VIBRATIONAL-RELAXATION IN INFRARED EXCITED SF6.AR-N(+) CLUSTER IONS

An attempt is made to characterize the increase in internal temperatur e that should accompany the partitioning of a single quantum of vibrat ional energy within a small SF6.Ar-n(+) cluster ion. For each value of n, the kinetic energy release associated with unimolecular (metastabl e) decay is used to establish an initial temperature for the cluster i on; similar to 950 cm(-1) of vibrational energy is then deposited into the nu(3) vibrational mode of the SF6 moiety (using a CO2 laser). Thi s step promotes additional dissociation which is accompanied by an inc rease in kinetic energy. From a model due to Klots [J. Chem. Phys. 58, 5364 (1973)] photofragment kinetic energies are predicted on the assu mption that energy from the photon is partitioned statistically and le ads to an overall increase in the temperature of each ion. Comparisons of experimental and calculated results clearly show that the infrared photoexcitation of SF6 in the nu(3) mode leads to incomplete energy r andomization. An improved description of the energy relaxation process is provided on the assumption that SF6 undergoes partial vibrational relaxation to either the nu(2) or nu(4) mode. The energy difference (s imilar to 300 cm(-1)) is then randomized throughout each cluster ion, and is reflected in the magnitude of the measured kinetic energy relea se accompanying the loss of a single argon atom. The estimated time sc ale for this process is an order of magnitude faster than the experime ntally measured time for the total relaxation of SF6 (nu(3) = 1) in an argon matrix. (C) 1998 American Institute of Physics.