Vibrational Excitation in Ion-Molecule Collisions: H+, H2+, He+, N+, Ne+, and Electrons on N2

Abstract

A study has been made of the relative band intensities of the $\Delta v=-1\mathrm{}$ sequence of the ${\mathrm{N}}_2^{+}$ first negative system excited by ${\mathrm{H}}^{+}$, ${\mathrm{H}}_2^{+}$, ${\mathrm{He}}^{+}$, ${\mathrm{N}}^{+}$, ${\mathrm{Ne}}^{+}$, and electrons. Projectile ion laboratory velocities from 6 × ${10}^6$ to 1.7 × ${10}^8$ cm/sec (100 eV to 13.5 keV) were used. At ion velocities greater than ${10}^8$ cm/sec, the relative band intensities were found to agree with those predicted by the Franck-Condon principle and found in excitation by 150-eV electrons. Below this velocity the relative population of higher ($v^{\prime }>0\mathrm{}$) vibrational states increased monotonically. At the lowest velocity used, the populations of the $v^{\prime }=0\mathrm{}$ and $v^{\prime }=1\mathrm{}$ vibrational states of ${\mathrm{N}}_2^{+}$ $B{{}_{}{}^2{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}}^{+}$ were found to be equal within experimental error while the higher-state populations increased many orders of magnitude above those predicted by the Franck-Condon principle. The vibrational excitation was found to be solely dependent on the projectile ion's laboratory velocity and independent of its chemical identity. No vibrational excitation was observed in the ${\mathrm{N}}_2$ second positive system excited under similar conditions. It is suggested that the excitation effects in the ${\mathrm{N}}_2^{+}$ first negative system occur by a mechanism involving perturbation of the target molecule's vibrational wave functions by the projectile ion. Implications of these results for other collision studies as well as atmospheric phenomena are discussed.