RADIATIVE ASSOCIATION REACTIONS OF SILICON AND TRANSITION-METAL CATIONS WITH THE PAH COMPOUNDS BENZENE, NAPHTHALENE, AND ANTHRACENE

Using the ion cyclotron resonance ion trap, reactions of Si+ with benzene, naphthalene, and anthracene were studied under low-pressure conditions of interest in interstellar modeling. Radiative association was efficient with all three hydrocarbons, occurring on roughly one-quarter of collisions. Association competed with charge transfer and/or condensation reactions in all three cases. In the naphthalene case it was inferred from collision-induced dissociation evidence and from reaction behavior that the association product ion at low pressure rearranges to a sigma complex or other intimately bonded structure, in contrast to the pi complex assigned under the high-pressure conditions in the SIFT. Reactions of the transition-metal ions Fe+, Cr+, and Mn+ with naphthalene were studied for comparison. Radiative association was also efficient for these ions. Association of the transition metal-naphthalene adducts with a further naphthalene molecule to yield the ''sandwich'' complex was also observed, although this type of complex was not formed for Si+ reacting with naphthalene.