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This paper summarizes the most recent knowledge concerning normal and three-body association ionmolecule reactions between small hydrocarbon ions m \ 1È6, n \ 0È9) and the neutral reactants (C m H ǹ ; H and Selected radiative association reaction rate coefficients have been calculated and estimated H 2 . based on three-body laboratory results. Some of the normal and radiative association rate coefficients have been incorporated into our "" new standard ÏÏ model of dense cloud interstellar chemistry. We Ðnd that atomic H provides an insufficiently appreciated mechanism to increase the level of saturation of ions, with m [ 3 in dense interstellar clouds despite the fact that the abundance ratio is C m H ǹ H/H 2 only 10~4. New model results show in particular that a signiÐcant abundance of the basic aromatic cyclic molecule benzene can now be produced in dense clouds.

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We report measurements of the reactions of a number of hydrocarbon ions with atomic and molecular hydrogen made using a selected ion flow tube (SIFT) operating at room temperature. Results, including branching ratios and rate coefficients, are reported for C m H n + ions (m = 2−6, n = 0−9). Highly unsaturated hydrocarbon ions undergo mainly H atom abstraction reactions with H2 forming C m H n +1 + + H products. More saturated ions are unreactive. Two types of reactions occur for H atoms:  H atom transfer to give C m H n - 1 + + H2 (if exothermic) and association (to give C m H n +1 +).