negligible role. The calculated lifetimes agree well with available experimental data. Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction In this Letter, we study the vibrational relax- ation of H atoms adsorbed on a Si1 0 0)-2  1) surface, which constitutes the core for explana- tions of scanning tunneling microscope STM) [1,2] and laser-induced [3] desorption experiments performed on this system. In more general terms hydrogen-covered Si surfaces are of interest for the passivation of semiconductor devices and as a microlab for fundamental theories and experi- ments on phase and energy transfer from an `ad- sorbate' to a `substrate' [4]. Experimentally, Guyot-Sionnest et al. [5] ®nd for the Si±H stretching mode in the Si1 0 0)- 2  1):H system at T  300 K an energy relax- ation lifetime s vib  T 1  of the ®rst vibrational state of s vib  1:2 ns [5]. At lower temperatures the damping rate depends on the quality of the sample; measurements of s vib at T  100 K with newly prepared samples yield s vib > 6 ns [5]. The exact dependence of the Si±H stretch lifetime on tem- perature is unknown. However, Avouris and co- workers [6] suggest on the basis of a perturbative multiphonon decay model, that s vib decreases ex- ponentially with increasing T. Isotopic substitution of H with D leads to somewhat shorter vibrational lifetimes [5]. The Si1 1 1)-1  1):H system behaves similar to the Si1 0 0)-2  1):H surface [4,5,7]. It has been suggested that for H-covered Si surfaces the major contribution to the decay of adsorbate vibrations comes from the coupling of 21 December 2001 Chemical Physics Letters 350 2001) 191±197 www.elsevier.com/locate/cplett * Corresponding author. Fax: +49-941-943-4719. E-mail address: peter.saalfrank@chemie.uni-regensburg.de P. Saalfrank). 0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0009-261401)01304-5