Vibrational relaxation in infrared excited SF6⋅Arn+ cluster ions

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Abstract
An attempt is made to characterize the increase in internal temperature that should accompany the partitioning of a single quantum of vibrational energy within a small SF6⋅Arn+ cluster ion. For each value of n, the kinetic energy release associated with unimolecular (metastable) decay is used to establish an initial temperature for the cluster ion; ∼950 cm−1 of vibrational energy is then deposited into the ν3 vibrational mode of the SF6 moiety (using a CO2 laser). This step promotes additional dissociation which is accompanied by an increase in kinetic energy. From a model due to Klots [J. Chem. Phys. 58, 5364 (1973)] photofragment kinetic energies are predicted on the assumption that energy from the photon is partitioned statistically and leads 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 ν3 mode leads to incomplete energy randomization. An improved description of the energy relaxation process is provided on the assumption that SF6 undergoes partial vibrational relaxation to either the ν2 or ν4 mode. The energy difference (∼300 cm−1) is then randomized throughout each cluster ion, and is reflected in the magnitude of the measured kinetic energy release accompanying the loss of a single argon atom. The estimated time scale for this process is an order of magnitude faster than the experimentally measured time for the total relaxation of SF6 (ν3=1) in an argon matrix.