Rotational quenching of H2CO by molecular hydrogen: cross-sections, rates, pressure broadening

We compute the rotational quenching rates of the first 81 rotational levels of ortho- and para-H2CO in collision with ortho- and para-H2, for a temperature range of 10-300 K. We make use of the quantum close-coupling and coupled-states scattering methods combined with the high accuracy potential energy surface of Troscompt et al. (2009a). Rates are significantly different from the scaled rates of H2CO in collision with He; consequently, critical densities are noticeably lower. We compare a full close- coupling computation of pressure broadening cross sections with experimental data and show that our results are compatible with the low temperature measurements of Mengel & De Lucia (2000), for a spin temperature of H2 around 50 K.

Collisional excitation of doubly deuterated ammonia NHD2 by para-H2

Collisional de-excitation rates of partially deuterated molecules are different from the fully hydrogenated species because of lowering of symmetry. We compute the collisional (de)excitation rates of ND2H by ground state para-H2, extending the previous results for He- lium. We describe the changes in the potential energy surface of NH3- H2 involved by the pres- ence of two deuterium nuclei. Cross sections are calculated within the full close-coupling ap- proach and augmented with coupled-state calculations. Collisional rate coefficients are given between 5 and 35 K, a range of temperatures which is relevant to cold interstellar conditions. We find that the collisional rates of ND2H by H2 are about one order of magnitude higher than those obtained with Helium as perturber. These results are essential to radiative transfer modelling and will allow to interpret the millimeter and submillimeter detections of ND2H with better constraints than previously.

Ab initio computation of the broadening of water rotational lines by molecular hydrogen

Theoretical cross sections for the pressure broadening by hydrogen of rotational transitions of water are compared to the latest available measurements in the temperature range 65-220 K. A high accuracy interaction potential is employed in a full close coupling calculation. A good agreement with experiment is observed above ~80 K while the sharp drop observed experimentally at lower temperatures is not predicted by our calculations. Possible explanations for this discrepancy include the failure of the impact approximation and the possible role of ortho-to-para conversion of H2.