FOURIER-TRANSFORM INFRARED-SPECTROSCOPY OF UF6 CLUSTERING IN A SUPERSONIC LAVAL NOZZLE - CLUSTER CONFIGURATIONS IN SUPERCOOLED AND NEAR-EQUILIBRIUM STATES

The Fourier transform infrared spectra of UF6 clusters seeded in Ar we re measured in a continuous supersonic Laval nozzle flow at various pr essures. The spectra of the clusters showed completely different featu res when the total pressure P(total) was changed from 3.0 to 6.0 Torr at a fixed male fraction of UF6, X(UF6) = 0.08. From measurements of t he temperature and UF6 monomer density in the nozzle flow, it was foun d that: the clusters grew in a supercooled state, i.e., at a supersatu ration ratio S much greater than 1, when P(total) = 3.0 Torr. In contr ast, UF6 clusters grew in a near-equilibrium state, i.e., at S approxi mate to 1, when P(total) = 6.0 Torr. A comparison between the measured and calculated spectra revealed that the distribution of the potentia l energy of the cluster configurations was similar to an inverted popu lation at P(total) = 3.0 Torr, and similar to the Boltzmann distributi on at P(total) = 6.0 Torr. The fact that the distribution of the poten tial energy of the cluster configurations was similar to an inverted p opulation in the supercooled state at P(total) = 3.0 Torr was explaine d qualitatively on the basis of the RRK theory. The spectra measured a t X(UF6) = 0.04 support our explanation. (C) 1997 American Institute o f Physics.