A study of the symmetric charge transfer reaction H+2+H2 using the high resolution photoionization and crossed ion–neutral beam methods

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Abstract
A new ion–molecule reaction apparatus, which combines the crossed ion–neutral beam method, high resolution photoionization mass spectrometry, and charge transfer detection, has been developed. Using this apparatus, we have examined the relative total charge transfer cross sections of H+2+H2 as a function of the vibrational state of H+2, v′0 =0–4, at the center-of-mass collision energy (Ec.m.) range of 0.38–200 eV. The relative total charge transfer cross sections measured at Ec.m. =8, 16, 22.5, and 200 eV are in general agreement with a recent theoretical calculation based on the semiclassical energy conserving trajectory formulation. The vibrational energy effects on the cross sections for the charge transfer and the H+3+H channels at low collision energies (Ec.m. ≤1 eV) were directly observed. The rotational states, J=0, 1, and 2, of H+2(v0=0) were also selected in this experiment. Within experimental uncertainties, the rotational excitations of H+2(v′0 =0) have no effect on the relative total charge transfer cross sections at Ec.m. =2 and 4 eV. By calibrating the nominal relative total charge transfer cross sections obtained with an ionizing photon energy of 18 eV (688 Å) to absolute total charge transfer cross sections determined previously using low energy electron impact ionization, absolute total charge transfer cross sections for v0 =0 and 1 in the kinetic energy range of Ec.m. =8–200 eV were estimated. The absolute total charge transfer cross sections thus obtained for v′0=0 and 1 are lower than the theoretical values by approximately a factor of 2. However, the kinetic energy dependence of the total charge transfer cross section is in agreement with the theoretical calculation. The final vibrational state distributions of the charge transfer products H+2 from the reaction H+2(v0=0) +H2(v″0=0) → H2(v′)+H+2(v″) at Ec.m. =4, 8, and 16 eV have been probed by charge transfer reactions H+2(v″)+N2 and H+2(v″)+CO. The results are consistent with the theoretical prediction that approximately 92% and 85% of the product H+2 ions formed at Ec.m. =8 and 16 eV are in the v″=0 state, respectively.