Noble gas clusters as matrices for infrared spectroscopy. From small clusters to the bulk-matrix limit: SF6Arn, SF6Krn, and SF6Xen with 100≲n≲10 000

Abstract

It is demonstrated that matrix-like spectroscopy may be carried out in the gas phase using molecular beams of clusters and the conditions under which bulk-matrix-like behavior is achieved are illustrated. At the same time, we obtain information on the structural evolution of noble gas clusters as a function of their size. Infrared spectra for SF6 attached to noble gas clusters of argon, krypton, and xenon were recorded using a free jet cluster source and a laser photofragmentation detection technique. When a dilute mixture of the chromophore in Ar and Kr is expanded at relatively low pressures, the clusters spectra show a feature characteristic of the SF6 solvated in a defective, unannealed matrix. This feature disappears at higher source pressures (larger sizes) at which the chromophore prefers to reside on the surface of the cluster. This can be demonstrated by producing neat clusters and depositing the chromophore on them. However, on producing still larger clusters, a different absorption appears which is accurately located at the same position as the main absorption in a well-annealed matrix of Ar or Kr. This behavior is related to the transition of clusters from a Mackay icosahedral structure, shown to be the most stable for smaller clusters, to the face-centered-cubic (fcc) structure which is observed in the bulk phase. This structural transition occurs at a nozzle stagnation pressure which corresponds to an average cluster size of about 2000 atoms for both Ar and Kr. Scattering studies performed on argon clusters suggest that the fcc-type clusters correspond to the largest sizes in the cluster size distribution present in the beams. A similar structural transition for xenon cluster was not established as the SF6 appears to solvate only slightly in Xe in the size range studied here.