Negative‐ion formation from positive ion by two‐electron transfer from an alkali‐metal target

To examine two‐electron transfer producing negative ions from positive ions, an apparatus using an alkali‐metal target has been developed. Positive ions formed by electron impact in an ion source collided with alkali‐metal vapor in a target chamber. Negative ions formed by two‐electron transfer have been analyzed with a single‐focusing mass spectrometer. Precursor positive ions and product negative ions have been identified from the apparent masses in the negative‐ion spectra. Using the target density dependence of the negative‐ion intensity, the processes of negative‐ion formation were determined to be double‐electron transfer in one collision or successive single‐electron transfer in two collisions. The cross section of these processes has been estimated from the peak area of positive‐ and negative‐ion spectra and the target density evaluated using the vapor‐pressure curve as a function of the temperature of the alkali‐metal cell. A cross section for He⁻‐ion formation in the Cs target with successive single‐electron transfer have been evaluated to be 1.40×10^−30±0.2 cm⁴ at a collision energy of 2.0 keV, which showed the fair agreement with the values reported by Donnally and Thoeming [Phys. Rev. 159, 87 (1967)]. The double‐electron transfer cross section for C₂⁻‐ion formation from a C₂⁺ ion with a Cs target has been evaluated as 7.03×10^−18±0.1 cm² at 3.0 keV. For O⁻‐ion formation from O₂⁺ ions, four broad peaks were observed due to the kinetic energy releases at dissociation which were 7.3, 3.4, 1.1, and 0.014 eV at full width at half maximum. Since the kinetic energy releases of the broad peaks corresponded with those by Peterson and Bae [Phys. Rev. A 30, 2807 (1984)], the peak extents have been explained as the dissociations of the excited neutral formed from exothermic neutralization. This two‐electron transfer from an alkali‐metal target offers a good advantage for the determination of the dissociation mechanism for excited neutral particles by means of the negative‐ion detection method.