Ž . Chemical Physics 228 1998 185–203 The role of the initial population of molecular vibrations in surface photochemistry S. Thiel, T. Kluner ) , M. Wilde, K. Al-Shamery, H.-J. Freund ¨ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4–6, D-14195 Berlin, Germany Received 25 August 1997; accepted 17 October 1997 Abstract In a one-dimensional wavepacket study the role of thermal population of molecular vibrations within surface photochem- Ž . istry is studied using a model potential adopted to the system NOrCr O 0001 . Simulations are made concerning the 2 3 temperature dependence of the efficiency for UV-laser induced desorption and of velocity distributions. The course of these observables as a function of temperature is strongly dependent on the lifetime of the excited state. The results are compared Ž . with experimental results on the temperature dependence of the non-thermal desorption of NO from the Cr O 0001 surface 2 3 after excitation at 6.4 eV. The experimentally observed increase of desorption cross-sections by about a factor of 2 when changing the surface temperature between 100 and 300 K is simulated when assuming an average resonance lifetime on the order of 10 fs. The experimentally found increase of translational energy with increasing surface temperature by ;100 mrs is also consistent with theoretical results. q 1998 Elsevier Science B.V. Keywords: Surface photochemistry; Wavepacket dynamics; Desorption cross-section; Velocity distributions 1. Introduction In the early days of laser-induced surface photo- chemistry one was interested in populating individ- ual adsorbate vibrations using infrared lasers to in- wx duce surface reactions 1 . The idea based on a concept proposed by Lethokov was to prepare a favourable precursor state for the reaction, via local excitation of the bond to be broken with the aim to control the route of elementary chemical reactions wx 2 . One application was suggested to be isotope separation. At this time only nanosecond lasers with a limited wavelength tunability were available. Nor- mally, more than one laser photon was needed to ) Corresponding author. excite the system above the dissociation threshold. It turned out that mainly surface heating was observed Ž as the energy redistribution processes IVR s intra- . molecular vibrational redistribution leading to phonon excitation were fast on the time scale of the laser pulses used. However, in the field of gas-phase photochemistry vibrationally mediated photochem- wx istry in small molecules has been demonstrated 3. The mild laser-induced thermal desorption using in- frared laser light found its application in analytical wx chemistry such as laser mass spectroscopy 4 . The surface molecular dynamicists, on the other hand, turned to electronically induced surface processes. Ž . DIET desorption induced by electronic transition was one of the major research activities. Particularly measurements on final quantum state distributions of 0301-0104r98r19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII S0301-0104 97 00326-1