New H and H 2 Reactions with Small Hydrocarbon Ions and Their Roles in Benzene Synthesis in Dense Interstellar Clouds

Abstract

This paper summarizes the most recent knowledge concerning normal and three-body association ion-molecule reactions between small hydrocarbon ions (CmH+n; m = 1-6, n = 0-9) and the neutral reactants H and H2. Selected radiative association reaction rate coefficients have been calculated and estimated based on three-body laboratory results. Some of the normal and radiative association rate coefficients have been incorporated into our "new standard" model of dense cloud interstellar chemistry. We find that atomic H provides an insufficiently appreciated mechanism to increase the level of saturation of CmH+n ions, with m > 3 in dense interstellar clouds despite the fact that the H/H2 abundance ratio is only 10-4. New model results show in particular that a significant abundance of the basic aromatic cyclic molecule benzene (c-C6H6) can now be produced in dense clouds.