In order to interpret the experimental results of the state resolved UV-laser-induced desorption of NO from NiO͑100͒ ͑rotational and vibrational populations, velocity distributions of the desorbing NO molecules, etc.͒, we have performed ab initio complete active space self-consistent field ͑CASSCF͒ and configuration interaction ͑CI͒ calculations for the interaction potential between NO and the NiO͑100͒ surface in the electronic ground state and for those excited states which are involved in the desorption process. The NiO͑100͒-NO distance and the tilt angle between the NO axis and the surface normal have been varied. A cluster model containing a NiO 5 8Ϫ-cluster embedded in a Madelung potential has been used for representing the NiO͑100͒ surface. The excited states which are important for the desorption process, are charge transfer states of the substrate-adsorbate system, in which one electron is transferred from the surface into the NO-2-orbital. The potential curves of these excited charge transfer states show deep minima ͑4 eV-5 eV͒ at surface/NO distances which are smaller than that in the ground state. The angular dependence of these potentials behaves similar as in the ground state. A semiempirical correction to the calculated excitation energies has been added which makes use of the bulk polarization of NiO. With this correction the charge transfer states are considerably stabilized. The lowest excitation energy amounts to about 4 eV which is in reasonable agreement with the onset of the laser desorption observed experimentally at about 3.5 eV. The density of the NO Ϫ-like states is rather high, so that probably several excited states are involved in the desorption process. The potential energy curves for all of these states are quite similar, but the transitions from the ground state into different excited charge transfer states show strongly differing oscillator strengths, which are also strongly dependent on the surface/NO distance. This fact is important for the dynamics of the deexcitation process in the sense of a selection criterion for the states involved. The magnitude of the oscillator strengths is large in comparison with the excitation of NO in the gas phase, which might be an indication for the possibility of optical excitation processes. One dimensional wave packet calculations on two potential energy curves using fixed lifetimes for the excited state in each calculation have been performed and enable us to estimate the mean lifetime of the excited state to be 15 fsрр25 fs. This implies that the dynamics of the system is dominated by the attractive part of the excited state potential.
Quantum state resolved velocity distributions of NO desorbing from single crystal metal and oxide surfaces after a nontherma1 excitation process with TJVphotons have been studied by several groups in the past. In order to achieve a "complete" experiment it is necessary to determine in addition the spatial distribution of the desorbing particles. We report on results of the determination of angular distributions of desorbing NO via a new experimental setup. Two systems have been studied in detail, i.e. NO on NiO(OO) and NO on NiO(11 1). A model proposed before is employed to explain the experimental results, i.e. the observation of bimodal velocity flux distributions. The bimodal signal is consistent with the existence of two desorption channels which are predicted to exhibit different angular distributions. This prediction is verified with the new experimental setup. The comparison of the two crystallographic planes of NiO allows us to address the problem of the influence of the magnetic properties of the substrate onto the population of different spin states of desorbing NO molecules. Finally we shall report on results gained with a CO detection system based on a (1+1 ') REMPI process employing VUV photons. Here the system CO/Cr2O3(1 1 1) is studied. With this setup the resolution of rotational states in the desorbing particles is easy to achieve in contrast to the widely used (2+1) REMPI process of the same transition. INTRODUCTIONWhile state resolved studies in photochemistry of gas phase systems are common standard by now', such studies on well characterized surfaces are still in an early stageZ3. One of the simplest possible photochemical processes at the surface involving bond breaking is the disrupture of the molecule surface bond followed by desorption of the molecule4. Due to the rather large photodesorption cross sections for molecules desorbing from oxide surfaces5'6, these system are particularly well suited to undertake state 182 ISPIE Vol. 2125 O-8194-1418-2/94/$6.OO Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/20/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx
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