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)Pi -> X-1 Sigma(+)) of CO and the optically forbidden O (S-5(o) -> P-3) 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 the A(1)Pi state, including cascade by electron impact on CO. The A(1)Pi 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 O I (S-5(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 similar 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 CO2 indicates that the O I (S-5(o)) state has a kinetic energy of similar to 1 eV by modeling the radial glow pattern with the published lifetime of 180 mu s for the O I (S-5(o)) state. Plain Language Summary Both Mars and Venus have upper atmospheres that are similar in composition: mostly CO2, 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.