Predissociation of the A2Σ+ (v′ = 3) state of the OH radical

Predissociation of electronically excited OHA2Σ+ (v′ = 3) is studied using velocity-map imaging of the atomic oxygen photofragments. Fine structure yields, angular distributions and alignment parameters are obtained for the O(3PJ), J = 2,1,0 products. Angular distributions for the O3P0 (J = 0) fragment, which has no angular momentum polarization, agree well with predictions from the angular distribution simulation computer routine by Kim et al. [J. Chem. Phys., 2006, 125, 133316] which calculates the anisotropy of photofragment recoil as a function of dissociation lifetime, excitation frequency, rotational level, and rotational constant. When angular momentum polarization (i.e. non-equilibrium population distributions of the magnetic sublevels) of the atomic fragments is present, the polarization sensitivity of the O(3PJ) (2 + 1) resonance enhanced multiphoton ionization (REMPI) detection scheme used to detect the O(3P2,1) products affects the measured angular distribution. Strong polarization effects are observed for the O(3P2,1) products and accounted for in a simple sudden limit model for the photodissociation. In agreement with the sudden limit predictions for pre-dissociation of OHA2Σ+ (v′ = 3) through the 4Σ− state, strongly aligned O(3P2) is found to be the major product.