Laser induced desorption of NO from NiO(100): Characterization of potential energy surfaces of excited states

In order to interpret experimental results such as velocity flux distributions and rotational/vibrational populations of the state resolved UV-laser induced desorption of NO from NiO(100) ab initio calculations at the configuration interaction (CI) and complete active space self consistent field (CASSCF) levels have been performed for the electronic ground state and those excited states which are important for the desorption process. The NO/NiO(100) system was described by a NiO58--cluster embedded in a Madelung field of point charges with NO adsorbed in the on-top position on the central Ni2+ ion. Two-dimensional potential energy surfaces for several electronic states have been calculated as a function of the N-Ni distance and the tilt angle of NO towards the surface normal. The excited states involved in the desorption process are charge transfer states in which one electron is transferred from the oxygen 2p-shell into the NO 2 pi-orbitals. The dependence of the potential energy surfaces on the N-Ni distance is dominated by a strong Coulomb attraction between the NO- ion formed as an intermediate and the hole created within the cluster. The angular dependence of the potentials favours an upright adsorption geometry if NO- is approaching the surface. This offers an explanation of the strong coupling between translation and rotation, which has been observed experimentally for the system NO/NiO(100), as well as the absence of such a coupling in the system NO/NiO(111).