Non-adiabatic processes at metal surfaces: R Brako and D M Newns, Vacuum, 32 (1), 1982, 39–50

Abstract

Single-electron capture (SEC) by NH32+ with rare gases (He, Ne, Ar, Kr, Xe) and benzene, NH32+ + G → NH3+ + G+, has been investigated using a forward geometry double-focusing mass spectrometer. The SEC cross-section goes through a maximum within a reaction window centered at an exothermicity of approximately 4.5 eV. This correspondes to a target-projectile distance for the charge transfer of about 3.2 Å. The average internal energy of NH3+-ions resulting from SEC has been obtained both via translational energy gain measurements and via an analysis of the daughter ion spectra. The agreement between both methods is satisfactory and shows that the lower the ionization energy of the target, the more probable the excitation of NH3+ to the à 2E state. With benzene (ionization energy = 9.25 eV) as the target gas, however, the excess energy is converted partly to internal energy of C6H6+, leading most probably to the D̃ 2B2u, Ẽ 2B1u and F̃ 2A1g states of C6H6+. A model based on the Landau-Zener non-adiabatic formalism, including one entrance channel (NH32+ + G) and two exit channels (NH3+ X̃ 2A2″ + G+ and NH3+ à 2E + G+) accounts qualitatively for the observed results in the case of exothermic single-electron capture. This model is not appropriate for slightly exothermic or endothermic charge transfer, i.e., in the present case, for the Ne and He target gases.