Laboratory studies of isotope exchange in ion-neutral reactions: interstellar implications

Abstract

The rare stable isotopes of several elements (e.g. D, ¹³ C and ¹⁵ N) have been detected in several interstellar molecules, and their abundance relative to the more common isotope is often enhanced above that in the solar-terrestrial environment. Important questions to answer are to what extent the isotopic ratios in the molecules are representative of those in the cloud matter as a whole, and whether fractionation of the heavier isotope into the molecules via ion—molecule interactions is a significant process. A laboratory study of isotope exchange in ion-molecule reactions has therefore been carried out, the results of which indicate that fractionation of heavy isotopes can occur very efficiently at low temperatures. Consideration is given in this paper to reactions in which H-D, ¹² C - ¹³ C, ¹⁴ N - ¹⁵ N and ¹⁶ O - ¹⁸ O exchange occurs, and it is shown how better estimates of the electron density and the temperature in interstellar clouds have been obtained from these laboratory data.