4 Conclusion

This paper is a continuation of the work of Phillips and co-workers (Green et al. 1993; Phillips et al. 1994, 1995, 1996) on the excitation rates of H₂O by H₂. Phillips et al. (1996) gave excitation rates in the energy range from 20 K to 140 K; we calculated these rates in the energy range from 5 K to 20 K. We also provided fits of the excitation rates that are valid in the energy range from 5 K to 20 K. The results presented in this paper, the excitation rates of other transitions, and the associated cross sections will be made available on our WEB site.

We carried out a thorough investigation of the energy dependence of the excitation cross sections, which showed the importance of the resonance structure at low temperature. The resonances are certainly both Feshbach and shape resonances, and the resonance structure is very strongly linked to the potential energy surface. An assessment of the accuracy of the PES and an improvement of the potential energy surface including the vibrational degrees of freedom are underway (P. Valiron et al., in preparation).

Acknowledgements

The authors thank C. Ceccarelli for pointing out the interest of these calculations and E. Roueff and P. Valiron for stimulating discussions. Most scattering calculations were performed at the IDRIS-CNRS (Institut du developpement et des ressources en informatique scientifique du Centre National de la Recherche Scientifique) under project 011472-CP:4.