Middle UV to near‐IR spectrum of electron‐excited SO2

Abstract

We investigated the electron impact–induced fluorescence spectrum of SO2 to provide excitation cross sections for modeling Io's emission spectrum and analyzing Cassini Imaging Science Subsystem observations. The electron‐excited middle‐ultraviolet visible optical near‐infrared (VOIR) emission spectrum of SO2 gas was generated in the laboratory and studied from 2000 to 11,000 Å at a resolution of Δλ ∼ 2.5 Å full width at half maximum (FWHM). The VOIR laboratory spectrum longward of 6000 Å consists entirely of S I, II and O I, II multiplets for electron impact energies above ∼15 eV. Between 2000 and 6000 Å, we find previously identified molecular bands from both SO and SO2. This work represents a significant improvement in spectral resolution over our earlier work done at 18 Å FWHM. From a measurement of the medium‐resolution spectrum, we provide detailed 25‐ and 100‐eV emission cross sections for spectral features from 2000 to 11,000 Å. On the basis of these data, we suggest future ground‐based and satellite telescopic observations in the VOIR that are of promise for understanding Io's atmosphere.