SOLVATION DYNAMICS OF AN EXCESS ELECTRON IN METHANOL AND WATER

Molecular dynamics simulation has been used to explore the nature of s olvation dynamics for an excess electron in methanol and in water. We perform the analysis within the linear response theory and show that n onlinear corrections are small in bath cases. The response function ch aracterizing solvent relaxation after electron photoexcitation and tha t following the subsequent nonradiative transition are modeled and fou nd to behave very similarly in methanol, in contrast to water. For met hanol, each is comprised of an extremely short Gaussian inertial compo nent of small amplitude and a bi-exponential diffusive decay. A relati vely fast similar to 1 ps exponential accounts for approximately half of the solvent relaxation and is followed by a slower similar to 7 ps relaxation of comparable magnitude, a solvation response that is rathe r similar to that reported previously for relatively large molecules i n methanol. Spectral densities of energy gap fluctuations for the equi librium ground and excited state trajectories show that translational motion dominates solvation. Relaxational processes in methanol have be en compared with the results for water. In contrast to methanol, libra tional motions of solvent molecules significantly influence aqueous so lvation dynamics, especially following excited state decay. This diffe rence is reflected in the relaxational processes, which are an order o f magnitude slower in methanol than in water. (C) 1998 American Instit ute of Physics. [S0021-9606(98)50639-3].