Charge-Transfer Excitation Produced in Collisions of He+ and N2+ with Alkali Atoms

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Abstract
Optical excitation produced in collisions of He+ and N2 + with alkali atoms has been investigated using a crossed beam apparatus. Studies of the spectralradiation were made in the near uv and visible wavelength region for ion kinetic energies 50–1600 eV. For He+ on K and Rb, the spectra observed were identified primarily with excited alkali ions resulting from electron capture into the ground state of He. The relative weakness of near‐resonant excitations and the generally small magnitude of the emission cross sections are explained by the fact that the active electron moves from a closed shell of K or Rb to the 1s shell of He; orbital overlap requires a close encounter in which the atoms are strongly perturbed. For N2 + on K, the predominant spectra are the bands of the second positive system in N2 resulting from electron capture into the excited C 3 Π u states of N2. Emission cross sections were large (10−17–10−16 cm2) having energy dependence characteristic of near‐resonant charge‐transfer reactions and the population distribution of the various vibrational states corresponded to the Franck–Condon factors for transitions between N2 +(X) and N2(C). These features arise because the outermost electron of the alkali moves to an excited orbital of N2 so that orbital overlap favors a transfer at large distances, a process in which the collision partners are only slightly perturbed.