Collisional excitation of doubly deuterated ammonia NHD2 by para-H2

TL;DR

This paper calculates collisional excitation rates of doubly deuterated ammonia (NHD2) by para-H2, providing essential data for astrophysical modeling of cold interstellar environments.

Contribution

It extends previous collisional rate calculations from helium to para-H2 for NHD2, improving the accuracy of radiative transfer models in space.

Findings

Collisional rates with H2 are about ten times higher than with helium.

Results cover temperatures from 5 to 35 K relevant to cold interstellar regions.

Provides data to better interpret millimeter and submillimeter observations of NHD2.

Abstract

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.