Photoinduced dissociation of water adsorbed on a Ag cluster

Theoretical electronic structure calculations are reported for the photoinduced dissociation of water adsorbed on a 31-atom silver cluster, Ag31 to produce H2 and adsorbed O. Both ground and excited electronic state processes are considered. The excited electronic state of interest is formed by photoemission of an electron from the Ag cluster and transient attachment of the electron to the adsorbed water molecule. A very large energy barrier for H2 formation (3.51 eV) is found for the ground state process while the barrier in the excited state is much smaller (0.47 eV). In the excited state, partial occupancy of an OH antibonding orbital facilitates OH stretch and the electron hole in the metal cluster stabilizes OH and O adsorbates. The excited state pathway for dissociation of water begins with the formation of an excited electronic state at 3.49 eV. Stretch of the OH bond up to 3.44 a.u. (1.82 Å) occurs with little change in energy. In this region of OH stretch the molecule must return to the ground state potential energy surface to dissociate fully and to form H2. Geometry optimizations are carried out using a simplex algorithm which allows the total energy to be calculated using configuration interaction theory.