Charge transfer reactions between D +, O +, Ar +, Kr + and Xe + with CH 4, C 2H 6 and C 3H 8 at relative energies 1–10 eV

Abstract

Fifteen asymmetric resonant charge transfer reactions, involving the five title atomic ions A + and the three title molecules B have been studied in the relative (center-of-mass) energy range 1–10 eV. Two different tandem mass spectrometers were used to measure limiting cross-sections at the highest energies. Time-of-flight and retarding-potential-analysis techniques were used to characterize the contribution from non-resonant processes, occurring with momentum transfer at low impact parameters. An attempt is made to correlate the magnitudes of the cross-sections for A + + B → products with molecular parameters of the original reactants: (i) the Franck—Condon factors for B → B +, and (ii) the cross-section for A + + A → A + A +. The Franck—Condon factors for B → B + are shown not to correlate quantitatively, although a propensity rule — low factors legislate low cross-sections — does apply. The dependence of the cross-section on the nature of A correlates qualitatively with the square root of the symmetric charge transfer cross-section for A + + A → A + A +. The incorporation of all these factors, within the context of Förster theory (which treats the transfer of excitation energy A ∗ + A → A + A ∗ ), is considered for charge transfer. Qualitative but not quantitative correlations are found, and a formal justification for this is noted. The implications of these results for the mechanism of these reactions at thermal energies are briefly considered.