INFRARED-SPECTROSCOPY OF SF6 IN AND ON ARGON CLUSTERS IN AN EXTENDED RANGE OF CLUSTER SIZES - FINITE-SIZE PARTICLES ATTAINING BULK-LIKE PROPERTIES

The spectroscopy of the nu-3 mode of SF6 seeded in or deposited on orgon clusters has been extended to larger cluster sizes using a new thermal detection photodissociation spectrometer. A spectral feature characteristic of an SF6 molecule in a tightly bound monomeric site has been observed at 937.9 cm-1 for clusters consisting of approximately 1000 atoms that are prepared by coexpanding a mixture of 0.025% SF6 in argon. This "matrix" feature, which is red-shifted by 10.1 cm-1 with respect to the SF6 gas-phase band center, disappears for larger clusters where an absorption characteristic of a chromophore residing on the surface of the cluster appears. However, on producing even larger clusters, i.e., above approximately 2000 atoms, a different absorption appears which is accurately located at the same position as the main SF6 absorption in a well-annealed matrix. This feature is blue-shifted by 0.7 cm-1 with respect to the earlier matrixlike feature, which itself corresponds closely to the absorption produced by SF6 in an unannealed (or moderately annealed) argon matrix. This behavior may be related to the transition of the clusters from a polyicosahedral structure, which has been shown to be more stable for smaller clusters, to a face-centered cubic (fcc) structure which is observed in the bulk phase. Finally, for SF6 molecules located at the surface of the cluster, we have resolved a line splitting of 1.5 cm-1. In addition, the intensity ratio of these two absorptions strongly suggests that, in this site, the chromophore is more than half buried in the cluster's surface.