Dissociative charge-transfer reactions of Ar+ with simple aliphatic hydrocarbons at thermal energy

A flowing-afterglow apparatus coupled with a low pressure chamber has been used to measure product ion distributions and rate constants in the charge-transfer reactions of Ar+ with CH4, C2Hn(n=2,4,6), and C3Hn(n=6,8) at thermal energy. Only parent cation is formed for C2H2 due to energy restriction. Major product channels are dissociative charge transfer followed by cleavage of C–H bond(s) for CH4, C2H4, C2H6, and C3H6, while by cleavage of a C–C bond for C3H8. A comparison of the product ion distributions with the photoelectron–photoion coincidence data for CH4, C2H4, and C2H6 leads us to conclude that the mean energies of precursor (pre)dissociative states are 15.3–15.5 eV, which are 0.3–0.5 eV below the resonance states. Thus the fractions of available energy deposited into internal modes of precursor parent ions at the instant of charge transfer are estimated to be 85%–95%, indicating that most of the CT reactions occurs without significant momentum transfer. The total rate constants for CH4, C2Hn(n=4,6), and C3Hn(n=6,8) are (0.78–1.1)×10−9 cm3 s−1, corresponding to 60%–92% of the calculated values from the Langevin theory. The rate constant for C2H2, 4.2×10−10 cm3 s−1, amounts to 38% of the kcalcd value. The small kobsd/kcalcd ratio is attributed to the lack of ionic states with favorable Franck–Condon factors for ionization.