Experimental verification of the breakdown of the electric dipole rotational selection rule in electron impact ionization–excitation of N2

Abstract

Variable energy (60–1500 eV) electron impact excitation of cold (∼10 K) N2 supersonic beams has been used to produce excited N+2 B 2Σ+u ions whose rotational energy distribution has been determined by spectroscopic examination of N2+ first negative system (B 2Σ+u→X 2Σ+g) emissions. For energies ≳800 eV, the rotational state distribution is found to be constant with electron energy. The distribution is non-Boltzmann with a peak at the N′=1 rotational state and a long tail to higher energies. At high electron energies, the electric dipole selection rule (‖ΔJ‖=1) is expected to be obeyed in the ionization–excitation process. Above 800 eV, this rule is followed, and the non-Boltzmann B 2Σu+ rotational distribution arises from the non-Boltzmann rotational distribution in the neutral supersonic N2 beam. Below 800 eV, however, breakdown of the ‖ΔJ‖=1 selection rule occurs with ‖ΔJ‖=3 transitions observed first, followed by much larger ΔJ transitions at low electron energies (<100 eV). Implications of this effect for geophysics and supersonic beam diagnostics, as well as the theoretical questions involved, are discussed.