INFRARED PHOTOFRAGMENTATION SPECTRA OF SIZE-SELECTED SF6-CENTER-DOT-AR-N(+) CLUSTER IONS

Results are presented of a detailed experimental study of the infrared photofragmentation patterns of size-selected SF6 . Ar-n(+) cluster io ns for n in the range 3 to 70. Line-tuneable CO2 and N2O lasers have b een used to excited the nu(3) vibrational mode of the SF6 molecule whi ch is followed by the loss of one and two argon atoms as the principal fragmentation routes. Which of the two processes is dominant depends quite strongly on the size of the cluster ion concerned, with very pro nounced fluctuations in the relative intensities of photofragments bei ng observed for cluster ions in the range SF6 . Ar-3(+) to SF6 . Ar-25 (+). Only for SF6 . Ar-3(+) is the fragmentation pattern markedly diff erent from that found for the other ions; an observation that supports an earlier conclusion regarding the relative ionisation energies of t he two constituents [Stace et al. J. Phys. Chem. 97, 11363 (1993)]. A summation of fragment ion intensities as a function of laser wavelengt h is used to determine infrared absorption profiles and these have bee n recorded for individual clusters containing up to 70 argon atoms. Cl usters containing fewer than 40 argon atoms appear to form single stru ctures, with both the absorption profile shapes and selected hole-burn ing experiments suggesting that the number of isomers is small. The pr esence of isomers only appears to become significant when the clusters contain more than 40 argon atoms. The observation of site splittings for the triply degenerate nu(3) vibrational mode of SF6, together with the comparatively narrow linewidths seen for clusters containing betw een 15 and 40 rare gas atoms, indicates the presence of ordered struct ures. Such a conclusion implies that the clusters are solidlike rather than liquidlike. Overall, the results demonstrate that there is a cle ar correlation between those criteria previously used to identify the presence of stable cluster ion structures, i.e., mass spectra and unim olecular fragmentation patterns, and the corresponding infrared fragme ntation patterns and absorption profiles. Of the ions studied, SF6 . A r-21(+) stands out as being particularly stable and worthy of future t heoretical attention. (C) 1995 American Institute of Physics.