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1. Laboratory Study of the Cameron Bands and UV Doublet in the Middle Ultraviolet 180–300 nm by Electron Impact upon CO 2 with Application to Mars

   https://doi.org/10.3847/1538-4357/ac88c8  

   Lee, Rena A. ; Ajello, Joseph M. ; Malone, Charles P. ; Evans, J. Scott ; Veibell, Victoir ; Holsclaw, Gregory M. ; McClintock, William E. ; Hoskins, Alan C. ; Jain, Sonal K. ; Gérard, Jean-Claude ; et al ( October 2022 , The Astrophysical Journal)

   We have observed electron impact fluorescence from CO₂to excite the Cameron bands (CBs), CO (a³Π →X¹Σ⁺; 180–280 nm), the first-negative group (1NG) bands, CO⁺(B²Σ⁺→X²Σ⁺; 180–320 nm), the fourth-positive group (4PG) bands, CO (A¹Π →X¹Σ⁺; 111–280 nm), and the UV doublet, CO₂⁺($\tilde{B}{}^2{\mathrm{\Sigma }}_u^{+}\to \tilde{X}{}^2{\mathrm{\Pi }}_g;$288.3 and 289.6 nm) in the ultraviolet (UV). This wavelength range matches the spectral region of past and present spacecraft equipped to observe UV dayglow and aurora emissions from the thermospheres (100–300 km) of Mars and Venus. Our large vacuum system apparatus is able to measure the emission cross sections of the strongest optically forbidden UV transitions found in planetary spectra. Based on our cross-sectional measurements, previous CB emission cross-sectional errors exceed a factor of 3. The UV doublet lifetime is perturbed through$\tilde{B}{}^2{\mathrm{\Sigma }}_u^{+}-\tilde{A}{}^2{\mathrm{\Pi }}_u$spin–orbit coupling. Forward modeling codes of the Mars dayglow have not been accurate in the mid-UV due to systematic errors in these two emission cross sections. We furnish absolute emission cross sections for several band systems over electron energies 20–100 eV for CO₂. We present a CB lifetime, which together with emission cross sections, furnish a set of fundamental physical constants for electron transport codes such as AURIC (Atmospheric Ultraviolet Radiance Integrated Code). AURIC and Trans-Mars are used in the analysis of UV spectra from the Martian dayglow and aurora.

    

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2. UV Study of the Fourth Positive Band System of CO and O i 135.6 nm From Electron Impact on CO and CO 2

   https://doi.org/10.1029/2018JA026308  

   Ajello, J. M. ; Malone, C. P. ; Evans, J. S. ; Holsclaw, G. M. ; Hoskins, A. C. ; Jain, S. K. ; McClintock, W. E. ; Liu, X. ; Veibell, V. ; Deighan, J. ; et al ( April 2019 , Journal of Geophysical Research: Space Physics)

   We have measured the 30 and 100 eV far ultraviolet (FUV) emission cross sections of the optically allowed Fourth Positive Group (4PG) band system (A¹Π → X¹Σ⁺) of CO and the optically forbidden O (⁵S^o → ³P) 135.6 nm atomic transition by electron‐impact‐induced‐fluorescence of CO and CO₂. We present a model excitation cross section from threshold to high energy for theA¹Π state, including cascade by electron impact on CO. TheA¹Π state is perturbed by triplet states leading to an extended FUV glow from electron excitation of CO. We derive a model FUV spectrum of the 4PG band system from dissociative excitation of CO₂, an important process observed on Mars and Venus. Our unique experimental setup consists of a large vacuum chamber housing an electron gun system and the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission Imaging Ultraviolet Spectrograph optical engineering unit, operating in the FUV (110–170 nm). The determination of the total Oi(⁵S^o) at 135.6 nm emission cross section is accomplished by measuring the cylindrical glow pattern of the metastable emission from electron impact by imaging the glow intensity about the electron beam from nominally zero to ~400 mm distance from the electron beam. The study of the glow pattern of Oi(135.6 nm) from dissociative excitation of CO and CO₂indicates that the Oi(⁵S^o) state has a kinetic energy of ~1 eV by modeling the radial glow pattern with the published lifetime of 180 μs for the Oi(⁵S^o) state.

    

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3. A Missing Piece of the E‐Region Puzzle: High‐Resolution Photoionization Cross Sections and Solar Irradiances in Models

   https://doi.org/10.1029/2023JA031870  

   Soto, Emmaris ; Evans, J. Scott ; Meier, R. R. ; Tashiro, Motomichi ; Sakib, MD. Nazmus ; Yiğit, Erdal ( April 2024 , Journal of Geophysical Research: Space Physics)

   Most ionospheric models cannot sufficiently reproduce the observed electron density profiles in the E‐region ionosphere, since they usually underestimate electron densities and do not match the profile shape. Mitigation of these issues is often addressed by increasing the solar soft X‐ray flux which is ineffective for resolving data‐model discrepancies. We show that low‐resolution cross sections and solar spectral irradiances fail to preserve structure within the data, which considerably impacts radiative processes in the E‐region, and are largely responsible for the discrepancies between observations and simulations. To resolve data‐model inconsistencies, we utilize new high‐resolution (0.001 nm) atomic oxygen (O) and molecular nitrogen (N₂) cross sections and solar spectral irradiances, which contain autoionization and narrow rotational lines that allow solar photons to reach lower altitudes and increase the photoelectron flux. This work improves upon Meier et al. (2007,https://doi.org/10.1029/2006gl028484) by additionally incorporating high‐resolution N₂photoionization and photoabsorption cross sections in model calculations. Model results with the new inputs show increased O⁺production rates of over 500%, larger than those of Meier et al. (2007,https://doi.org/10.1029/2006gl028484) and total ion production rates of over 125%, while production rates decrease by ∼15% in the E‐region in comparison to the results obtained using the cross section compilation from Conway (1988,https://apps.dtic.mil/sti/pdfs/ADA193866.pdf). Low‐resolution molecular oxygen (O₂) cross sections from the Conway compilation are utilized for all input cases and indicate that is a dominant contributor to the total ion production rate in the E‐region. Specifically, the photoionization contributed from longer wavelengths is a main contributor at ∼120 km.

    

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

   https://doi.org/https://doi.org/10.1029/2019JA027546  

   Joseph M. Ajello1, J. Scott ( January 2020 , Journal geographic)

   null (Ed.)

   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 1Πg → X 1Σg+) 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 w 1Δu → a 1Πg), which are followed by a cascade induced transition a 1Πg → X 1Σg+. 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 1Πg state are enhanced by CIETs, and the emission cross sections of the LBH band system for direct and cascading-induced excitation are provided at 40, 100, and 200 eV 

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5. The Photoionization Dynamics, Electronic Spectroscopy, and Excited State Photochemistry of AlCO and AlOC

   https://doi.org/10.3847/1538-4357/ac75be  

   Esposito, Vincent J. ; Francisco, Joseph S. ( July 2022 , The Astrophysical Journal)

   The high cosmic abundance of carbon monoxide (CO) and the ubiquitous nature of aluminum-coated dust grains sets the stage for the production of weakly bound triatomic molecules AlCO (X²Π) and AlOC (X²Π) in circumstellar envelopes of evolved stars. Following desorption of cold AlCO and AlOC from the dust grain surface, incoming stellar radiation in the 2–9 eV wavelength range (visible to vacuum ultraviolet) will drive various photochemical processes. Ionization to the singlet cation state will cause an immediate Al–X (X = C, O) bond dissociation to form Al⁺(¹S) and CO (X¹Σ⁺) coproducts, whereas ionization to the higher-lying triplet states will lead to stabilization of AlCO⁺(X³Π) and AlOC⁺(X³Π) in deep potential wells. In competition with ionization is electronic excitation. Excitation to the spectroscopically bright 1²Π and 2²Σ⁺states will lead to either highly Stokes-shifted fluorescence, or photodissociation to yield Al (²D) + CO (X¹Σ⁺) products via nonadiabatic pathways, making AlCO and AlOC good candidates for electronic experimental studies. These many photoinduced pathways spanning orders of magnitude of the electromagnetic spectrum will lead to the depletion of AlCO and AlOC in astronomical environments, potentially explaining the lack of observational detection of these molecules. Furthermore, these results indicate new catalytic pathways to the freeing of aluminum atoms trapped in solid aluminum dust grains. Additionally, the results herein implicate an ion–neutral reaction as a possible important pathway in [Al, C, O] cation formation.

    

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