EXPERIMENTAL AND THEORETICAL-STUDY OF INHOMOGENEOUS ELECTRON-TRANSFERIN BETAINE - COMPARISONS OF MEASURED AND PREDICTED SPECTRAL DYNAMICS

New tunable pump and probe data are reported for the S1 --> S0 reverse electron transfer in betaine in solution. The observed dynamics are a dramatic function of probe wavelength due to energy deposition into s olute and solvent molecular degrees of freedom from the electron trans fer. The data indicate that in nonpolar solvents, this energy is prima rily deposited into intramolecular modes. In polar solvents, most of t he energy is deposited into intramolecular modes, but some is specific ally deposited into polar collective motions of the solvent, i.e. the solvent coordinate. Modem theories of electron transfer that include d iffusive motion along the solvent coordinate as a driving mechanism fo r electron transfer predict this specific energy deposition into the s olvent coordinate. In this paper, we extend a model by Walker et al. t o predict the spectral dynamics due to solvent coordinate heating caus ed by the electron transfer and compare with experimental results. We show that the diffusive electron transfer mechanism creates a displace d distribution along the solvent coordinate that is characterized by a n absorption spectrum that is red-shifted from the equilibrium ground- state distribution, in agreement with experimental observations. The m odel is not in quantitative agreement with experiment, however, becaus e it neglects the contributions of vibrational heating of the intramol ecular modes to the spectral dynamics. This type of vibrational heatin g is evident in the observed dynamics in nonpolar solvents.