Vibrational predissociation versus intramolecular vibrational energy redistribution (IVR) in rare gas⋯dihalogen complexes: IVR identified in Ar⋯I2(B, ν') using velocity-map imaging.

The competition between multiple pathways sampled during the energetic relaxation of excited molecules can be difficult to experimentally decipher. The rare gas···dihalogen van der Waals complexes have remained key systems for exploring the competition between relaxation pathways, such as intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP). As these mechanisms can yield the same products, the relaxation pathways traversed are often deduced from the excitation spectra or product-state distributions. In addition to a brief perspective on IVR and VP in rare gas⋯dihalogen complexes, we present new results obtained using time-of-flight velocity-map imaging (VMI) on T-shaped Ar⋯I2(B, ν', n' = 0) complexes that illustrate how contributions from these two pathways can be separated. The angular anisotropies of the ion images collected for the I2(B, ν < ν') fragments indicate the products for certain Ar⋯I2(B, ν', n' = 0) levels are weighted along the direction perpendicular to the laser-polarization axis. These distributions are consistent with prompt dissociation of the T-shaped excited-state complexes, likely via direct VP. The distributions measured for other Ar⋯I2(B, ν', n' = 0) levels are preferentially along the laser-polarization axis. These initially prepared levels must undergo IVR with nearly resonant Ar⋯I2(B, ν < ν', n > 0) intermolecular vibrational levels that sample linear Ar-I-I orientations during dissociation.