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

¹ Université Pierre et Marie Curie, LPMAA UMR CNRS 7092, Case 76, 4 place Jussieu, 75252 Paris Cedex 05, France e-mail: marie-lise.dubernet@obspm.fr
² Observatoire de Paris-Meudon, LERMA UMR CNRS 8112, 5 place Jules Janssen, 92195 Meudon Cedex, France
³ Departamento Molecular and Infrared Astrophysics, Consejo Superior de Investigaciones Cientificas, C/ Serrano 121, 28006 Madrid, Spain
⁴ Institut UTINAM, UMR CNRS 6213, 41bis avenue de l'Observatoire, BP 1615, 25010 Besançon Cedex, France

Received: 25 July 2008
Accepted: 17 December 2008

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 = 0, +2, as well as with H₂(j₂ = 2) and = 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 = 0,-2 and j₂ = 2 with = +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  K and  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.