ROLE OF VIBRATIONAL ENERGY MIGRATION UPON V-]V TRANSFER IN MATRIX-ISOLATED CO

Vibrational energy transfer rates for several exothermic and endothermic processes have been measured in isotopically-enriched matrix-isolated CO between 9 and 25 K using the laser excited vibrational fluorescence technique. The experimental data are interpreted in the light of the recent theories for microscopic and macroscopic energy transfer. The dependence of the macroscopic rates upon the donor concentrations shows that V - V processes are limited by the quantum hopping motion of the donor excitation. The hopping model square rood dependence of the macroscopic rate constants upon the microscopic probabilities provides a satisfactory explanation for the observed weak dependence of the rate constants for non-resonant transfer on the number of phonons involved. A fitting of the rate constants calculated from the hopping theory to the experimental values leads to microscopic probabilities for one phonon assisted transfer ≈ 300 times smaller than for resonance transfer. Such a factor is accounted by mechanical anharmonicity provided the experimental vibrational shift originated entirely from the dynamical contribution, which may not be the case. Electrical anharmonicity, which has been recently shown to account for non-resonant energy transfer in solid N2 offers an interesting alternative explanation for the magnitude of the phonon assisted transfer rates. Finally a study of the time evolution of the 12C16O vibrational populations shows that in addition to the migration of the v = 1 donor excitation the hopping motion of the acceptor energy is limiting the energy transfer rate for the first two steps of the up the ladder excitation. This effect tends to disappear for the upper steps as the acceptor excitation becomes locally trapped. © 1979.