Middle ultraviolet and visible spectrum of SO2 by electron impact

Abstract

Electron‐impact‐induced fluorescence spectra of SO2 in the middle ultraviolet and visible wavelength regions (200–600 nm) have been measured in the laboratory using a crossed beam experiment at three electron impact energies. The emission spectra at 8, 18, and 98 eV exhibit a broad and continuous emission region extending from 225 to near 600 nm with a peak emission close to 330 nm. The quasicontinuous SO2 bands arise primarily from direct excitation of SO2. At 18 and 98 eV, simultaneous excitation and dissociation of SO2 produces distinct vibrational bands from SO and from atomic emission lines from S I, S II, O I, and O II that are superimposed on the SO2 electronic transitions. The laboratory spectra were compared to green/violet color ratios obtained at Io by the Galileo Orbiter Solid State Imaging experiment. The laboratory spectra were also applied to the Cassini Imaging Subsystem to determine which filter combinations are particularly sensitive to electron energy, if the atmospheric gas present in the auroral atmosphere is solely or primarily SO2.