2007A&A...465..647M

Doubly deuterated ammonia has been found in various dense interstellar environments. We extend our previous study of singly deuterated ammonia NH₂D collisional excitation by helium to doubly deuterated ammonia ND₂H. The presence of two deuterium atoms in the system modifies the symmetry and the expansion coefficients of the intermolecular potential energy surface. We establish the corresponding collision equations. Cross sections are calculated in the coupled-state (CS) approximation and collisional rate coefficients are given between 5 and 100K, a range of temperatures that is relevant to interstellar conditions. These results may be used to interpret the available millimeter and submillimeter observations of ND₂H.