Excitation ofN2+ions by collisions with rare-gas atoms

Abstract

Collisions of 0.3-5-keV (4-20\ifmmode\times\else\texttimes\fi{}${10}^{6}$ cm/sec) ${\mathrm{N}}_{2}^{+}$ ions with He and Ne targets have been studied by spectroscopic observation of the ${\mathrm{N}}_{2}^{+}$ first-negative-emission spectrum. Vibrational excitation within the collisionally excited $X^{2}\ensuremath{\Sigma}_{g}^{+}\ensuremath{\rightarrow}B^{2}\ensuremath{\Sigma}_{u}^{+}$ transition in ${\mathrm{N}}_{2}^{+}$ was studied by a measurement of the relative band intensities of the $\ensuremath{\Delta}v=\ensuremath{-}1$ sequence of the first-negative system. Rotational excitation was examined by observation of the rotational line intensities of the (0,0) first-negative band ($\ensuremath{\lambda}=3914$ \AA{}A). Although the ${\mathrm{N}}_{2}^{+}$ 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 ${\mathrm{N}}_{2}^{+}$ emission spectra produced by high-velocity ($v>1.2\ifmmode\times\else\texttimes\fi{}{10}^{7}$ 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 \ifmmode^\circ\else\textdegree\fi{}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.