SPECTROSCOPY, POLARIZATION AND NONADIABATIC DYNAMICS OF ELECTRONICALLY EXCITED BA(AR)(N) CLUSTERS - THEORY AND EXPERIMENT

Molecular Dynamics simulations using a surface-hopping method for tran sitions between different electronic states are employed to study the dynamics following photoexcitation of the Ba(Ar)(125) cluster. The res ults are used to interpret spectroscopic experiments on large, size-di stributed Ba(Ar)(n) clusters. The dynamics of the coupled electronic-n uclear motions in the cluster involves transitions between three poten tial energy surfaces, corresponding to the nearly-degenerate p-states of the excited Ba atom. Ejection of excited Ba atoms, adsorbed on the surface of the cluster, can take place. The focus in comparing theory and experiment is on the emission spectrum from the excited clusters, on the polarization of this radiation, and on the polarization of ligh t emitted by excited Ba atoms ejected from the cluster. Based on the g ood agreement found between theory and experiment, a comprehensive pic ture of the excited state dynamics is given. It is found that upon exc itation, energy is rapidly redistributed in the cluster and no direct ejection of Ba occurs. Electronic relaxation to the lowest P-state occ urs, and the latter dominates the cluster emission spectrum and polari zation. The electronic state relaxation is mostly complete within t le ss than or similar to 10 ps. Ejection of Ba atoms occurs as a rare and delayed event when a dynamical fluctuation creates a ''hot spot'' at the Ba site, with a non-adiabatic excitation to the highest electronic level. The results show the feasibility of near-quantitative understa nding of non-adiabatic processes in large clusters. (C) 1996 American Institute of Physics.