Predissociation dynamics of I-2(B) in liquid CCl4 observed through femtosecond pump-probe measurements: Electronic caging through solvent symmetry

Direct observations of the solvent induced electronic predissociation of I-2(B) in liquid CCl4 are made using femtosecond pump-probe measurements in which fluorescence from spin-orbit excited I*I* pairs, bound by the solvent cage, is used as detection. Data is reported for initial preparations ranging from the B state potential minimum, at 640 nm, to above the dissociation limit, at 490 nm. Analysis is provided through classical simulations, to highlight the role of solvent structure on: recombination, vibrational relaxation, and decay of coherence. The data is consistent with an anisotropic I-2(X)-CCl4 potential which, in the first solvent shell, leads to an angular distribution peaked along the molecular axis. The roles of solvent structure and dynamics on electronic predissociation are analyzed. The data in liquid CCl4 can be understood in terms of a curve crossing near upsilon = 0, at 3.05 Angstrom<R-c<3.8 Angstrom, and the final surface can be narrowed down to 2(g) or a(1(g)). This nonadiabatic u-->g transition is driven by static and dynamic asymmetry in the solvent structure. The role of solvent structure is demonstrated by contrasting the liquid phase predissociation probabilities with those observed in solid Kr. Despite the twofold increase in density, predissociation probabilities in the solid state are an order of magnitude smaller, due mainly to the high symmetry of the solvent cage. The role of solvent dynamics is evidenced in the energy dependent measurements. Independent of the kinetic energy content in I-2, electronic predissociation in liquid CCl4 proceeds with a time constant equal to the molecular vibrational period. A modified Landau-Zener model, in which the effective electronic coupling is taken to be a linear function of vibrational amplitude fits the data, and suggests that cage distortions driven by the molecule enhance its predissociation probability. A nearly quantitative reproduction of the observations is possible when using the recently reported off-diagonal DIM surface that couples the B(O-u(+)) and a(1g) states [Batista and Coker, J. Chem. Phys. 105, 4033 (1996)]. (C) 1997 American Institute of Physics. [S0021-9606(97)00544-8].