Rotational Excitation in Ion-Molecule Collisions: The SystemH2++N2

A study has been made of the (0, 0) and (0, 1) bands of the ${\mathrm{N}}_2^{+}$ first negative system excited by collisions between ${\mathrm{H}}_2^{+}$ and ${\mathrm{N}}_2$. ${\mathrm{H}}_2^{+}$ projectile ion energies from 0.4 to 3.0 keV were used. An improved multistage apparatus produced sufficient ion current to allow the rotational lines up to $K^{\prime }=50\mathrm{}$ to be resolved and measured at all projectile ion energies. The energy distribution among rotational states was found to be grossly different from a Boltzmann distribution. The system could not be described by a single rotational temperature although the energy distribution among the high rotational states had a characteristic temperature in excess of 3000°K. The excitation of high rotational states was found to increase with decreasing projectile ion velocity. The effect of overlapping branches on the rotational line intensities was calculated and found to be an extremely important effect under these conditions. A review of previous experiments in the light of these data suggests that rotational excitation is a widespread phenomenon in ion-molecule collisions.