Laboratory studies of the reactions of HCO(+) (and DCO/+/) and N2H(+) (and N2D/+/) with D (and H) atoms - Interstellar implications

Abstract

Measurements were made of the rate coefficients, k, for the forward (kf) and reverse (kr) ion-atom reactions HCO(+) + D yields DCO(+) + H and N2H(+) + D yields N2D(+)+ H at 300K and 120 K. At 300 K, the kf for both reactions are 2-3 times greater than the respective kr. At 120 K, the kf exceed the kr by more than an order of magnitude, the kf having markedly increased and the kr decreased relative to the 300 K values. In the light of these data, it is predicted that the kf for both of the above reactions will approach the collisional limiting values and the kr will be negligibly small at the low temperatures of dense interstellar clouds. Thus fractionation of deuterium will occur via the above ion-atom reactions in those dense interstellar clouds in which a significant fraction of the deuterium is in the atomic form in accordance with a very recent postulate.