ISOTOPIC FRACTIONATION IN LOW-TEMPERATURE ION MOLECULE-EXCHANGE REACTIONS - ENRICHMENT OF NE-22 IN NEN+ CLUSTERS FORMED BY ASSOCIATION IN AN IONIZED FREE JET

Cationic clusters of neon atoms (Nen+) formed by association of neutrals onto seed ions in an ionized supersonic expansion are found to favor incorporation of the heavier isotope (22Ne) by as much as a factor of 15 (in the dimer and trimer ions) when compared to a simple statistical distribution based on natural abundances. This enrichment is attributed to the small difference in zero-point energies among species formed with the two major isotopes of neon (20Ne and 22Ne), which is of the same order as the collisional energy of particles in the expanding jet. This enrichment is anticipated by current models of isotope exchange which are invoked to explain the anomalous isotope abundance patterns in interstellar clouds. © 1990 American Institute of Physics.