Laboratory Study of the Cameron Bands and UV Doublet in the Middle Ultraviolet 180–300 nm by Electron Impact upon CO2 with Application to Mars

Abstract

We have observed electron impact fluorescence from CO2 to excite the Cameron bands (CBs), CO (a 3Π → X 1Σ+; 180–280 nm), the first-negative group (1NG) bands, CO+ (B 2Σ+ → X 2Σ+; 180–320 nm), the fourth-positive group (4PG) bands, CO (A 1Π → X 1Σ+; 111–280 nm), and the UV doublet, CO2 + ( 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 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.