VIBRATIONAL SPECTROSCOPY AND PREDISSOCIATION OF UF6 CLUSTERS IN A SUPERSONIC LAVAL NOZZLE

The FT-IR spectra of clusters of UF6 seeded in argon were measured in a continuous supersonic Laval nozzle Row. We observed absorption peaks of UF6 clusters on both the red- and the blue-shifted sides of the mo nomer peak (around 627 cm(-1)) at a temperature below 130 K. We assign ed these peaks using calculated results on the frequency shifts and tr ansition strength of the IR spectra of UF6 clusters. Clusters as large as 10 in size were found to be for med under our experimental conditi ons. Vibrational predissociation of UF6 clusters was observed after a single shot from a Raman laser tuned to a vibrational mode of the UF6 clusters (614.8 cm(-1)) in the Laval nozzle. We monitored the concentr ation of UF6 monomers in the ground state with a diode laser spectrome ter, and qualitatively interpreted the time-scale of the predissociati on of the UF6 clusters and the recombination of UF6 monomers subsequen t to the predissociation in the nozzle. After the UF6 clusters had dis sociated in less than 1 ns, the excited monomers formed by the dissoci ation were deactivated with a time constant of about 0.3 mu s via vibr ational relaxation induced by collisions with carrier gas (argon) atom s. The recombination of UF6 monomers produced by the vibrational predi ssociation competed with vibrational relaxation. The time constant of the recombination (about 6 mu s) was found to be much larger than that of the relaxation. (C) 1997 Elsevier Science B.V.