UV Study of the Fourth Positive Band System of CO and O i 135.6 nm From Electron Impact on CO and CO₂

Abstract

We have measured the 30 and 100 eV far ultraviolet (FUV) emission cross sections of the optically allowed Fourth Positive Group (4PG) band system (A 1 Π → X 1 Σ + ) of CO and the optically forbidden O ( 5 S o → 3 P) 135.6 nm atomic transition by electron-impact-induced-fluorescence of CO and CO 2 . We present a model excitation cross section from threshold to high energy for the A 1 Π state, including cascade by electron impact on CO. The A 1 Π 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 CO 2 , 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 O I ( 5 S 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 O I (135.6 nm) from dissociative excitation of CO and CO 2 indicates that the O I ( 5 S o ) state has a kinetic energy of~1 eV by modeling the radial glow pattern with the published lifetime of 180 μs for the O I ( 5 S o ) state.Plain Language Summary Both Mars and Venus have upper atmospheres that are similar in composition: mostly CO 2 , CO, and N 2 are dominant molecular gases, with nearly identical UV spectra. The modeling studies of atmospheric UV emissions cannot presently be accurately conducted to the same accuracy as the planetary UV measurements, which avail themselves with state-of-the-art calibrated spectrographs. This dichotomy in accuracy between planetary observation and model occurs because the atomic and molecular emission cross sections with uncertainties of certain transitions are greater than 100%. Furthermore, the analysis is complicated by the spectral blending of the various emissions of the low-resolution spectral spaceborne instruments. We present in this paper a UV laboratory instrument unique in the world at the University of Colorado that can measure for the first time the excitation mechanisms with accurate emission cross sections of both allowed and optically forbidden transitions that are occurring in a planetary atmosphere. Key Points: • We measured 30 and 100 eV emission cross sections of Fourth Positive band system of CO and O I (135.6 nm) from electron impact on CO and CO 2 • We conducted a laboratory experiment measuring single-scattering electron-impactinduced-fluorescence FUV spectra from excitation of CO and CO 2 • Fragment kinetic energy measurement for O I ( 5 S o ) atoms found to be around~1 eV from both CO and CO 2 molecular dissociation Correspondence to: et al. (2019). UV study of the Fourth Positive band system ...

Characteristics of Mars UV Dayglow Emissions From Atomic Oxygen at 130.4 and 135.6 nm: MAVEN/IUVS Limb Observations and Modeling

“…This value is then in very good agreement with the latest measurements by Ajello et al () that provide new results at 30 eV and at 100 eV, even though the value at 30 eV is about 30% higher in the Ajello measurement than in the scaled Mumma cross section. The shape of the cross section at 135.6 nm has been measured by Ajello () and is now rescaled by a factor of 0.18, taking into account the measurements by Ajello et al (). The electron impact excitation cross section reaches a maximum value of 1.9 × 10 −19 cm 2 at 115 eV.…”

“…e + CO 2 : For the electron impact on CO 2 resulting in 130.4‐nm emission, the cross section from Mumma et al () is adapted with a scaling factor of 0.61 as suggested by Itikawa (), resulting in a maximum value of 6.1 × 10 −19 cm 2 at 115 eV. This value is then in very good agreement with the latest measurements by Ajello et al () that provide new results at 30 eV and at 100 eV, even though the value at 30 eV is about 30% higher in the Ajello measurement than in the scaled Mumma cross section. The shape of the cross section at 135.6 nm has been measured by Ajello () and is now rescaled by a factor of 0.18, taking into account the measurements by Ajello et al ().…”

“…The peak cross section for the 130.4‐nm triplet at 110 eV amounts to 8.0 × 10 −19 cm 2 , and the cross section for the 135.6‐nm doublet is estimated to be smaller by a factor of 0.85 (6.8 × 10 −19 cm 2 at 100 eV). According to Ajello et al () the cross section for the O( 3 S) production (130.4 nm) is slightly higher (8.8 × 10 −19 cm 2 at 100 eV) but even significantly lower for the O( 5 S) state (135.6 nm): 1.8 × 10 −19 cm 2 at 100 eV. The values are therefore comparable with the cross sections for electron impact on CO 2 , but at the respective altitudes between 100 and 150 km, the CO density is at least an order of magnitude smaller than the CO 2 density.…”

“…Other prominent features in the figure are, for example, the dominant Lyman‐α line at 121.6 nm, atomic carbon lines at 127.8, 156.1, and 165.7 nm, a CII line at 133.5‐nm and several lines of the CO 4th positive bands. A more complete list can be found, for example, in Ajello et al (). Figure b displays the corresponding limb profiles of the 130.4‐nm (blue) and 135.6‐nm (red) emissions.…”

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

“…Each source was treated as a free parameter and constrained with the requirement that fit coefficients be nonnegative. Molecular sources were modeled using AURIC rovibrational band models for CB, 4PG, and 1NG (Ajello et al., 2019; Stevens et al., 2015). The rotational temperature used for the three band models was 300 K. Comparisons of the measured electron‐impact‐fluorescence CO total spectra (black) with the MLR best fit (red) are shown in Figures 4a, 4b, and 4c for 15, 30, and 100 eV, respectively.…”