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[1] Integral cross sections for electron impact excitation out of the ground state (X 1 S g + ) to, a 1 Å g , w 1 D u , and C 3 Å u states in N 2 are reported at incident energies ranging between 10 and 100 eV. These data have been derived by integrating differential cross sections previously reported by this group. New differential cross section measurements for the a 1 Å g state at 200 eV are also presented to extend the range of the reported integral cross sections for this state, which is responsible for the emissions of the Lyman-Birge-Hopfield band system (a 1 Å g ! X 1 S g + ). The present results are compared and critically evaluated against existing cross sections. In general, the present cross sections are smaller than previous results at low impact energies from threshold through the excitation function peak regions. These lower cross sections have potentially significant implications on our understanding of UV emissions in the atmospheres of Earth and Titan.

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 ...

We present a spectral analysis of the far ultraviolet (FUV: 1150–1900 Å) disk airglow observations of Titan's atmosphere by the Cassini Ultraviolet Imaging Spectrograph (UVIS). The FUV spectrum consists of emissions from the Lyman‐Birge‐Hopfield (LBH) band system of N2 excited by photoelectrons (a 1Πg → X 1Σg+), N I multiplets from solar photodissociative excitation of N2, resonantly scattered solar H Ly‐α and sunlight reflected by N2 in the mesosphere‐stratosphere and modified by aerosols (e.g., tholins) and hydrocarbon absorption. Below 1450 Å, the strongest emissions arise from H Ly‐α with an intensity of 208 Rayleighs (R), LBH bands with an intensity of 43 R, and the N I multiplets with a combined intensity of 16 R. Above 1450 Å, most of the UVIS signal is due to reflected sunlight. Mixing ratios of tholins, C2H2, C2H4 and C4H2 have been derived from the reflected sunlight using a Rayleigh scattering model. The derived mixing ratios are in good agreement with Voyager infrared observations and with FUV photochemical models, assuming solar energy deposition above 1450 Å occurs near 250 km (Wilson and Atreya, 2004). We also present the first geometric albedo measurement of Titan from 1500–1900 Å.