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

Abstract

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 (a 1 Π g →X 1 Σ þ g ) of N 2 excited by 30-200 eV electrons. The cascading transition begins with two processes: radiative and collision-induced electronic transitions (CIETs) involving two states (a′ 1 Σ − u and w 1 Δ u → a 1 Π g ), which are followed by a cascade induced transition a 1 Π g →X 1 Σ þ g 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 N 2 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 1 Π 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.

Laboratory Study of the Cameron Bands, the First Negative Bands, and Fourth Positive Bands in the Middle Ultraviolet 180–280 nm by Electron Impact Upon CO

“…The experimental apparatus comprised of a large (0.3 m in length) electrostatic electron gun system and the IUVS OEU housed in a large (1.5 m diameter and 2.35 m length) vacuum chamber referred to as the multi-optical beam instrument (MOBI). This experimental setup has been described in detail previously (Ajello et al, 2017;Kanik et al, 2003;Noren et al, 2001), and an experimental schematic is given in Ajello et al (2017Ajello et al ( , 2020. The experimental procedure is a replication of Ajello et al (2019) wherein an electron beam with an energy resolution of ∼1 eV was passed through static CO gas with a chamber pressure of 1 × 10 −5 Torr (n = 3 × 10 11 cm −3 ) and gas swarm temperature of 300 K. The cylindrical emission glow profile produced about the electron beam was measured up to a radial distance of ∼400 mm, moving upward from the electron beam centered initially at 0 mm, using the IUVS OEU mounted to a vertically moving stage, to fully observe and measure optically allowed and forbidden transitions.…”

“…Cameron Band Total Emission Cross Sections From Electron Impact of CO From 180 to 280 nm With an Uncertainty of 35% (Ajello et al, 2020) laboratory temperature. The CB volume emission rate in the cylindrical glow about the electron beam falls off a little faster than 1/r due to the exponential factor with lifetime.…”

“…Most of these metastable molecules drift out of the field of view as a result of thermal motion. It was shown in the past for N 2 that it is possible to determine what fraction of the total emission cross section is actually observed (Ajello et al, 2017(Ajello et al, , 2020Kanik et al, 2003) by using a large apparatus with an imaging detector, and a knowledge of the velocity distribution and the effective lifetime of the metastable state.…”

“…We have started a laboratory aeronomy program at the University of Colorado to study electron impact fluorescence of the CB in the MUV from two parent gases, CO and CO 2 , to match and model the spectra of past, present, and future spacecraft equipped with MUV capabilities to observe the upper atmosphere of Mars and Venus (100-300 km) where both gases are present and abundant (Fox, 2008;Gérard et al, 2019;Krasnopolsky & Feldman, 2002). A thorough understanding of CO CB emissions by the same laboratory technique as performed for the LBH bands (Ajello et al, 2020) from a large chamber will lend greater precision in the spectral analysis of solar system objects. These laboratory measurements of both cross sections and spectra are an important step in allowing forward modeling to determine the excitation rates and volume emission rates in the thermospheres of Mars and Venus (Evans et al, 2015;Gérard et al, 2011Gérard et al, , 2019Hubert et al, 2010;Jain et al, 2015;Stevens et al, 2015).…”

Global‐Scale Observations of the Limb and Disk Mission Implementation: 1. Instrument Design and Early Flight Performance

“…This suggests that the flight spectrum includes contributions from cascade as well as from direct excitation (Eastes, 2000). Direct excitation is the dominant source of emission from the ~1 mm emitting region of the lamp where cascade effects would be negligible (Ajello et al, 2020).…”