Solventinduced nonadiabatic transitions in iodine: An ultrafast pump–probe computational study

The solvent-induced electronic predissociation ͓B→a1 g ͑ 3 ⌸͔͒ following an ultrafast X→B transition in molecular iodine is studied using a classical ensemble representation of Heisenberg's equations of motion. An N electronic state quantum mechanical Hamiltonian is used to derive ͑coupled͒ equations of motion for the population ͑and the coherence͒ of the different electronic states as well as classicallike coupled equations for the nuclear dynamics ͑of both the molecule and the solvent͒ on each electronic state. The ultrafast excitation of the intermediate B state creates a coherent vibrational motion in this bound state. The localized nature of the solvent-induced B – a1 g ͑ 3 ⌸͒ coupling results in a steplike depletion of the excited B state population and hence in a bulletlike appearance of population on the dissociative a1 g ͑ 3 ⌸͒ state twice per vibrational period. The depletion of the B state population and the appearance of products on the a1 g ͑ 3 ⌸͒ state are discussed as a function of solvent density and polarizability. The magnitude of the nonadiabatic B – a1 g ͑ 3 ⌸͒ coupling depends both on the molecule–quencher separation and on the quencher's polarizability. It is found that at all reduced densities the small Ar atom is the most effective quencher ͑when compared to either Kr and/or Xe͒. We attribute this unexpected trend to the local density of atoms around the solute molecule. For all the rare gas solvents the local density around the iodine molecule does not quite scale with the global one and there is an observed tendency for the solvent to cluster around the solute in a T-shaped configuration. It is this close-packed configuration that compensates for the smaller polarizability of the Ar atom and hence provides for a more effective quenching. These arguments are used to explain the experimental results which demonstrate that for a series of homologous alkanes the extent of predissociation scales with the length of the molecular chain although the global polarizability density remains roughly constant.