Energetics of charged transfer in collisions of Ar+ ions with H2, D2, and CO2

The energetics of charge transfer in collisions of low‐energy Ar⁺ ions with H₂, D₂, and CO₂ molecules have been investigated in ion beam‐gas scattering experiments, which utilize a special feature of collision kinematics. The kinetic energy (KE) distributions of product ions which emerge from their collision sites at 90° with respect to the projectile beam direction were determined by electrostatic analysis, using a parallel‐plate analyzer. At this angle, exothermic transfer channels dominate the observed distributions, and the coupling between projectile and product ion kinetic energies is minimized. Information concerning the contributing transfer channels, and therefore concerning the internal states of the product ${\mathrm{H}}_2^{+}$ , ${\mathrm{D}}_2^{+}$ , and ${\mathrm{CO}}_2^{+}$ ions, has been deduced from the measured KE distributions. For the Ar⁺–H₂, D₂ systems, measurements were carried out at a number of projectile ion KE's, over the range 20<E₁<150 eV. For the Ar⁺–CO₂ system, runs were made at E₁≃6 eV and E₁ =20.1 eV. In addition, a single distribution for the Ne⁺–CO₂ system was obtained, at E₁=50.1 eV. The distributions for Ar⁺–D₂ and Ar⁺–H₂ at the lower collision energies revealed a surprising isotope effect, in that the former showed contributions of fairly energetic ions, whereas the latter consisted solely of ions with near‐thermal energies. The Ar⁺–CO₂ data at E₁=6 eV gave evidence of ${\mathrm{CO}}_2^{+}$ ions with energies 0≤E₃≤∼1 eV, from which it was inferred that transfer proceeds via a number of exothermic transfer channels, having internal energy changes Q over the entire accessible range 0<Q≤2.16 eV. The effect of the thermal motions of the target particles (Doppler broadening) has been discussed in some detail. This effect is particularly important in these studies, since the kinetic energies of the product ions were small (for the most part, less than a few tenths of an electron volt).