Femtosecond wave packet and chemical reaction dynamics of iodine in solution: Tunable probe study of motion along the reaction coordinate

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Abstract
One- and two-color time-domain probing of the resonant dichroic response of iodine in n-hexane following femtosecond B-X excitation at 580 nm is described. The detected signals contain both ground and excited state vibrational coherence contributions to the third-order polarization. The dichroic response can be separated into positive and negative amplitude contributions: B-X absorption and stimulated emission are positive but absorption from the B-state can yield either positive or negative signals depending on the direction of the transition moment. Wave packet motion on both the ground and excited states of iodine is studied with a frequency tunable femtosecond probe. It is shown that the positive signals can be interpreted as B-X dichroic response using the classical Franck principle. The classical Franck principle also provides information about the potential probed in absorption from the B state. From the probe wavelength dependent delay in the signal appearance, it is concluded that the absorptive signal for blue probe wavelengths arises from a repulsive state reached by solvent-induced predissociation of the B state. Dephasing of B state vibrational coherence results from this solvent-induced predissociation of iodine. We discuss the evolving reaction in terms of possible dissociative potential energy curves a1g(3Π) and a′0g+(3Σ−). The time evolution of the bluest probe dichroism signals is representative of continuing atom separation; the experiments have not yet probed large enough internuclear separations to evidence a buildup of dissociated product or momentum reversal, i.e., caging.