Quenching transitions for the rovibrational transitions of water: Ortho-HO in collision with ortho- and para-H.

Abstract

This study presents a complete quantum mechanical calculation of rovibrational quenching of water molecules from the first bending mode (approximately 1595 cm⁻¹) through collisions with both ortho- and para-H₂. The full coupled-channel formalism was applied within the rigid bender approximation, decoupling rotational and vibrational bases of water. Rotational quantum numbers of H₂ up to 4 (para) and 3 (ortho) were included, and rate coefficients were converged over a kinetic temperature range of 50–500 K using a well-validated full-dimensionality water–hydrogen potential energy surface. The projectile's rotational state was found to exert a dominant influence, producing rate differences spanning orders of magnitude across channels. Overall quenching rate coefficients were approximately 10⁻¹³ cm³ s⁻¹, one to three orders of magnitude below purely rotational quenching rates. These values are intended for astrophysical modeling of warm, dense environments such as protostellar and planet-forming regions accessible to infrared space observatories.

Mechanism

Using full coupled-channel quantum dynamics with the rigid bender approximation on a validated potential energy surface, the rotational state of H₂ was shown to drive order-of-magnitude variations in rovibrational quenching rate coefficients of water.