Middle ultraviolet and visible spectrum of SO₂ by electron impact

Abstract

[1] Electron-impact-induced fluorescence spectra of SO 2 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 SO 2 bands arise primarily from direct excitation of SO 2 . At 18 and 98 eV, simultaneous excitation and dissociation of SO 2 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 SO 2 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 SO 2 .

Molecular and atomic emission during single- bubble cavitation in concentrated sulfuric acid

“…Note that these emissions are very similar to the emissions observed during electron impact on SO 2 . 12 Weak O 2 ϩ emission is also observed in the spectrum at ϳ230 nm. 13 Since the thermal conductivity of He is much higher than that of Ar and Xe, peak temperatures are expected to be lower; small molecules would therefore dissociate to a lesser degree and would contribute more strongly to the SBSL spectrum.…”

Spatially resolved HST/STIS observations of Io’s dayside equatorial atmosphere

“…We follow the same methods for fitting the continuum emission signature outlined in Jessup et al (2004), thus we assume the spectral distribution measured by Ajello et al (2002) for an electron impact energy of 9 eV (see Fig. 5, Jessup et al, 2004).…”

HST/STIS observations and simulation of Io’s emission spectrum in Jupiter shadow: Probing Io’s Jupiter-facing eclipse atmosphere

“…This is taken to be the corresponding average of complementary half-disk spectra, which are also shown. Laboratory SO 2 dissociative excitation cross sections of SO 2 (e.g., Ajello et al, 2002) extend down only to 2000 Å. However, a pair of laboratory spectra showing S I lines in emission is shown offset (Ajello, personal communication), which were generated by 25 and 200 eV electrons impacting SO 2 vapor (the brighter is 200 eV).…”

“…However, we conclude that the solar contribution to the NUV-visible CCD extracted spectrum below 3000 Å is negligible because: (1) during eclipse Io is not directly illuminated by solar light and (2) based on the Cassini observations of Io in eclipse (Geissler et al, 2004), the contribution from refracted sunlight is expected to be low (see the detailed arguments in the Discussion). For comparison, scaled laboratory spectra of SO 2 at 9 eV and 11 eV are overplotted (the pair of light lines; Ajello et al, 2002). These differ essentially only in the neighborhood of the SO emission, bracketing it.…”

“…The respective 1-r propagated statistical error spectra for Io's nominal spectrum are plotted near the bottom (shallow concave solid lines). Also plotted for comparison are two nearly overlapping scaled laboratory spectra for SO 2 excited by impact of electrons at 9 eV and 11 eV (thin lines peaking near 3150 Å and bracketing the observed SO band; Ajello et al, 2002). These show the sensitivity to electron energy at the SO band although not actually fitted to the data.…”