Computation of orbiting cross-sections from ab initio potential energy surfaces for recombination of atomic oxygen

Recombination of atomic oxygen to O₂ can be modeled within the direct simulation Monte Carlo (DMSC) framework using the two-step binary collision model (TSBC). Formation of an orbiting pair, (OP) is the first step in the TSBC recombination process, where two O atoms revolve around their common center of mass. So far, the OP formation cross-section has been obtained assuming detailed balance between the dissociated O atoms and the O₂ molecule, without any regards to the physical nature of orbiting pairs. However, preliminary calculations reveal that an OP formation depends on collisional parameters like interaction impact parameter and relative translational energy. In the present work, we therefore compute OP formation cross-section and OP lifetimes based on classical scattering theory, and compare them with those obtained using (quasi-)classical trajectory calculations. These values are employed in computing the recombination rate coefficient for O₂ formation from O+O+O interactions. A good agreement is obtained between the present calculations and recombination rate coefficient values computed by applying detailed balance to dissociation cross-section values obtained from QCT calculations.