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

Rate coefficients for rotational transitions in HDO and D2O induced by H2 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–H2 interaction potential of Valiron et al. The HDO–H2 and D2O–H2 rate coefficients are compared to the corresponding rate coefficients for HDO–He and H2O–H2, 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 (⁠⁠) transition.