Computation of inelastic rotational rates and collision cross-sections for astrophysically important systems: H2+ H2O and the water maser

Abstract

The water maser radiation from stellar regions which is observed on Earth arises from the stimulated emission of radiation from rotationally excited states of the water molecule. In order to model and understand the physical conditions pertaining in the regions from which the radiations originates it is necessary to know the rates with which the rotational energy level populations of the water molecules are changed. One of the main mechanisms, other than stimulated emission, through which such changes occur is by collisions of the water molecules with molecular hydrogen, which is the most abundant species in the regions of interest. The purpose of the present paper is to report preliminary results for the rotationally inelastic cross-sections and of rate constants for collisions between H₂ and H₂O molecules which are needed in the modelling of the water maser. The paper also summarises the ab initio computation of the interaction potential and its fitting to an appropriate analytic form suitable for use in the subsequent quantum-mechanical scattering calculations of the rotationally inelastic cross-sections.