Dynamics of liquid state chemical reactions: Photodissociation dynamics and geminate recombination of molecular iodine in liquid solution

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Abstract
The dynamics of photodissociation, cage breakout, and subsequent recombination of molecular iodine in model Lennard‐Jones solvents designed to simulate liquid carbon tetrachloride and ethane is studied via classical stochastic trajectory simulations based on the molecular timescale generalized Langevin equation (MTGLE) of motion for liquid state chemical reaction dynamics [S. A. Adelman, J. Chem. Phys. 73, 3145 (1980)]. Also presented for comparison purposes are parallel trajectory studies based on a matrix Langevin equation characterized by friction coefficients which depend on the I2 internuclear separation R. These studies show that dynamical solvent effects arising from molecular timescale correlated solute/solvation shell motions play a basic role in the I2 photodynamics. The nature and magnitude of these dynamical solvent effects are governed by the MTGLE force constants, especially, the basic force constants ω2e0(R) and ω2c1(R). Dynamical solvent effects in particular: (i) can lead to the production of quasibound pairs of iodine atoms even if the static or mean I2 effective interaction is repulsive. The fundamental vibrational frequency of the quasibound pairs is governed by ω2e0(R) which determines the modification of the static effective interaction due to instantaneous elastic cage restoring forces exerted on the I2 molecule; (ii) largely determine the efficiency of cage breakout (and hence the nongeminate yield) which occurs during the initial stages of the photolysis process. This breakout efficiency depends on a competition between ω2e0(R) [large ω2e0(R) retards breakout] and ω2c1(R) [large ω2e0(R)promotes breakout]; (iii) determine, via the strength of solute‐shell coupling as measured by ω2c1(R), the rate and qualitative character of the energy relaxation of highly vibrationally excited nascent I2 product. Moreover, the interplay between the efficiencies of the initial cage breakout step and the final vibrational stabilization step of the photolysis process determines the observable picosecond timescale transient electronic absorption spectrum. Because of this interplay, systems with very different underlying dynamics can exhibit similar transient absorption spectra (on the timescale of typical experiments). This is illustrated by the similarities in the absorption spectra of I2 in liquid CCl4 (high geminate yield and relatively inefficient vibrational stabilization) and I2 in liquid C2H6 (low geminate yield and relatively efficient vibrational stabilization). Finally, simplified methods to handle the complicating effects of nonlinear solvent response to very fast initial solute motion and of nonadiabatic motion involving several I2 electronic states are described.