Excitation of Nitrogen by Electrons: The Lyman–Birge–Hopfield System of N2

Abstract

The intensity of vacuum ultraviolet emission produced by a beam of electrons in nitrogen has been measured to determine the probability of exciting the Lyman–Birge–Hopfield (LBH) bands of N2. Measurements included spectral scans of the region 1250–2150 Å and scans across the electron beam with a photometer sensitive in the range 1230–1800 Å. Some 60 spectral features were attributed to LBH bands. Emission in these bands persisted in a glowing region outside the electron beam. At low gas pressure (0.1 μ), relative band intensities are in rough agreement with values calculated by assuming the Franck–Condon principle applies in excitation and emission. At higher pressure, the intensity was increased by secondary processes, and bands with lower υ′ were enhanced relative to other bands. Photometer scans at low pressure yielded a profile of the glow around the beam which was fairly consistent with a profile calculated by assuming a radiative lifetime of 80 μsec for the parent state of the LBH. Based on the calculated profile and spectrometer and photometer observations, an apparent cross section of 1.1 ± 0.2 × 10−18 cm2 was obtained for excitation of the LBH system by 900-eV electrons. The dependence on energy of representative bands indicated total LBH cross sections for 100- and 2000-eV electrons of 6.7 × 10−18 and 5.2 × 10−19 cm2, respectively.