Rotational excitation of 45 levels of ortho/para-H 2O by excited ortho/para-H 2 from 5 K to 1500 K: State-to-state, effective, and thermalized rate coefficients

Abstract

Aims. This work deals with the rotational excitation of ortho/para-H 2O with para/ortho-H 2 so that thermalized de-(excitation) rate coefficients up to 1500 K for the first 45th level of ortho/para-H 2O 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-H 2 versus para-H 2 was also analyzed. Results. For collision with para-H 2, the rotation-rotation process is found to be the dominant process for inelastic transfer for some water transitions, implying that calculations must include the j 2 = 2 level. An important result of this paper is that j 2 = 1 and j 2 = 2 effective rate coefficients are very similar so that either j 2 = 1 or j 2 = 2 need to be calculated for astrophysical applications. In addition, at high temperature ratios of j 2 = 2 (1) over j 2 = 0, effective rate coefficients converge towards one to within a few percent. This study confirms that j 2 = 3 effective rate coefficients are within 20% to j 2 = 1 effective rate coefficients. Conclusions. For astrophysical applications, these results imply that future collisional excitation of light molecules with H 2 should be carried out with para-H 2, including j 2 = 2, so as to obtain correct effective j 2 = 0 effective rate coefficients and using the j 2 = 2 effective rate coefficients for all excited j 2 effective rate coefficients. In contrast, collisional excitation of heavy molecules with H 2 might be restricted to para-H 2 with j 2 = 0 and to ortho-H 2 with j 2 = 1, using the j 2 = 1 rate coefficients for all excited j 2 effective rate coefficients. These conclusions should simplify the future methodological choice for collisional excitation calculations applied to interstellar/circumstellar media. © 2011 ESO.