Nature of Simple Photodissociation Reactions in Liquids on Ultrafast Timescales.

This review article, to appear in Annual Reviews of Physical Chemistry (1988), describes recent advances in understanding the ultrafast dynamics of diatomic photodissociation reactions in liquids. Such reactions form a basis for conclusions about reactant-solvent energy exchange, and for the nature of solvent caging effects in bimolecular reactions. Emphasis is on the prototype iodine photodissociation and recombination process, which has been investigated, by the authors and others, using picosecond spectroscopy and through extensive reaction dynamics simulations. Following excitation, the iodine molecule dissociates or predissociates in 1-2 ps. The atoms then escape through the solvent, or recombine very rapidly (2-10 ps). Subsequent electronic and vibrational relaxation of the excited, recombined molecule is slower, taking 50 ps to 3 ns, depending on the solvent.