Volume 136, number 1 CHEMICAL PHYSICS LETTER8 24 April 1987 UV LASER-STIMULATED RESONANT DESORPTION FROM METAL SURFACES: NO/Ni(lOO) D. WEIDE, P. ANDRESEN Max-Planck Institut fiir Striimungsforschung, Bunsenstrasse 10, D-3400 Giittingen I, Federal Republicof Germany and H.-J. FREUND Institutfiir Physikalische und Theoretische Chemie, Vniversitiit Erlangen-Niirnberg, Egerlandstrasse 3, D-8520 Erlangen, Federal Republicof Germany Received 5 January 1987; in final form 18 February 1987 NO chemisorbed on a Ni( 100) surface at 170 K, has been desorbed with UV laser light ( ArF, 193 nm = 6.42 eV) in the energy range of the most intense electronic absorption of the NO/Ni( 100) system. The rotational and vibrational state distribution of the desorbing molecules is probed via laser-induced fluorescence. The desorbing molecules are vibrationally and rotationally excited. The rotational distribution is markedly non-Boltzmann. The velocity distributions are narrow and very different for various rotational states. They cannot be fitted to a Maxwell distribution. We believe that these results provide evidence for the resonant nature of the light-absorption-desorption process for NO on a Ni( 100) surface. 1. Introduction Light-induced gas-surface interactions have received considerable attention in recent years [ l-31. For UV photon-induced desorption studies the experiments and results have been critically reviewed and the general conclusion was that for prototype gas-metal systems such as CO on nickel or tungsten, the observed photodesorption was mainly thermal in nature [ 4,5]. A resonant effect, if present at all, was assumed to have a cross section of less than 1O-22 cm2, which is equivalent to less than low8 molecules desorbed per photon [ 11. Only very few exceptions were noted where quantum yields above 10-s have been measured at sufficiently high excitation energy, i.e. E,,, = 5 eV [ 61. These higher yields were ascribed to resonance effects and have been interpreted within a model suggested by Menzel and Gomer [ 71 and Redhead [ 81 (the MGR model), originally pro- posed in the context of electron-stimulated desorp- tion. Specifically, the MGR model consists of a two- step process: Desorption is initiated by an electronic excitation from the ground state of the adsor- bate-substrate complex to a repulsive excited state of the system. As a consequence, the adsorbed parti- cle begins to move away from the surface, and, if deexcitation does not occur sufficiently rapidly, desorbs as a neutral species or an ion depending on the nature of the excitation process. In the electron-stimulated desorption studies of Bums and co-workers [ 91, quantum state distribu- tions measured via laser-induced fluorescence (LIF) indicated that NO desorbs from polycrystalline Ni by a resonant process which can be interpreted in terms of the MGR model. However, there have only been a few investigations in which lasers were used to induce the desorption process [ l-31. In most of these studies the desorbing neutrals were analyzed with a mass spectrometer, in some cases also by LIF. Laser detection has the pronounced advantage that time-of-flight distributions can be determined state selectively. In recent years a series of studies have been carried out on the collisional excitation of NO on various surfaces via laser-induced fluorescence 106 0 009-2614/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)