SOLVENT-INDUCED NONADIABATIC TRANSITIONS IN IODINE - AN ULTRAFAST PUMP-PROBE COMPUTATIONAL STUDY

The solvent-induced electronic predissociation [B-->a1(g)((3) Pi)] fol lowing an ultrafast X-->B transition in molecular iodine is studied us ing a classical ensemble representation of Heisenberg's equations of m otion, An N electronic state quantum mechanical Hamiltonian is used to derive (coupled) equations of motion for the population (and the cohe rence) of the different electronic states as well as classicallike cou pled equations for the nuclear dynamics (of both the molecule and the solvent) on each electronic state, The ultrafast excitation of the int ermediate B state creates a coherent vibrational motion in this bound state. The localized nature of the solvent-induced B-a1(g)((3) Pi) cou pling results in a steplike depletion of the excited B state populatio n and hence in a bulletlike appearance of population on the dissociati ve a1(g)((3) Pi) state twice per vibrational period, The depletion of the B state population and the appearance of products on the a1(g)((3) Pi) state are discussed as a function of solvent density and polariza bility. The magnitude of the nonadiabatic B-a1(g)((3) Pi) coupling dep ends both on the molecule-quencher separation and on the quencher's po larizability. It is found that at all reduced densities the small Ar a tom is the most effective quencher (when compared to either Kr and/or Xe). We attribute this unexpected trend to the local density of atoms around the solute molecule. For all the rare gas solvents the local de nsity around the iodine molecule does not quite scale with the global one and there is an observed tendency for the solvent to cluster aroun d the solute in a T-shaped configuration. It is this close-packed conf iguration that compensates for the smaller polarizability of the Ar at om and hence provides for a more effective quenching. These arguments are used to explain the experimental results which demonstrate that fo r a series of homologous alkanes the extent of predissociation scales with the length of the molecular chain although the global polarizabil ity density remains roughly constant. (C) 1996 American Institute of P hysics.