Guided-Ion Beam (GIB) measurements of the Ar ϩ ϩ Ar symmetric charge-transfer (SCT) system are presented for ion energies ranging from 0.2 to 300 eV. Two methods are applied to distinguish primary and secondary ions: (i) based on isotopic-labeling, (ii) based on significant laboratory velocity differences. The absolute cross sections measured with these methods are in excellent agreement at energies above 1 eV. The experimental results are compared with semi-classical calculations performed with various published Ar2 ϩ potentials. The calculations including spin-orbit effects lie within 10% of the isotopeselected and attenuation measurements at all investigated ion energies. The present results lie significantly above the simple Rapp and Francis model [1]. Important errors in the latter approach are pointed out and a correct one-electron model is proposed. First measurements of the differential cross section at 0.5 eV collision energy are briefly mentioned.
Absolute integral cross sections are measured in the collision energy range between 0.1 to 3.5 eV for the N2++CH4 and N2++CD4 reactions using the universal guided ion beam apparatus. The reaction branching ratio, CX3+:CX2+:N2X+ (X=H or D), is found to be 0.86:0.09:0.05 and 0.88:0.07:0.05 for the N2++CH4 and N2++CD4 reactions, respectively. The CH3+/CH2+ ratio is constant over the whole collision energy range and very similar to the one obtained for the almost isoenergetic Ar++CH4 reaction. Axial velocity distributions of the product ions are measured by time of flight at collision energies between 0.1 and 3.5 eV. The results provide direct insight into the reaction dynamics. The dissociative charge transfer channels, leading to CH3+ and CH2+ product ions, occur via an electron jump combined with some exchange of momentum between the colliding partners. The H (D) transfer leading to N2H+ can be described as a direct process, similar to a spectator stripping mechanism. Various isotope effects are observed, the dominant being that the cross sections for reaction with CH4 are up to 20% bigger than the corresponding ones for CD4.
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