Rotational excitation in the electron impact ionization of supercooled N2

Electrons of controlled energy have been used to ionize and electronically excite N2 cooled to a translational temperature of <2 °K in a supersonic expansion either of pure N2 or 10% N2/Ar. This source likely provides N2 molecules ’’frozen out’’ in their N′′ = 0 and 1 rotational levels. The rotational state distribution of B 2Σ+uN+2 was determined by monitoring the rotationally resolved emission spectrum. Observation showed that even at high (∼500 eV) impact energies, the N+2 rotational distributions are inconsistent with selection rules (ΔN = ±1) for dipole excitation from N′′ = 0 and 1. For lower electron impact energies the angular momentum content of the excited N+2 ion increases sharply. These observations are rationalized by a model which invokes interactions between the ion and the ejected electron.