Rotational and Vibrational Excitation of Nitrogen by Electron Impact

Abstract

Rotational excitation of nitrogen by low-energy electron impact has posed an unsolved problem for more than three decades. Early analysis of the results of swarm experiments in nitrogen found that the data could be matched remarkably well by assuming that the energy dependences of the Δj = 2 cross sections from threshold to a few tenths of an eV are given by a simple formula based on the Born approximation. Moreover, the quadrupole moment (the only adjustable parameter in the formula) which gave the best fit to the data was commensurate with existing experimental values. This finding posed an enigma, since the quadrupole Born expression is known to incorrectly represent the interaction potential for scattering except within a few meV of threshold. We have analysed new swarm data, taken in a dilute mixure of nitrogen in neon, using theoretical rotational and momentum transfer cross sections based on a solution of the Schrödinger equation using static, exchange, and polarisation potentials. This work explains the long-standing enigma and provides the basis for a subsequent analysis in which theoretical vibrational excitation cross sections are also investigated using the new swarm data for the mixture.