Rotational excitation of ortho-H2O by para-H2 (j2 = 0, 2, 4, 6, 8) at high temperature

Abstract

Aims. Our objective is to obtain the best possible set of rotational (de)-excitation state-to-state and effective rate coefficients for temperatures up to 1500 K. We present state-to-state rate coefficients among the 45 lowest levels of o-H2O with H2(j2 = 0) and Δj2 = 0, +2, as well as with H2(j2 = 2) and Δj2 = 0, −2. In addition and only for the 10 lowest energy levels of o-H2O, we provide state-to-state rate coefficients involving j2 = 4 with Δj2 = 0, − 2a ndj2 = 2 with Δj2 =+ 2. We give estimates of effective rate coefficients for j2 = 6, 8. Methods. Calculations are performed with the close coupling (CC) method over the whole energy range, using the same 5D potential energy surface (PES) as the one employed in our latest publication on water. We perform comparisons with coupled states (CS) calculations, with thermalized quasi-classical trajectory (QCT) calculations using the same PES and with previous quantum calculations obtained between T = 20 K and T = 140 K with a different PES. Results. We find that the CS approximation fares extremely badly even at high energy for j2 different from zero. Comparisons with thermalized QCT calculations show large factors at intermediate temperatures and factors from 1 to 3 at high temperature for the strongest rate coefficients. Finally we stress that scaled collisional rate coefficients obtained with He cannot be used in place of collisional rate coefficients with H2.