Relaxation channels of vibrationally excited Van der Waals molecules

Abstract

Energy flow patterns of vibrationally excited Van der Waals molecules, such as I₂*⋯ He, HCl*⋯ Ar, N₂O*⋯ H₂O, C₂H₄*⋯ C₂H₄ and HF*⋯, HF, are reviewed. These complexes are described as A—B ⋯ C, where A—B* is a vibrationally excited chemically bonded molecule attached by a Van der Waals bond to C, an atom or molecule. Relaxation of the initially prepared excited complex can proceed by at least four channels: (a) A—B*⋯ C → A—B + C +ΔE_V–T, (b) A—B*⋯ C → A—B†+ C†+ΔE_V–T,R, (c) A—B*⋯ C → A—B + C*+ΔE_V–V, (d) A—B*⋯ C →[A—B ⋯ C]*. In (a), energy from the excited chemical bond breaks the Van der Waals bond and A—B (now relaxed) and C fly away with translational energy ΔE_V–T. This is the vibration–translational channel. If A—B†; and C†; contain rotational energy as in (b), the relaxation is by the vibration–translation rotation channel. Vibrational excitation of the fragments, such as C* in (c), appears in the vibration–vibration channel. Finally, in (d) energy initially localized in A—B* flows throughout the complex by exciting isoenergetic internal modes without breaking the Van der Waals bond. This is the intra molecular relaxation channel which produces [A—B ⋯ C]*, which may some time later dissociate to yield A—B + C with translational, rotational and possibly vibrational excitation.

Taking recently studied vibrationally excited Van der Waals molecules as examples, simple theoretical models are applied to assess the relative importance of these four relaxation channels. Our results are summarized with a set of propensity rules.