Reactive Quenching of OH A 2Σ+ in Collisions with Molecular Deuterium via Nonadiabatic Passage through a Conical Intersection

Abstract

The D and H atom products from collisional quenching of OH A 2Σ+ (v = 0) by D2 have been examined through Doppler spectroscopy using two-photon (2 2S ← ← 1 2S) laser-induced fluorescence. A bimodal Doppler profile is observed for the D atoms, indicating that two different velocity distributions result from the OH A 2Σ+ + D2 → D + HOD reaction. Nearly 40% of the products are H atoms produced in the OH A 2Σ+ + D2 → H + D2O reaction with a single Gaussian profile. The two components of the D atom kinetic energy distribution are characterized by translational temperatures of approximately 1200 and 10 000 K and on average account for 4% and 30% of the available energy. The H atom products accommodate about 37% of the available energy and are described by a 13 000 K temperature. The translational energy distributions of the H/D atom products are attributed to two dynamical pathways through the strong nonadiabatic coupling region at the HO−D2 conical intersection. The narrow “cold” distribution of D atoms arises from an abstraction reaction in a direct passage through the conical intersection region. The broad statistical distribution observed for both D and H atom products suggests that the HO−D2 collision pair lives long enough on the excited-state surface for energy to randomize before evolving through the conical intersection that leads to products.