Collisional excitation of doubly deuterated ammonia ND2H by helium

International audienceContext: Doubly deuterated ammonia has been found in various dense interstellar environments. Aims: We extend our previous study of singly deuterated ammonia NH2D collisional excitation by helium to doubly deuterated ammonia ND2H. Methods: 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. Results: Cross sections are calculated in the coupled-state (CS) approximation and collisional rate coefficients are given between 5 and 100 K, a range of temperatures that is relevant to interstellar conditions. Conclusions: .These results may be used to interpret the available millimeter and submillimeter observations of ND2H