Molecular Resonant Charge-Transfer Processes; H2+–H2 and N2+–N2

Abstract

A study has been made of the competition between exothermic ion—molecule reactions and resonant charge‐transfer processes in collisions of diatomic molecule ions and their parent molecules; H₂⁺–H₂ and N₂⁺–N₂. Experimental values of resonant charge‐transfer cross sections are shown to be smaller than values obtained from calculations using an impact parameter method. Much better agreement between experiment and theory is obtained if competition between ion—molecule reaction and charge transfer is assumed and experimental values of both channels of reaction are summed prior to comparison with theory. The mechanisms of the respective processes are illuminated by an examination of the velocity dependences of charge‐transfer cross section for both diatomic systems. With H₂⁺–H₂ where reaction producing H₃⁺ is a probable process, in low‐velocity collisions, the velocity dependence of charge transfer is essentially that of collisions of ions with unperturbed rectilinear trajectories. In a similar low‐energy region, N₂⁺–N₂ is not energetically capable of participating in ion—molecule reactions and the velocity dependence of charge‐transfer cross section is steeper, characteristic of both orbiting collisions and the larger‐impact‐parameter rectilinear trajectory collisions. Experimental cross sections for charge transfer and ion or atom transfer reflect probability of reaction but their velocity dependences clearly isolate the range of impact parameters operative for the respective processes.