C+(2P)+H2(D2,HD)→CH+(CD+)+H(D). I. Reaction cross sections and kinetic isotope effects from threshold to 15eV c.m.

Total cross sections for the reactions of carbon (1 + ) ions with isotopic molecular hydrogen (H2, D2, and HD) to form methyliumylidene (CH+ and CD+) have been measured using guided ion beam techniques. Cross sections are reported as a function of the translational energy of the reactants from the reaction threshold up to 15 eV c m . The true cross sections are shown to rise sharply from thresholds given by the thermochemical endothermicities. Inter- and intramolecular isotope effects in the threshold region can be attributed to the different endothermicities due to vibrational zero-point energies. At higher energies, an unexpected intermolecular isotope effect is found. Thermal reaction rates (300 K) derived from the data are 1.2X10-16 for H2 , 2.3 X 10 -17 for D2 , and 1.2X 10 -16 for HD (17% CH+, 83% CD+), all in units of cm3 s -1 .