State-to-state dynamics of predissociation in OH–Ar: experiment and theory

Abstract

Long-lived predissociative levels of OH–Ar which lie as much as 350 cm^–1 above the OH (A ²σ⁺, v= 0, 1)+ Ar dissociation limits have been identified experimentally and computed based on an adjusted semiempirical potential-energy surface for OH (A ²σ⁺)+ Ar. The rotational state distributions of the OH (A ²σ⁺) photofragments have been probed using a novel variation of the stimulated emission pumping technique and exhibit a surprising degree of selectivity. The highest energetically available OH (A ²σ⁺) rotational channel is always accessed, yet low rotational levels are conspicuously absent from the product distributions. Predissociation lifetimes and product state distributions have also been calculated using the finite-range scattering wavefunction method. The theoretical results suggest two main mechanisms for internal rotational predissociation involving Coriolis coupling and the potential anisotropy. Comparison of the energies, lifetimes, and product rotational distributions of the OH–Ar predissociative levels observed experimentally with those obtained from the theoretical calculations provides a sensitive test of the short-range interaction of this highly anisotropic intermolecular potential-energy surface. Product rotational distributions measured following vibrational predissociation of OH–Ar also give new insight into the short-range part of the OH (A ²σ⁺)+ Ar intermolecular potential.