1996A&A...309L...1L

Deuterated molecules in interstellar clouds constitute a unique source of information on three interstellar parameters: the deuterium/hydrogen abundance ratio [D]/[H], the fractional ionisation in dense clouds, and the galactic ionizing flux (Dalgarno & Lepp 1984, Millar 1990). The molecular ion H₂D⁺ plays a key role in the formation of deuterated molecules. Dissociative recombination with electrons is a major loss channel and a potential source of deuterium atoms. An ion storage ring has been used to obtain the total recombination rate coefficient, and the probabilities for decay into H+H+D, HD+H and H₂+D. Using them in a model of the cloud L1529 we derive from observational data on HCO⁺ and DCO⁺ an upper limit to the fractional ionization of 2x10^–6, a [D]/[H] ratio of 1.7x10^–5 and an atomic D/H ratio of 2.7x10^–3.