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

We have analyzed medium-resolution (full width at half maximum, FWHM = 1.2 nm), Middle UltraViolet (MUV; 180-280 nm) laboratory emission spectra of carbon monoxide (CO) excited by electron impact at 15, 20, 40, 50, and 100 eV under single-scattering conditions at 300 K. The MUV emission spectra at 100 eV contain the Cameron Bands (CB) CO(a (3)Pi -> X (1)Sigma (+)), the fourth positive group (4PG) CO(A (1)Pi -> X (1)Sigma (+)), and the first negative group (1NG) CO+(B (2)Sigma (+) -> X (2)Sigma) from direct excitation and cascading-induced emission of an optically thin CO gas. We have determined vibrational intensities and emission cross sections for these systems, important for modeling UV observations of the atmospheres of Mars and Venus. We have also measured the CB glow profile about the electron beam of the long-lived CO (a (3)Pi) state and determined its average metastable lifetime of 3 1 ms. Optically allowed cascading from a host of triplet states has been found to be the dominant excitation process contributing to the CB emission cross section at 15 eV, most strongly by the d (3)Delta and a' (3)Sigma (+) electronic states. We normalized the CB emission cross section at 15 eV electron impact energy by multilinear regression (MLR) analysis to the blended 15 eV MUV spectrum over the spectral range of 180-280 nm, based on the 4PG emission cross section at 15 eV that we have previously measured (Ajello et al., 2019, ). We find the CB total emission cross section at 15 eV to be 7.7 x 10(-17) cm(2). Plain Language Summary Carbon monoxide (CO) is an important molecule in the atmospheres of many planetary bodies, including Mars and Venus. These two planets have nearly identical UV spectra, exhibiting CO, CO2, and N-2 as dominant components of the upper atmospheres. Thorough investigation of atmospheric emissions relies on spectral models of individual emission features, which depend on laboratory-based measurements of the emission cross sections and metastable lifetimes. In this paper, we trace the history of the emission cross section work for electron impact on CO, identify previous experimental systematic flaws, and present corrected values for the emission cross sections of key Middle UltraViolet (MUV; 180-280 nm) emission features over several electron impact energies (15-100 eV). We give special emphasis to the Cameron bands (CBs), the strongest molecular band system in the MUV dayglow observed on Mars. A successful spectral study has neither been accomplished nor been possible until now, with our unique experimental regime at the University of Colorado, which can routinely study optically forbidden transitions. Our emission cross section results for the optically forbidden CB system are vital for increased precision, especially in investigations utilizing electron transport codes (e.g., AURIC, Trans-Mars) to retrieve composition and emission rate measurements in upper atmospheres. Key Points Key Measured 15, 20, 40, 50, and 100 eV emission cross sections of Cameron Bands (CO) and 1NG Bands (CO+) from electron impact fluorescence of CO Developed rovibrational spectral models of Cameron Band, 1NG, and 4PG systems based upon optically thin middle ultraviolet laboratory spectra of e + CO Measured lifetime of the CO(a (3)Pi) state to be 3 +/- 1 ms

Automated summary

This study analyzed laboratory emission spectra of carbon monoxide (CO) excited by electron impact in the Middle UltraViolet (MUV) range and determined vibrational intensities and emission cross sections for various systems, which are important for modeling UV observations of the atmospheres of Venus and Mars. The results provide valuable insights for understanding the excitation processes of CO molecules under electron impact, particularly in the context of spectral observations in the upper atmospheres of different planets.