Rate Coefficients for Rotational State-to-State Transitions in H$_2$O + H$_2$ Collisions as Predicted by Mixed Quantum/Classical Theory (MQCT)

Abstract

A new database of collisional rate coefficients for transitions between the rotational states of H$_2$O collided with H$_2$ background gas is developed. The goal is to expand over the other existing databases in terms of the rotational states of water (200 states are included here) and the rotational states of hydrogen (10 states). All four symmetries of ortho and para water combined with ortho, and para hydrogen are considered.The mixed quantum/classical theory of inelastic scattering implemented in the code MQCT is employed. A detailed comparison with previous databases is conducted to ensure that this approximate method is sufficiently accurate. Integration over collision energies, summation over the final states of H$_2$ and averaging over the initial states of H$_2$ is carried out to provide state-to-state, effective, and thermal rate coefficients in a broad range of temperatures.The rate coefficients for collisions with highly excited H$_2$ molecules are presented for the first time. It is found that rate coefficients for rotational transitions in H$_2$O molecules grow with the rotational excitation of H$_2$ projectiles and exceed those of the ground state H$_2$, roughly, by a factor of 2. These data enable more accurate description of water molecules in high-temperature environments, where the hydrogen molecules of background gas are rotationally excited, and the H$_2$O + H$_2$ collision energy is high. The rate coefficients presented here are expected to be accurate up to the temperature of $\sim$ 2000 K.