Rotational excitation of 45 levels of ortho/para-H₂O by excited ortho/para-H₂ from 5 K to 1500 K: state-to-state, effective, and thermalized rate coefficients

¹ CAB, INTA-CSIC, Crta, Torrejón km 4, 28850 Torrejn de Ardoz, Madrid, Spain
² Observatoire de Paris, LUTH UMR CNRS 8102, 5 Place Janssen, 92195 Meudon, France
e-mail: marie-lise.dubernet@obspm.fr
³ Université Pierre et Marie Curie, LPMAA UMR CNRS 7092, Case 76, 4 Place Jussieu, 75252 Paris Cedex 05, France
⁴ Institut UTINAM, UMR CNRS 6213, 41 bis avenue de l’Observatoire, BP 1615, 25010 Besançon Cedex, France

Received: 8 September 2011
Accepted: 10 October 2011

Abstract

Aims. This work deals with the rotational excitation of ortho/para-H₂O with para/ortho-H₂ so that thermalized de-(excitation) rate coefficients up to 1500 K for the first 45th level of ortho/para-H₂O are provided. Results are available in BASECOL with state-to-state rate coefficients, their fitting coefficients, and effective rate coefficients. In addition, we provide a routine that combines all data in order to create thermalized rate coefficients.

Methods. Calculations were 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 previous papers. The current CC results were compared 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. The relative strengths of water excitation rate coefficients when water is excited with ortho-H₂ versus para-H₂ was also analyzed.

Results. For collision with para-H₂, the rotation-rotation process is found to be the dominant process for inelastic transfer for some water transitions, implying that calculations must include the j₂ = 2 level. An important result of this paper is that j₂ = 1 and j₂ = 2 effective rate coefficients are very similar so that either j₂ = 1 or j₂ = 2 need to be calculated for astrophysical applications. In addition, at high temperature ratios of j₂ = 2 (1) over j₂ = 0, effective rate coefficients converge towards one to within a few percent. This study confirms that j₂ = 3 effective rate coefficients are within 20% to j₂ = 1 effective rate coefficients.

Conclusions. For astrophysical applications, these results imply that future collisional excitation of light molecules with H₂ should be carried out with para-H₂, including j₂ = 2, so as to obtain correct effective j₂ = 0 effective rate coefficients and using the j₂ = 2 effective rate coefficients for all excited j₂ effective rate coefficients. In contrast, collisional excitation of heavy molecules with H₂ might be restricted to para-H₂ with j₂ = 0 and to ortho-H₂ with j₂ = 1, using the j₂ = 1 rate coefficients for all excited j₂ effective rate coefficients. These conclusions should simplify the future methodological choice for collisional excitation calculations applied to interstellar/circumstellar media.