The dissociation dynamics of $He⋯I 35Cl(B,v′=2,3)$ complexes with varying amounts of internal energy

The $He⋯I 35Cl$ intermolecular vibrational levels with $n′=0–6$ that are bound within the $He+ICl(B,v′=3)$ potential [A. B. McCoy, J. P. Darr, D. S. Boucher, P. R. Winter, M. D. Bradke, and R. A. Loomis, J. Chem. Phys. 120, 2677 (2004)] are identified in laser-induced fluorescence experiments performed at very low temperatures within a supersonic expansion. Comparisons of the positions and intensities of these lines with the excitation spectra, calculated using potential surfaces to describe the interactions between the helium atom and ICl in its ground and excited state, assist in the assignments. Based on these comparisons the excited state potential was rescaled so that the experimental and calculated $J′=0$ energies agree to within the experimental uncertainties for all but the lowest, $n′=0,$ intermolecular level. Two-laser, action, and pump-probe spectroscopy experiments indicate that the bound $He⋯I 35Cl(B,v′=3)$ intermolecular vibrational levels undergo vibrational predissociation forming rotationally excited $I 35Cl(B,v′=2,j′)$ products with distributions that depend upon the initial intermolecular vibrational level excited. Action spectra recorded in the ICl $B-X,$ 2-0 region while monitoring the $Δv=0,$ $I 35Cl(B,v′=2)$ channel reveal two additional dissociation mechanisms for the $He⋯I 35Cl(B,v′)$ excited state complexes: rotational predissociation of discrete metastable states lying slightly above the $He+I 35Cl(B,v′=2)$ asymptote and direct dissociation that occurs when the linear conformer is excited to the continuum of states above the same asymptote. The rotational predissociation pathway forms $I 35Cl(B,v′=2,j′)$ products in all of the rotational states energetically accessible. The direct dissociation mechanism yields very cold rotational product state distributions; for instance, the average rotational energy in the product state distribution measured when the linear complexes are prepared 20 cm⁻¹ above the dissociation limit is only 1.51 cm⁻¹, representing only 7.6% of the available energy.