The Fourier transform infrared spectra of UF6 clusters seeded in Ar were measured in a continuous supersonic Laval nozzle flow at various pressures. The spectra of the clusters showed completely different features 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 the temperature and UF6 monomer density in the nozzle flow, it was found that: the clusters grew in a supercooled state, i.e., at a supersaturation ratio S much greater than 1, when P(total) = 3.0 Torr. In contrast, UF6 clusters grew in a near-equilibrium state, i.e., at S approximate to 1, when P(total) = 6.0 Torr. A comparison between the measured and calculated spectra revealed that the distribution of the potential energy of the cluster configurations was similar to an inverted population at P(total) = 3.0 Torr, and similar to the Boltzmann distribution at P(total) = 6.0 Torr. The fact that the distribution of the potential energy of the cluster configurations was similar to an inverted population in the supercooled state at P(total) = 3.0 Torr was explained qualitatively on the basis of the RRK theory. The spectra measured at X(UF6) = 0.04 support our explanation. (C) 1997 American Institute of Physics.