ULTRAFAST STUDIES OF CHEMICAL-REACTIONS IN LIQUIDS - VALIDITY OF GAS-PHASE VIBRATIONAL-RELAXATION MODELS AND DENSITY DEPENDENCE OF BOUND ELECTRONIC STATE LIFETIMES

The dynamics of the I2 geminate recombination reaction in liquid Xe are monitored with transient picosecond absorption spectroscopy as a function of solvent density and temperature. Specifically, the I2 X state vibrational relaxation and the nonradiative electronic curve crossing from the deeply bound A' and A states to the X state are measured over a Xe density range which spans the entire liquid phase region of the phase diagram. The density dependence of the X state vibrational relaxation is shown to be consistent with the isolated binary collision (IBC) model of vibrational relaxation through a procedure which, unlike previous studies of IBC theory, requires no estimate of the collision frequency. This verification procedure is performed by ascertaining whether vibrational energy versus time plots at different solvent density can be overlapped by a linear scaling of the time axis. The consistency with IBC theory demonstrated in this system is surprising because of the low I2 vibrational frequency (170-214 cm-1) and the large amplitude oscillations involved in the relaxation process. Contrary to theoretical predictions of the A'/A state lifetime based upon Langevin modeling in the high density Kramers limit, the A'/A state lifetime shows very little dependence on solvent density although both viscosity and activation barrier increase substantially with solvent density. This finding suggests that a non-Markovian description of the solvent frictional effects and a model of nonadiabatic curve crossing must be included in order to correctly calculate the excited state lifetime.