Ion–Molecule Reactions in Alpha-Particle-Irradiated Methane and Water Vapor

Ion–molecule reactions in methane and water vapor are investigated for the various primary, intermediate, and secondary species formed in parent gases bombarded by 1–3.5-MeV alpha particles. Reaction rates for CH4+ and CH3+ colliding with methane gas and for H2O+ and OH+ colliding with water vapor are given and compared to similar data obtained by others using electron ionization. Single alphas are detected, counted, and their number compared to the number of ions formed in the reaction chamber. A pulsed time-of-flight type of mass spectrometer is used to vary ion–molecule reaction times and to separate ions according to their mass to charge ratios. The well-defined “zero” time of interaction in this pulse counting experiment allows one to estimate a “lifetime” for shortlived intermediates; the results indicate a lifetime near 0.18 μsec for the excited (C2H7+)* ion formed as a collision complex of CH3+ and CH4. These experiments also reveal that collisions between either the OH+ or H2O+ ions with water vapor result in intermediates that have lifetimes far shorter than the 10−7 sec measured for (C2H7+)*.