Energy partitioning following the IR photofragmentation of SF6·(NO)n+ cluster ions

Abstract

Using two separate IR lasers, we have attempted to characterise the increase in internal temperature that should accompany the partitioning of a single quantum of vibrational energy within small SF₆·(NO)_n⁺ cluster ions. An initial temperature is established by measuring a kinetic-energy release associated with the unimolecular (metastable) decay of each ion. Using a CO₂ laser, ca. 950 cm^–1 of the vibrational energy is deposited into the SF₆ moiety and with a CO laser, the (NO)_n⁺ moiety is excited with ca. 1700 cm^–1 of vibrational energy. In both cases, the ions are observed to photodissociate and the corresponding kinetic-energy releases are measured. Using Klots' model (J. Chem. Phys., 1973, 58, 5364), an attempt is made to predict the photofragment kinetic energies on the assumption that the energy of each photon is partitioned statistically and contributes to an overall increase in temperature for each ion. The results show that at the higher photon energy, events are dominated by angular momentum conservation; a factor that is not an integral part of the model. In contrast, the photoexcitation of SF₆ at ca. 950 cm^–1 appears to result in incomplete energy randomisation.