The complete UV spectrum of SO₂ by electron impact, 2, The middle ultraviolet spectrum

Abstract

We have measured the middle ultraviolet (MUV) electron impact induced fluorescence spectrum of SO2 from 200 to 430 nm in a crossed beam experiment. The spectrum is dominated by two features at the experimental resolution of 0.5 nm. These two features are referred to as MUV 1 and MUV2. MUV 1 is the SO(A 3H --• X3X -) band system extending from 240 to 265 nm produced by dissociative I I excitation. MUV2 is a blend of the SO2 (•(A2),/•(B1)--• •(1A1)) and SO•-(C(2B2)--• •(2A 1)) molecular band systems in the range 2• •.30 nm. In addition, the excitation function measurements of MUV2 indicate that low-energy electrons effectively prepare SO2 in one or more electronically excited triplet states that involve the SO2 (d(3B1)) state by direct excitation and/or cascading. A candidate triplet band system for this emission process is the E-t? electronic transition. This emission process is the largest contributor to the MUV2 emission cross section at low electron impact energy. The peak cross section for MUVl occurs at 20 eV with a value of 25.0 _+ 5.5 x 10 -19 cm 2. The peak cross section for MUV2 arises at 9 eV with a value of 368 -+ 81 x 10 -19 cm 2. The laboratory measurements of the excitation functions of both MUV features were made from 0 to I keV. The emission cross sections are an important p• of the total inelastic cross section of SO2 needed in modeling the Io atmosphere. The laboratory results give a plausible explanation of the Io auroral hot soots observed by Voyager 2. INTRODU•ION In 1979 the Voyager 1 spacecraft observed intense auroral glows of SO2 issuing from plumes associated with volcanoes on Io [Cook et al., 1981]. The detection threshold in the full disk images was estimated to be 30 kR in the 200-rim-wide clear filter at wavelengths from the UV to the visible. The emission was seen from both polar re•ons and the •lumihated disk. Although the auroral activity scenario suggests electron impact excitation of SO2 as the prim• mechanism, there has been no discussion as to the molecular states involved in the dectron impact induced fluorescence process. Other direct evidence for SO2 in the atmosphere of Io comes from the Voyager IR experiment observation of the fundamental •'3 band. Inversion of the the•al emission band spectrum led to the first estimate of the column abundance [Pearl et al., 1979]. Recent publications describing observational data of Io have been •ted in the preceding paper [Ajello eta!., this issue], referred to as paper 1. Paper 1 has discussed the extreme ultraviolet (EUV) spectrum, from 40 to 120 nm, and the l•x •traviolet (FUV) spectrum, from 120 to 200 nm, produced by electron impact excitation of SO2. It is our goal in paper 2 to continue the study into the middle ultraviolet (MUV), a range we define as extending from 200 to 430 rim. This spectral region is to be e_xplored in 1995 by the Galileo spacecraft dm'ing its satellite tour of Jupiter [Hord et al., 1•]. In addition, F•arth-based satellites equipped Mth UV spectrometers plan to study lo in portions of the FUV arm MUV region. ...

Eclipse Spectroscopy of Io's Atmosphere

“…Red glows observed in the equatorial plumes are therefore likely to be due to either dissociatively excited oxygen derived from SO 2 or to the optical tail of ultraviolet band emission of collisionally excited SO 2 (Geissler et al 1999, Ajello et al 1992. The nearly uniform red limb brightening, however, must arise from a globally distributed atmospheric constituent.…”

Electron impact dissociative excitation of O2: 2. Absolute emission cross sections of the OI(130.4 nm) and OI(135.6 nm) lines

“…The presence of the spin‐forbidden emissions at 135.6 nm or 630.0 nm indicates the existence of a strong electron excitation source as opposed to the solar fluorescence excitation mechanism possible for OI(130.4 nm). For example, in the Jovian system the emissions of OI(135.6 nm) and OI(130.4 nm) tend to dominate the FUV airglow spectrum on Io [ Ajello et al , 1992a, 1992b; Clarke et al , 1994], Europa [ Hall et al , 1995, 1998], and Ganymede [ Hall et al , 1998; Feldman et al , 2000]. Even on Earth, the intensity of the FUV spectrum contains 90% atomic oxygen emissions [ Torr et al , 1995].…”

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

“…An excellent agreement between the earlier work of Ajello et al [2002] and the present work for the same wavelength range is shown in Figure 5. The two broad molecular structures centered near 2550 Å and 3600 Å have been previously labeled MUV1 (2385–2670 Å) and MUV2 (2670–6000 Å) and thoroughly discussed by Ajello et al [1992b]. In brief, the two excitation processes for MUV1 are as follows: Many excitation processes contribute to MUV2.…”

“…We have been supporting the NASA missions engaged in Io monitoring through a laboratory program that measures electron impact–induced spectra and derives cross sections of SO 2 in the UV [ Vatti Palle et al , 2004; Ajello et al , 1992a] and Visible [ Ajello et al , 1992b, 2002] and atomic O in the UV [ Noren et al , 2001; Johnson et al , 2003]. Our atomic and molecular data have been used most recently in the analysis of the Galileo Solid State Imaging observations of Io [ Geissler et al , 1999], Cassini spacecraft observations of Io by the Imaging Science Subsystem (ISS) [ Porco et al , 2003; Geissler et al , 2004], and the Ultraviolet Imaging Spectrograph [ Esposito et al , 2005].…”