On the efficient generation of Arn–SH free-radical clusters via ultraviolet photolysis of closed-shell Arn–H2S complexes

Abstract

Detailed investigations are described for the generation of open-shell radical complexes formed by unimolecular photolysis of closed-shell van der Waals precursors. As a specific test case, ultraviolet photolysis of slit-jet cooled Arn–H2S (n⩽2) complexes at both 248 and 193 nm are shown to yield Ar–SH and Ar2–SH radical cluster species with surprisingly high efficiencies. Analysis of the laser induced fluorescence (LIF) spectra indicates that the radical complexes are produced with extensive van der Waals stretch/bend and overall rotational excitation, which is consistent with a simple ballistic model of the dissociation dynamics. The LIF spectra obtained as a function of expansion distance downstream provide clear evidence for remarkably efficient cooling of the newly formed radical cluster species by low-energy collisions with jet-cooled inert gas atoms at ≈10 K. Spectrally resolved Ar–SH and Ar2–SH LIF signals have been investigated as a function of Ar composition, which yields information on relative branching ratios for fragmentary (e.g., Ar2–H2S→Ar–SH+H+Ar) and nonfragmentary (e.g., Ar2–H2S→Ar2–SH+H) photolysis events.