Rotational excitation of mono- and doubly-deuterated water by hydrogen molecules

Rate coefficients for rotational transitions in HDO and D2O induced by H-2 collisions below 300 K are presented. Calculations have been performed at the close-coupling and coupled-states levels with the deuterated variants of the H2O-H-2 interaction potential of Valiron et al. The HDO-H-2 and D2O-H-2 rate coefficients are compared to the corresponding rate coefficients for HDO-He and H2O-H-2, respectively. Significant differences are observed. In particular the new HDO rate coefficients are found to be significantly larger (by up to three orders of magnitude) than the corresponding HDO-He rate coefficients. The impact of the new HDO rate coefficients is examined with the help of non-LTE radiative transfer calculations. A number of potential HDO maser lines are finally identified, in particular the 80.6 GHz (1(1,0)-1(1,1)) transition.