State-resolved UV Photodissociation of OH: High-lying Excited States, Shape Resonances, and Thermal Cross Sections

Abstract

The hydroxyl radical (OH) is a key intermediate in oxygen chemistry and one of the primary tracers of ultraviolet (UV)-driven molecular processing in astrophysical environments, including photodissociation regions, protoplanetary disks, and planetary atmospheres. Accurate photodissociation cross sections of OH are therefore essential for modeling UV-regulated oxygen chemistry and molecular destruction processes. However, existing theoretical studies remain limited in their treatment of highly excited electronic states and thermally populated rovibrational levels. In this work, high-level MRDCI calculations of potential energy curves and transition dipole moments were performed for an extended manifold of excited electronic states, including several previously neglected high-lying channels. Vibrational excitation mainly shifts the spectral onset through changes in Franck–Condon overlap, whereas rotational excitation can strongly restructure the UV photodissociation spectrum through centrifugal barriers that support quasi-bound states and shape resonances, particularly in the 12Σ+ ← 12Π transition. LTE-averaged cross sections from 0 to 10,000 K show that thermal population of excited rovibrational levels substantially enhances the photodissociation cross section up to about 5000 K, after which the temperature dependence becomes weaker. Channel-resolved analysis further shows that the 42Π ← 12Π channel provides the dominant contribution in the UV region between Lyα and the Lyman limit, highlighting the previously underappreciated role of high-lying electronic states in OH photodissociation. The present results provide a state-resolved and temperature-dependent description of OH photodissociation suitable for astrochemical and radiative transfer modeling of UV-irradiated astrophysical environments.