Electron impact study of the 100 eV emission cross section and lifetime of the Lyman‐Birge‐Hopfield band system of N2: Direct excitation and cascade

We have measured the 100 eV emission cross section of the optically forbidden Lyman‐Birge‐Hopfield (LBH) band system (a1Πg→ X1Σ+g) of N2by electron‐impact‐induced fluorescence. Using a large (1.5 m diameter) vacuum chamber housing an electron gun system and the Mars Atmosphere and Volatile EvolutioN mission Imaging Ultraviolet Spectrograph optical engineering model, we have obtained calibrated spectral measurements of the LBH band system from 115 to 175 nm over a range of lines of sight to capture all of the optical emissions. These measurements represent the first experiment to directly isolate in the laboratory single‐scattering electron‐impact‐induced fluorescence from both direct excitation of the a1Πgstate and cascading contributions to the a1Πgstate (a′1Σ−uand w1Δu→ a1Πg→ X1Σ+g). The determination of the total LBH emission cross section is accomplished by measuring the entire cylindrical glow pattern of the metastable emission from electron impact by imaging lines of sight that measure the glow intensity from zero to ~400 mm radial distance and calculating the ratio of the integrated intensity from the LBH glow pattern to that of a simultaneously observed optically allowed transition with a well‐established cross section: Ni120.0 nm. The “direct” emission cross section of the a1Πgstate at 100 eV was determined to be σemdir= (6.41 ± 1.3) × 10−18cm2. An important observation from the glow pattern behavior is that the total (direct + cascading) emission cross section is pressure dependent due to collision‐induced cascade transitions between close‐lying electronic states. Measured 100 eV total emission cross section of the optically forbidden Lyman‐Birge‐Hopfield (LBH) band system of N2by electron‐impact‐induced fluorescenceLaboratory experiment isolating single‐scattering electron‐impact‐induced fluorescence from direct excitation and cascading contributions to the a1Πgstate of N2Lifetime for the optically forbidden a1Πgstate is found to be ~50 ± 15 μs