The UV Spectrum of the Lyman-Birge-Hopfield Band System of N2 Induced by Cascading from Electron Impact

We have measured in the laboratory the far ultraviolet (FUV: 125.0-170.0 nm) cascade-induced spectrum of the Lyman-Birge-Hopfield (LBH) band system (a1Πg→X1Σg+) of N₂ excited by 30-200 eV electrons. The cascading transition begins with two processes: radiative and collision-induced electronic transitions (CIETs) involving two states (a'1Σu- and w¹Δ_u → a¹Π_g), which are followed by a cascade induced transition a1Πg→X1Σg+ at the single-scattering pressures employed here. Direct excitation to the a-state produces a confined LBH spectral glow pattern around an electron beam. We have spatially resolved the electron-induced glow pattern from an electron beam colliding with N₂ at radial distances of 0-400 mm at three gas pressures. This imaging measurement is the first to isolate spectral measurements in the laboratory of single-scattering electron-impact-induced fluorescence from two LBH emission processes: direct excitation, which is strongest in emission near the electron beam axis; and cascading-induced, which is dominant far from the electron beam axis. The vibrational populations for vibrational levels from v' = 0-2 of the a ¹Π_g state are enhanced by radiative cascade and CIETs, and the emission cross sections of the LBH band system for direct and cascading-induced excitation are provided at 40, 50, 100, and 200 eV.