role of vibrational energy migration upon v → v transfer in matrix isolated co》.pdf

Chemical Physics 41 (1979) 329-343
D North-Holland Publishing Company
ROLE OF VIBRATIONAL ENERGY MIGRATION UPON V + V TRANSFER
IN MATRIX ISOLATED CO
Henri DUBOST and Renee CHARNEAU
Laboratoire de Photophysique Mol&, CIniversitP Paris&d.
91405Orsay, France
Received 28 December 1978
Vibrational energy transfer rates for several exothcrmic 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 concentration shows that V-V processes are limited by the quantum hopping motion
of the donor excitation. The hopping model square root 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 expe-
rimental values leads to microscopic probabilities for one phonon assisted transfer = 300 times smaller then for -
fer. Such a factor is accounted by mechanical anharmonicity provided the experimental vibrational shift originates 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 Na offers an interesting alternative explanation for the magnitude of the
phonon assisted transfer rates. Finally a study of the time evolution of the *2C’60 vibrational populations shows that in ad-
dition to the migration of the u = 1 donor excitati