Isotope effect in charge-transfer collisions of slow H+ and D+ ions with H2, HD, and D2 molecules

Marked characteristics in charge-transfer cross sections between collisions of H+ and D+ ions with H2, HD, and D2 molecules, the so-called isotope effect, are observed in the energy range from 0.18 to 1.0 keV/u. The observed cross-section ratios σ(D++D2)/σ(H++H2) of charge transfer in D++D2 and in H++H2 collisions are found to be 0.665 at 0.18 keV/u and gradually increases, finally approaching unity at higher energies. Similar behavior and magnitude within the error bars of the cross-section ratios σ(H++HD)/σ(H++H2) and σ(D++HD)/σ(H++H2) have been observed, although the present calculation predicts a sizable difference for heteronuclear molecules. The cross section differences due to the target isotope effect are pronounced even in the high-eV to low-keV region. On the other hand, charge-transfer cross-section ratios for the same target but different projectile isotopes, e.g., σ(H++HD)/σ(D++HD) and σ(D++D2)/σ(H++D2), are constant near unity, which indicates a minor role of collision-induced vibrations as compared to the target vibrational spacing. It is understood that the isotope effect in the charge transfer of ion-molecule collisions originates from the combination of a small offset in binding and vibrational energies and the different spaces occupied by the wave functions of the target H2, HD, and D2 molecules