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

Optical excitation produced in collisions of He+ and N2+ with alkali atoms has been investigated using a crossed beam apparatus. Studies of the spectral radiation 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 3Πu 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.