VIBRATIONAL-ENERGY RELAXATION OF HOD IN LIQUID D2O

Molecular Dynamics simulation is used to study the vibrational relaxat ion of the first excited state of the O-H stretch for HOD dissolved in D2O. The technique applied is based on a Landau-Teller type formula, in which the solvent contribution is computed classically, while the q uantum nature of the solute enters through the transition moments of t he molecular normal modes. The experimental result for the relaxation time (approximate to 8 ps) is accounted for, and the pathway to the gr ound state is determined. The relaxation proceeds through a sequence o f intramolecular transitions initially facilitated by the solute;inter nal anharmonicities. In particular, the anharmonicity allows an initia l and rate-determining transfer to the first overtone of the HOD bend; a corresponding harmonic force field calculation in which this step i s precluded yields a relaxation time that is three orders of magnitude larger. The excess energy is removed by the bath modes, which include rotations and translations of all molecules, including the solute. Re laxation by Coriolis coupling plays a minor but non-negligible role, w hile the centrifugal coupling contribution to the relaxation is neglig ible. (C) 1996 American Institute of Physics.