PHOTODISSOCIATION OF F2 AND MOBILITY OF F-ATOMS IN CRYSTALLINE ARGON

Experimental studies on the photodissociation of F2 in 12 K crystalline argon solids are reported, and compared with molecular dynamics (MD) simulations of the same system. At excess energies above 2.5 eV, the dissociation probability is near unity, in agreement with theory. At the longest experimentally accessible wavelength for photodissociation, 450 nm, which corresponds to a dissociation excess energy of 1.16 eV, the permanent dissociation probability is reduced to 0.05. This is an order of magnitude smaller than that predicted by the simulations. Possible sources for this discrepancy are discussed. Long range migration of F atoms, predicted by MD simulations, is demonstrated to occur both upon photodissociation of F 2, and upon relaxation of argon fluorides. In the latter case, it is shown that with the initial impulse, the F atoms migrate on average a length of ∼70 Å in the lattice. The thermal recombination of F atoms in both solid Ar and Kr are also characterized. The experimental studies are conducted in free standing crystals of argon, by following the photogeneration of F atoms. This is accomplished by monitoring the argon fluoride emissions upon charge transfer excitation over the ArF(F←X) excitation at 193 nm. Two distinct charge transfer states contribute to the observed emission. The first, with an emission maximum near 315 nm and a lifetime of 180(± 10) ns, is assigned to Ar2F. The second with a maximum at 355 nm and a lifetime of 700 (± 50) ns is assigned to Ar6F stabilized by trapping of F atoms in interstitial sites. Pump-probe measurements indicate a strong transient absorption from both of these states, precluding the possibility of observing any stimulated emission over these exciplexic transitions in the solid state. © 1990 American Institute of Physics.