High-resolution far ultraviolet spectrum of electron-excited SO₂

Abstract

The high-resolution UV capabilities (λ/Δλ = 10⁵) of the Hubble Space Telescope (HST) equipped with the Space Telescope Imaging Spectrograph (STIS) reflects a need for a high-resolution laboratory UV spectral data base for comparison with observation. For the purpose of interpreting the astronomical observations of Io by HST the electron-excited UV spectrum of SO₂ gas has been studied from 800–1700 Å at medium (λ/Δλ ∼ 10³) and high resolution (λ/Δλ ∼ 5 × 10⁴). The far ultraviolet (FUV) laboratory spectrum consists entirely of S I, II and O I, II multiplets. From a measurement of the medium resolution spectrum at 1.5 Å FWHM we are able to provide new detail in the 200 eV emission cross sections for all the FUV features. At 30 and 100 eV electron-impact energy we have measured high resolution emission spectra of the fine structure lines of the atomic multiplets at 1256 Å from S II, at 1304 Å from O I, 1479 Å from S I and at 1429 Å from S I at 100 eV electron-impact energy. At 100 eV electron-impact energy we compare the optically thin laboratory spectrum of the atomic multiplet S I at 1479 Å with the model spectrum of electron excitation of atomic sulfur. In addition, we compare a HST STIS observation of Io with the G140M grating (FWHM ∼ 0.5 Å) at 1479 Å with a laboratory spectrum after transmission through a layer of atomic S. We have also compared our laboratory spectrum to FUV observations by STIS reported by Roesler et al. [1999] . The relative intensities of the strongest lines of S I and O I are compared with the laboratory emission cross sections for SO₂ near 40 eV (3 Ryd) electron energy. The Doppler line profiles of individual fine structure lines of atomic sulfur and oxygen were used to ascertain the kinetic energy distribution from dissociative excitation at 30 and 100 eV. At 100 eV electron-impact energy we find the kinetic energy distribution of the oxygen and sulfur atoms to be in the range of 2 eV to 3 eV. At 30 eV electron-impact energy we place an upper limit on the kinetic energy of sulfur and oxygen atoms as <1 eV. We provide the first measurement of the O I] 1356 Å absolute excitation function and the estimate of the cross section ratio O I] 1356 Å/1304 Å as a function of energy. We establish the absolute cross section of O I 1304 Å as 2.3 ± 0.5 × 10^-18 cm² at 100 eV and the estimated cross section of O I] 1356 Å as 2.1 ± 1 × 10^-18 cm² at 100 eV.