Collisional excitation of monodeuterated ammonia NH₂D by helium

Laboratoire Univers et Théories and UMR 8102, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92195 Meudon, France e-mail: [leandre.machin;evelyne.roueff]@obspm.fr

Received: 13 July 2006
Accepted: 21 August 2006

Abstract

Context.Observations of deuterated molecules are useful probes of the physical and chemical conditions in star-forming regions. However their detailed interpretation is hampered by the lack of knowledge of the corresponding collisional rate coefficients.

Aims.We extend our previous study of ammonia collisional excitation by helium to monodeuterated ammonia NH₂D, which has been found to be abundant in a variety of environments.

Methods.We introduce the principal isotopic effects in the collisional equations by calculating the relevant angular dependence of the intermolecular potential energy surface.

Results. Cross sections are calculated in the coupled states approximation, and collisional rate coefficients are given from 5 to 100 K, a range of temperatures that is relevant in the interstellar medium.

Conclusions.These results allow to include justifiable collisional excitation rate coefficients of NH₂D by He in the analysis of the emitted radiation for the first time