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 Ar-n-H2S (n less than or equal to 2) complexes at both 248 and 193 nm are shown to yield Ar-SH and Ar-2-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 approximate to 10 K. Spectrally resolved Ar-SH and Ar-2-SH LIF signals have been investigated as a function of Ar composition, which yields information on relative branching ratios for fragmentary (e. g., Ar-2-H2S-->Ar-SH+H+Ar) and nonfragmentary (e. g., Ar-2-H2S-->Ar-2-SH+H) photolysis events.