A drift chamber study of the reaction ArH++H2 →H+3 + Ar and related reactions

Ion–molecule reactions in argon– hydrogen mixtures were studied as a function of total and partial pressures, using a drift chamber mass spectrometer. The rate coefficient of the title reaction was found to depend on pressure, decreasing as the total pressure is increased from a value of 3.5×10−10 at low pressure to 3.5×10−11 cm3 molecule −1 s−1 at pressures greater than 0.8 Torr. This behavior is explained by the presence of vibrationally excited ArH+ ions formed in the reaction Ar+ + H2, and their deactivation by collisions with argon atoms. The thermalized ArH+ ions react with hydrogen more slowly than the excited ArH+ ions first formed. Rate coefficients for these processes and for several other ion reactions occurring in argon–hydrogen mixtures are given.