Infrared photofragmentation spectra of size-selected Krn+SF6 cluster ions

Abstract

Presented here are the results of an experimental study of the infrared photofragmentation patterns of size-selected Kr_n⁺SF₆ cluster ions for n in the range 2⩽n⩽50. The ν₃ (T_1u) vibrational mode of the SF₆ molecule in each cluster ion has been excited using a line-tunable CO₂ infrared laser, and the subsequent fragmentation patterns were monitored for the loss of one and two krypton atoms and also for the loss of the SF₆ chromophore. For ions containing more than four krypton atoms, the dominant fragmentation process observed was the loss of a single krypton atom, with the loss of SF₆ being the least intense of the fragmentation channels for most clusters. For one ion, Kr₃⁺SF₆, significantly different behaviour was observed with the loss of SF₆ being the dominant fragmentation route, which suggests a stable Kr₃⁺ trimer. Fragmentation intensities were monitored as a function of laser wavelength in order to determine infrared absorption profiles for individual Kr_n⁺SF₆ cluster ions. The observation of site-symmetry splittings of the triply degenerate ν₃ mode and the relatively narrow linewidths suggest the presence of asymmetric, but ordered, structures. With only a maximum of three absorption features apparent for clusters consisting of less than thirty five krypton atoms, the number of structural isomers appears to be small. Comparisons are made with results previously obtained for the Ar_n⁺SF₆ series of cluster ions (J. Chem. Phys., 1995, 103, 5177), and differences in spectra are attributed to the sensitivity of the SF₆ molecule to its environment. Additional interpretation comes from comparisons with computer simulation studies on neutral clusters with the compositions Ar_nSF₆ and Kr_nSF₆ (J. Chem. Phys., 1991, 95, 6271). The appearance of SF₆ loss as a minor fragmentation channel for clusters in the region of K₁₂⁺SF₆ is interpreted in terms of stacked structures.