High energy density systems in cryogenic media: The production and reaction of atoms and radicals

The lifetime and reactive behavior of energetic species in low temperature rare gas media have been investigated. A technique for measuring the diffusion coefficient of reactive species has been developed and applied in detail to probe O atoms in Xenon matrices. O atoms have long lifetimes in xenon matrices at 32 K. For initial concentrations on the order of one part in 10,000 approximately half of the initial atoms are unreacted on a timescale of one week. The long timescale for this process is discussed in relationship to the shorter timescales observed for atom and radical diffusion under conditions involving constant photolysis of matrices. The lifetime excited state species are important in understanding the storage and reactivity of energetic species in low temperature cryogenic media. Isolated Binary Collision (IBC) models attempt to predict lifetimes of excited states in condensed media based on the probability of collision deactivation of the species in the gas phase and the local density of the condensed phase media. The predictions of this theory have been tested for a number of systems. While the predictions have been shown to generally be very good for non-polar systems undergoing vibration-translation/rotation relaxation processes, some possible anomalies are noted for the hydrogen halides in heavy rare gases. Possible sources of these anomalies and their ramifications are discussed.