A low energy crossed beam study of the reactions of C+ with CH4

We present a crossed molecular beam study of the reactions of C+ with CH4 over the relative energy range from 0.66 to 2.38 eV. CH+3 production involves a direct rebound collision mechanism. Production of C2H+3 and C2H+2 proceeds through unimolecular decay of long-lived collision complexes. Hydrogen atom emission proceeds through a rotating ethylene cation and yields a polarized angular distribution of reaction products. The kinetic energy distributions for vinyl cation production place nearly three times as much of the available energy in translation as a statistical model predicts. At low collision energy, C2H+2 production proceeds through molecular hydrogen elimination and the angular distribution for reaction products is polarized perpendicularly to the initial relative velocity vector. At a higher collision energy of 2.38 eV, the complex decays in a fraction of a rotational period. The lifetime of 0.03 ps inferred from the asymmetry in the angular distribution is in good agreement with a statistical calculation incorporating nonadiabatic couplings among the excited and ground states of C2H+4. The recoil distribution at this energy is dramatically narrower than at the lower collision energy and is consistent with the sequential ejection of two hydrogen atoms through a metastable C2H+3 intermediate.