Production of secondary electrons in nitrogen by fast electrons and simultaneous excitation of N2 bands

Abstract

Band emissions from the second positive (2PG) and first negative (1NG) systems of nitrogen were excited by secondary electrons in the pressure range 10−4–1 Torr. The secondary electrons were produced in ionizing collisions between N2 molecules and fast primary electrons. Band intensities were monitored inside and outside the interaction volume of the primary electron beam, enabling one to separate the contributions from secondary and primary electrons to the total excitation. 2PG bands were produced solely by secondary electrons whereas 1NG bands were excited by secondary and/or primary electrons depending on the interaction volume viewed. The intensity ratio of the 2PG(0,0) band to the 1NG(0,0) band excited by slow secondary electrons was found to be about 0.5 in the pressure range 10−3–1 Torr at a primary electron energy of 1000 eV. The pronounced difference in the energy dependence of the excitation cross section of the 2PG and 1NG bands was utilized to infer the characteristics of the energy spectrum of secondary electrons φ(E). Assuming an analytic form φ(E)αE−n for the energy spectrum, a ``spectral index'' n≃1.9 was found using the measured intensity ratio. The energy dependence φ(E)αE−1.9 applies to secondary electron energies between 11 and approximately 200 eV and was found to be in good agreement with binary encounter theory. The present results were compared with atmospheric measurements of auroral emissions and electron spectra.