Charge transfer cross sections in argon ion–diatomic molecule collisions

Abstract

Charge-transfer cross sections have been examined for Ar+–H2, N2, O2, CO, and NO collisions, in the energy range 0.5–3.0 keV, using time-of-flight techniques in which fast neutral Ar products from the electron transfer processes are measured. Cross sections in Ar+–H2 and N2 systems are comparatively large and decrease with ion kinetic energy while the Ar+–O2, CO, and NO cross sections increase with incident ion kinetic energy. The Ar+ ion beam was composed of 2P3/2 and 2P1/2 states with no evidence of long-lived, electronically excited states participating in these reactions. A multistate impact parameter treatment of atomic ion–diatomic molecule collisions is presented and applied to the Ar+(2P3/2, 2P1/2)−H2(X 1Σ+g, v0′=0) system. The resulting coupled differential equations have been solved numerically, with convergence of the cross sections achieved by inclusion of a comparatively large number of possible product channels in the wavefunction expansion of the system. At low ion kinetic energies both the energy defects and vibrational overlaps determine the population of the product vibrational states while at high kinetic energies the vibrational overlaps control the product-ion vibrational-state distribution. Small-angle scattering dominates the charge-transfer reactions with forward inelastic processes becoming more intense as the ion kinetic energy is increased.