THE COMPLETE UV SPECTRUM OF SO2 BY ELECTRON-IMPACT .1. THE VACUUM ULTRAVIOLET-SPECTRUM

We have measured the vacuum ultraviolet (VUV) emission cross sections of SO2 from 40 to 200 nm in the laboratory in a crossed beam experiment. The wavelength range comprised the extreme ultraviolet (EUV) spectrum, from 40 to 120 nm, and the far ultraviolet (FUV) spectrum, from 120 to 200 nm. The EUV and FUV emission spectra induced by electron impact consist of atomic multiplets of S I, II, and III and O I, II, and III. The studies included the measurement of excitation functions of the strongest multiplets from 0 to 1 keV impact energy and the identification of dissociation processes from the analysis of the threshold excitation function. The O I (98.9 nm) resonance multiplet (g3P-3D) is the strongest EUV feature from electron impact induced fluorescence of SO2 with a peak cross section of 7.82 +/- 1.59 x 10(-19) cm2 at 125 eV electron impact energy. The O I (130.4 nm) multiplet (g3P-3S) resonance transition is the strongest feature in the FUV, with a peak cross section of 1.98 +/- 0.44 x 10(-18) cm2 at 90 eV. The strongest feature of S I in the VUV is the S I (147.9 nm) multiplet (g3P-3D) resonance transition, with a peak cross section of 1.52 +/- 0.33 x 10(-18) cm2 at 90 eV. Dissociation-excitation cross sections of SO2 are important contributors to the total inelastic cross section of SO2 gas interacting with electrons. These cross sections are used in neutral cloud theory models of the Io atmosphere. The laboratory spectrum at low energy (approximately 22 eV) matches a recently obtained IUE FUV spectrum of Io. The comparison represents evidence of a thin or intermediate atmosphere of SO2 that is directly impacted by the plasma torus. The existence of an atmosphere on Io embedded in the corotating torus plasma is an important remote sensing target for the Hubble Space Telescope, Goddard High Resolution Spectrograph and Faint Object Spectrograph, and Galileo EUV and FUV spectrometers, among other present and planned VUV studies of the Jovian system.