Accurate determination of the relative concentrations of ammonia isotopologues in a cold, electrostatically guided molecular beam

Abstract

Buffer-gas cooling and electrostatic velocity selection techniques are combined in this work to produce a continuous molecular beam with rotational and translational temperatures comparable to those in the interstellar medium (ISM). A number of different mixtures of the isotopologues of ammonia (ND3, ND2H, NH2D and NH3) in varying ratios are used as the source gas, and the molecular beam emanating from the quadrupole guide velocity selector is characterised using mass spectrometry and Monte Carlo trajectory simulations. The guiding of the mixed isotopologues ND2H and NH2D is demonstrated for the first time. This approach establishes both the relative concentrations of each species in the beam and the internal rotational state distributions after buffer-gas cooling and guiding. Such information is essential for experiments in which the beam is used in reaction rate measurements (e.g., for ion–molecule reactions) and facilitates the study of reactions relevant to deuterium fractionation in the ISM, such as the competition between H and D transfer with mixed species including ND2H and NH2D.