Excitation of N₂⁺ ions by collisions with rare-gas atoms

Collisions of 0.3-5-keV (4-20×10⁶ cm/sec) N₂⁺ ions with He and Ne targets have been studied by spectroscopic observation of the N₂⁺ first-negative-emission spectrum. Vibrational excitation within the collisionally excited X²Σ_g⁺→B²Σ_u⁺ transition in N₂⁺ was studied by a measurement of the relative band intensities of the Δv=-1 sequence of the first-negative system. Rotational excitation was examined by observation of the rotational line intensities of the (0,0) first-negative band (λ=3914 ÅA). Although the N₂⁺ ions used for these experiments were highly vibrationally and rotationally excited by the ion-source discharge, the initial-energy distribution could be inferred from the N₂⁺ emission spectra produced by high-velocity (v>1.2×10⁷ cm/sec) collisions. At these velocities, populations of vibrational levels in the upper state were independent of projectile-ion velocity and the rotational-energy distribution corresponded to a Boltzmann distribution at 3700 °K. At lower velocities, the population of high vibrational states increased with decreasing velocities and deviations from a Boltzmann distribution among rotational states were observed. Excitation to high rotational states increased monotonically with decreasing projectile-ion velocity. No differences in the rotational and vibrational excitation produced by He and Ne were observed. These results show that for some ion-molecule collisions there is vibrational and rotational excitation in the direct electronic excitation (non-charge-transfer) channel.