Charge-transfer processes in collisions of ${\mathrm{H}}^{+}$ ions with ${\mathrm{CH}}_4,$ ${\mathrm{C}}_2{\mathrm{H}}_2,$ ${\mathrm{C}}_2{\mathrm{H}}_6,$ and ${\mathrm{C}}_3{\mathrm{H}}_8$ molecules have been studied based on joint experimental and theoretical approaches in the collision energy from 0.2 to 4.0 keV. In the present experiment, the initial growth rate method was employed for determination of cross sections, while in the present theory, the molecular orbital expansion method was utilized for ${\mathrm{CH}}_4$ and ${\mathrm{C}}_2{\mathrm{H}}_2$ molecules. Both experimental and theoretical results indicate that if one assumes vibrationally excited molecular ions $({\mathrm{CH}}_4^{+},$ ${\mathrm{C}}_2{\mathrm{H}}_2^{+},$ ${\mathrm{C}}_2{\mathrm{H}}_6^{+},$ and ${\mathrm{C}}_3{\mathrm{H}}_8^{+})$ in the exit channel, then charge-transfer processes are sometimes found to become more favorable due to an accidental resonant condition. This is the clear indication of the role of vibrational excited states for charge transfer, and is found to be particularly so as the molecular size increases.

• Received 3 January 2000

DOI:https://doi.org/10.1103/PhysRevA.62.062715