COMPARISON OF BENDING, C-C STRETCHING, AND COLLISION ENERGY EFFECTS ON THE REACTION OF C2H2+ WITH D2

We report the effects of vibrational excitation and collision energy on the cross sections and branching ratios for reaction of acetylene cations with D2, using two different guided-ion-beam instruments. Two major reaction channels are observed, both of which are nearly thermoneutral. Hydrogen atom exchange is slightly exoergic due to zero point energy, and is inhibited by both collision and vibrational energy. Formation of the two isotopic "C2H3+" products is enhanced by collision energy and C-C stretching vibration, but not by bending vibration. The branching ratios at low collision energies are consistent with reaction via an intermediate complex, and Rice-Ramsberger-Kassel-Marcus (RRKM) analysis is used to extract further information. At collision energies above 1 eV, D-atom transfer by a direct mechanism is also observed as a route to C2H2D+ production. Comparison of our results using both the Stony Brook and Freiburg instruments is made with the state-selected experiments of Honma, Kato, Tanaka, and Koyano [J. Chem. Phys. 81, 5666 (1984)], who previously studied both the C2H2+ + D2 and C2D2+ + H-2 isotopic reactants. Our results for C2H2+ + D2 are consistent with their C2D2+ + H-2 data for all collision energies and with their C2H2+ + D2 data at 0.2 eV collision energy. We do not reproduce the anamalous vibrational effects they reported for 1 and 2 eV collision energies.