Middle ultraviolet and visible spectrum of SO 2 by electron impact

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A7, 1099, /2001JA000122, 2002 Middle ultraviolet and visible spectrum of SO 2 by electron impact J. M. Ajello, D. L. Hansen, L. W. Beegle, C. A. Terrell, I. Kanik, G. K. James, and O. P. Makarov Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA Received 3 May 2001; revised 14 September 2001; accepted 17 September 2001; published 9 July [1] Electron-impact-induced fluorescence spectra of SO 2 in the middle ultraviolet and visible wavelength regions ( 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. INDEX TERMS: 0310 Atmospheric Composition and Structure: Airglow and aurora; 6218 Planetology: Solar System Objects: Jovian satellites; KEYWORDS: ultraviolet, sulfur dioxide, cross sections, Io, spectroscopy, Galileo 1. Introduction [2] Collisions between electrons and SO 2 gas play a significant role in the physics of Io s aurora. Magnetospheric electrons penetrate into the atmosphere of Io as a result of the relative motion of Io in its orbit and the Io plasma torus. The interaction produces a combined UV ( nm) and middle ultraviolet (MUV)/visible ( nm) auroral glow from SO 2 and its dissociation products: S, O, SO, and O 2 as well as H, S 2, Na, Cl, and K[Wong and Smyth, 2000; Roesler et al., 1999; Clarke et al., 1994; Geissler et al., 1999, 2001; Spencer et al., 1999]. The laboratory data to model the observed emission from Io over the full spectral range of the Galileo and Cassini imaging systems from 200 to 1200 nm have been unavailable until now. We present here electron-impactinduced fluorescence spectra of SO 2 gas from 200 to 600 nm to aid in the observation and modeling of the SO 2 auroral emission from Io [Bhardwaj and Michael, 1999; Geissler et al., 1999; Saur and Neubauer, 2000]. The instrument responses from Galileo remote sensing instruments (UV spectrometer and solid state imaging (SSI)), Space Telescope Imaging Spectrograph, and Cassini Imaging Subsystem (ISS) completely overlap the wavelength range of this study. All of these missions had planned observations to study Io auroral activity in the MUV while in eclipse [Geissler et al., 1999; P. E. Geisler, private communication, 2001; Trafton et al., 2000]. Copyright 2002 by the American Geophysical Union /02/2001JA [3] A review of laboratory electron impact cross sections for modeling UV ( nm) emission processes of SO 2 prior to 1992 has been given by Ajello et al. [1992a, 1992b]. However, a definitive identification and wavelength extent of all the electronic transitions generating the broad emission features found in the MUV and visible wavelength range ( nm) was not possible at the time because of low S/N ratio in all previous investigations. [4] In this paper our principal goal is to present calibrated electron-impact-induced emission spectra of SO 2 in the MUV at 8, 18, and 98 ev electron impact energies over the extended wavelength range nm. We use these three monoenergetic electron impact fluorescence spectra as a basis for a simple spectroscopic model to estimate the Galileo SSI green/violet filter color ratio and the Cassini ISS color ratios in the MUV/visible spectral region. The variation of the color ratio with energy may be an important criterion for remote sensing, since the laboratory spectra, obtained here, indicate that the ratio of intensities from certain wavelength intervals is dependent on electron energy. 2. Experimental Procedure [5] The experimental apparatus consists of an electron impact collision chamber mounted in tandem with a 1-m UV/visible monochromator. It has been described in detail before [Shemansky et al., 1995]. In brief, the spectrometer is an Acton 1-m vacuum monochromator with an EMR E- PMT detector. The measurements reported here represent the first observations with this apparatus in the visible spectral region beyond 450 nm. The detector and grating SIA 2-1

2 SIA 2-2 AJELLO ET AL.: EMISSION SPECTRUM OF SO 2 BY ELECTRON IMPACT Figure 1. Electron-impact-induced fluorescence spectrum of SO 2. Calibrated spectra were obtained at 8-, 18-, and 98- ev electron impact energy and a spectral resolution of 1.8 nm over the wavelength range nm. Data points were acquired every 0.1 nm. Data points are shown with a five-point smooth. pair that allowed these spectral measurements to be extended into the visible are, respectively, a low-noise trialkali E-PMT photocathode detector and a 300 g/mm Al/MgF 2 holographic grating blazed at 700 nm with a linear dispersion of 3.33 nm/mm. [6] Calibration of the system for spectral sensitivity was measured both with and without a cutoff filter. A Melles- Griot GG385 cutoff filter having zero transmission below 355 nm was installed between the interaction region and spectrometer to eliminate second-order effects. A standard blackbody deuterium lamp and a tungsten-halogen lamp served as the sources for wavelengths between 200 and 400 nm and between 340 and 600 nm, respectively. The full width at half maximum (FWHM) (and one-tenth maximum) of instrument sensitivity without filter extends from 265 to 505 nm (220 to 610 nm). Energy calibration of the impact electrons was accomplished by measuring the excitation function for the SO [A X] (0, 0) feature at 256 nm and comparing the measured values to the thermochemical threshold of 10.4 ev for this transition. 3. Experimental Results [7] Figure 1 shows spectra of SO 2 taken with incident electron energies of 8, 18, and 98 ev at 1.8-nm resolution. In general, the spectra consist of two broad features centered at 255 and 330 nm, which we have previously referred to as MUV1 and MUV2, respectively [Ajello et al., 1992b]. The 8-eV spectrum consists entirely of the broader MUV2 feature with no resolved structure. The absence of MUV1 in the 8-eV spectra is due to the impact energy being below the lowest threshold (10.43 ev) for formation of SO(A 3 )+ O( 3 P), which is the prominent A 3! X 3 feature seen at 255 nm, MUV1. [8] Ajello et al. [1992b] have identified the electronic transitions that would contribute to the MUV2 emission as SO 2 ~X 1 A 1 þe! SO2 ~A 1 A 2 ; ~B 1 B 1 ; ~a 3 B 1 : ð1þ The spectroscopy of these states has been recently considered by Zellmer [1992]. Studies by Johnson et al. [1987a, 1987b] demonstrated that excitation processes are dominated by the production of the optically forbidden triplet states. The ã, Ã, and B states each have an excitation energy of <8 ev, making them the most likely candidates for emission at this electron energy. The transitions have never been separated in highresolution experiments because of perturbations to the molecular energy levels. [9] With the appearance of MUV1 and other distinct emission features in Figure 1, the spectra grow in wavelength extent at 18- and 98-eV electron impact energies. The spectra agree well with the previous measurements [Miller and Becker, 1987; Ajello et al., 1992b]. However, the observed peak intensity ratios of MUV1/MUV2 lie between the ratios observed in the spectra from the two previous experimental studies. The MUV2 quasicontinuum is the fundamental underlying spectral structure at all electron energies with a long wavelength extent of near 600 nm. At 18-eV electron impact energy the features identified on top of the underlying continuum below 400 nm wavelength are mostly SO(A,B) vibrational bands. At 98-eV electron impact energy the additional processes of dissociative (ionization) excitation produce a myriad of hundreds of O I, O II, S I, and S II features mostly above 380 nm and up to 600 nm [Kiehling et al., 2001]. Clearly, the spectra at all electron impact energies extend beyond the previously reported value of 460 nm [Miller and Becker, 1987]. The spectra reported herein are acquired at higher S/N and twice the spectral resolution of Miller and Becker, which allows for the identification of vibrational structure and analysis of the full extent of the visible tail of the MUV2 emission. [10] Table 1 gives the emission cross sections of the MUV1 and MUV2 features and is based on emission cross sections given by Ajello et al. [1992b] for MUV1. The MUV1 cross section is defined as the emission cross section from SO(A, B! X) between and nm including the SO 2 quasicontinuum. The MUV2 quasicontinuum extends from 250 to 600 nm at 8 ev. The values of the MUV2 cross sections have changed drastically (100%) from our earlier work because of the improved instrument spectral calibration. In addition, the total MUV2 emission cross section has changed because the extent of the band system is to almost 600 nm. In the case of electron impact excitation at 98 ev, the peak MUV2 emission wavelength has shifted to 360 nm with the added emission contribution of the atomic features. We have shown that dissociative ionization becomes strong above 40-eV electron impact energy [Ajello et al., 1992a]. In Table 1 we have separated the total emission cross section at 98 ev of cm 2 into a value of cm 2 from SO and SO 2 molecular features and a value of cm 2 from dissociative ionization features, mostly lying longward of 380 nm. [11] In order to conclusively determine the band systems contributing to the SO 2 electron-impact-induced fluorescence spectrum, a higher-resolution spectrum was measured at 0.8 nm FWHM from 230 to 400 nm for 98 ev. Figure 2 shows that there is good agreement between the higherresolution spectrum and the band head assignments from Pearse and Gaydon [1976] for the SO B 3! X 3 and SO A 3! X 3 transitions. As significantly, there is less

3 AJELLO ET AL.: EMISSION SPECTRUM OF SO 2 BY ELECTRON IMPACT SIA 2-3 Table 1. Spectral Assignment for Electron-Impact-Induced Fluorescence Spectrum of SO 2 and Cross Sections (s) of Selected Transitions Range, nm Assignment s, cm 2a S II (5p 2 F 4s 2 D) b 0.1 [98 ev] S I (4s 3 D 1 S) b 0.2 [98 ev] S II (4f G) b 0.2 [98 ev] (MUV1) 21.0 c [98 ev], [SO (A 3! X 3 ), 23.8 c [18 ev] SO (B 3! X 3 )] (MUV2) 100 [98 ev], [SO 2 (ã, Ã, ~B)! SO 2, 91 [18 ev], [SO (A 3! X 3 )] SO (B 3! X 3 ) Plus over 100 candidate O I, O II, S I, and S II emissions b 135 d [8 ev] 60 [98 ev - molecular] 126 c [98 ev] 203 c [18 ev] 301 c [8 ev] 40 [98 ev - atomic] a Electron impact energy valued given in brackets. b From NIST web site ( list of atomic physics transitions and from [Kiehling et al., 2001]. c From Ajello et al. [1992b]. d Estimated from 18 ev/8 ev cross-section ratio of Ajello et al. [1992b]. agreement for the spectral features of the SO + A 2! X 2 r band system with a threshold near 20 ev [Murakami et al., 1982]. We can conclude that, if present, these transitions are at least a factor of 10 weaker than the SO emissions. [12] Figure 3 shows the location of O I, O II, S I, and S II atomic lines obtained at 1.8-nm resolution from 360 to 560 nm at 98-eV electron impact energy. Approximately 25 distinct emission features are observed. Each of the O I lines at 394.7, 395.4, 436.8, 503.7, and nm mentioned by Miller and Becker [1987] is clearly visible. A recent laboratory paper by Kiehling et al. [2001], which measured proton impact emission cross sections of SO 2, has observed scores of strong O I, O II, S I, and S II lines without blending from the SO 2 quasicontinuum of electronic transitions found in low-energy electron impact emission experiments. The same strongest features are identified in Figure 3 by the peak position. Scores of other weaker atomic lines pool together overlaying the MUV2 quasicontinuum. In addition, near 220 nm we identify, in Table 1, weak emissions from a few S I and S II lines. Miller and Figure 2. Electron-impact-induced fluorescence of SO 2 obtained at 98 ev with 0.8-nm resolution from 230 to 400 nm and the band assignments from Pearse and Gaydon [1976] for the SO (B 3! X 3 ) and SO (A 3! X 3 ) vibrational transitions. Uncalibrated data points were acquired every 0.18 nm and are shown without smoothing.

4 SIA 2-4 AJELLO ET AL.: EMISSION SPECTRUM OF SO 2 BY ELECTRON IMPACT Figure 3. Electron-impact-induced fluorescence spectrum of SO 2 obtained at 98 ev with 1.8-nm resolution from 375 to 550 nm. Calibrated data points are shown with a five-point smooth. Tick marks at ordinate value of 1.10 indicate positions of strong atomic multiplets. Becker have pointed out that the cross sections to the atomic lines are individually very small: on the order of cm 2 for most features. 4. Discussion [13] The quasicontinuum emission spectrum, MUV2, of SO 2 arises from a number of singlet and triplet states producing a complexity that is enhanced by perturbations Table 2. Revised MUV2 Emission Cross Sections and MUV2/ MUV1 Color Ratios Cross Section, cm 2 Energy, ev MUV1 MUV2 MUV2/MUV1 Ratio between the allowed and forbidden transitions. This spectrum is the basic underlying background spectrum to all MUV observations regardless of the electron impact energy. As the electron impact energy is increased above 10 ev, it is possible to access more dissociative and ionization states in SO 2, and the emission from excited states of SO adds distinct molecular bands to MUV2 between 220 and 400 nm and produces the second strong emission feature, MUV1, with an onset energy of 10.4 ev. The highly excited UV transitions found in the far ultraviolet (FUV) have upper states that are the terminal states for the visible cascade atomic features found in this study. The emission cross sections of the FUV transitions are very intense beginning with the onset of dissociative ionization, typically above 35 ev electron impact energy. The O, O +, S, and S + cascade emissions produce strong distinct structures on MUV2 above 35 ev. Pure dissociative excitation begins in the neighborhood of 20 ev and may weakly affect the spectra in the range of ev [Ajello et al., 1992a]. The most notable dissociative feature is MUV1, formed from a combination of SO B 3! X 3 and SO A 3! X 3 transitions near nm. These bands tend to mostly enhance the short-wavelength extent of MUV2. The spectral wavelength distribution in MUV2 is energy dependent. The peak MUV2 emission shifts from a wavelength of 320 nm (at 8 ev) to 360 nm (at 98 ev) with the onset of dissociative ionization. [14] Currently, major questions raised by observational measurements concern the identity of the emitting species in the atmosphere around Io and the distribution function of the exciting particles. Torus electrons can excite S, O, and SO, in addition to SO 2. The atmosphere of Io has a large diurnal variation in SO 2 column density and SO abundance with respect to SO 2 [Wong and Smyth, 2000]. Three distinct components make up Io s MUV/visible aurora. The bright-

5 AJELLO ET AL.: EMISSION SPECTRUM OF SO 2 BY ELECTRON IMPACT SIA 2-5 Figure 4. Electron-impact-induced fluorescence spectra with 3.6-nm resolution of SO 2 obtained at nine electron impact energies ranging from 9 to 100 ev over the wavelength range nm. Calibrated data points are not smoothed. est glows are blue glows likely resulting from SO 2 emissions [Geissler et al., 1999, 2001] close to the equator near the sub- and anti-jupiter points from centers of volcanic activity, with the red and green glows resulting from atomic O and Na emission. Direct excitation of SO should be weak compared to SO 2 emissions known to be abundant in volcanic plumes. If this is the case, these SO 2 emissions could provide a convenient thermometer for determining the temperature of electrons exciting emissions. This possibility was indicated by Ajello et al. [1992b], who provided the energy dependence of MUV1 and MUV2. The ratio of MUV2/MUV1 is monotonically decreasing with increasing energy, first very sharply from 10 to 20 ev and very slowly Table 3. Comparison of Laboratory and Galileo Solid State Imaging (SSI) Subsystem Green ( nm)/violet ( nm) (G/V) Color Ratios Source Location G/V Ratio Laboratory 8 ev 0.2 Laboratory 18 ev 0.2 Laboratory 98 ev 0.41 SSI Io Acala 0.35 SSI Io Prometheus 0.45 thereafter until 100 ev. Since the electron temperature in the vicinity of Io is dominated by low-energy electrons, the MUV2/MUV1 cross section ratio is very diagnostic of energy. We list the MUV2/MUV1 cross-section ratio and the corrected MUV2 cross sections in the energy range of ev in Table 2. We see that the emission cross-section ratio is 35.2 at 11 ev, decreasing to 2.9 by 25 ev. In Figure 4 we pictorially show the same energy variation information obtained from nine calibrated spectra of electron-induced fluorescence of SO 2 over the energy range from 9 to 100 ev. By carefully measuring the MUV1 and MUV2 peak intensities spectroscopically from Io with the Hubble Space Telescope, for example, the mean electron energy could be determined. This peak ratio is 1.9 at 18 ev in Figure 1 compared to a cross-section ratio of 3.8 in Table 1. [15] Color ratios within MUV2, itself, are not as diagnostic, because of the complicated wavelength behavior of the many underlying transitions at any excitation energy. We can quantitatively demonstrate this analysis technique by calculating the monoenergetic 8-, 18-, and 98-eV color ratios for comparison with the Galileo SSI violet and green filter response [Belton et al., 1992] and with the Cassini filters (C. Porco et al., The Cassini Imaging System, manuscript in preparation, 2002,)(hereinafter referred to as Porco et al., in preparation, 2002) for an SO 2 -only atmosphere.

6 SIA 2-6 AJELLO ET AL.: EMISSION SPECTRUM OF SO 2 BY ELECTRON IMPACT Table 4. Cassini Imaging Subsystem Color Ratios for Electron Impact Excitation of SO 2 Cassini Filter Central l [FWHM], nm Ratio of Filter to UV3 8eV 18eV 98eV UV1 260 [40] UV2 300 [60] UV3 335 [70] Blue 450 [150] Narrow Blue 440 [30] Green 565 [145] The green/violet color ratios for the three energies are shown in Table 3 compared to particular Galileo observations while Io was eclipsed by Jupiter [Geissler et al., 1999]. The numbers in Table 3 represent the ratios of the signals, in photon counts, received within the FWHM of the respective filters. The laboratory ratios at 8 and 18 ev are nearly identical with a value of 0.2, compared to 0.41 for 98 ev. In the wavelength region described inclusively by the violet and green filters ( nm), the two electron impact energies (8 and 18 ev) produce spectra with similar relative intensities. The error of the laboratory measurement ratio is 25% as a root-sum-square error due to background uncertainty in the green filter (15%) and in the violet filter (12%), relative calibration (10%), signal statistics over the band pass (10% for green and 5% for violet), and measurement uncertainties and digitization of pressure (1%) and of electron current (2%). The brightest visible Io aurora glows are closely associated with centers of volcanic activity, for example, Acala and Prometheus. Geissler et al. [1999] find green/violet ratios of 0.35 and 0.45 for Acala and Prometheus, respectively. Our laboratory experiments show that an electron energy source impacting on a predominantly SO 2 atmosphere could be used to explain the observations. The Galileo filters, however, are not optimally located in a spectral range that discriminates the threshold for excitation of the spectral region near MUV1, a region that would be most sensitive to electron energy. [16] Our laboratory experiments cannot detect the longlived O 1 S state that produces visible radiation at O I nm at 8-eV threshold energy. The addition of that intensity within the SSI green filter band pass should increase the laboratory model of the green/violet observational ratio. The excitation cross section of O 1 S from SO 2 has been recently measured by Kedzierski et al. [2000] to be cm 2 at 100 ev. In the Galileo green filter band pass our experiments show that this region contains 8% of the emission cross section of MUV2. We estimate a total emission cross section of cm 2 at 100 ev and cm 2 at 20 ev within the green filter band pass from the SO 2 quasicontinuum, not including O 1 S. The O 1 S cross section falls rapidly with decreasing energy, attaining a value of about cm 2 near 20 ev. The inclusion of the O 1 S cross section in the color ratio calculation would more than double the laboratory green/violet color ratio at 100 ev, taking the ratio outside the observed Galileo values. However, the inclusion of O 1 S cross section in the 20-eV color ratio would raise the green/violet ratio to 0.3. The laboratory match of green/violet color ratios with the color ratios from volcanic centers of activity would be excellent for lowenergy electrons in the energy range 8 20 ev. [17] A recent paper by Oliversen et al. [2001] describes the distribution function of electrons in Io s atmosphere as a mixture of 5-eV thermal, rapidly varying 30-eV nonthermal, and 100-eV nonthermal components. Electron impact excitation of atomic O to the optically forbidden 1 S state is very efficient for low-energy electrons. The presence of atomic O in the atmosphere would also increase the measured intensity in the green filter. Even though atomic O is globally distributed [Roesler et al., 1999], its presence is modeled to be only a minor daytime species in the lower atmosphere [Wong and Smyth, 2000]. The cross section for production of the O 1 S state has a peak value of cm 2 near 10 ev [Itikawa and Ichimura, 1990]. Finally, direct and dissociative excitations of SO, which models show becomes the dominant gas near the surface at local midnight, could also contribute to the green and/or violet filter signals. These cross sections have not been measured. However, the contribution to the signal level due to SO from the volcanic activity is suspected to be small compared to excitation of SO 2 [Geissler et al., 1999]. Thus other constituents, particularly atomic O, may contribute to the green SSI intensity. [18] Cassini has recently flown by Jupiter, acquiring ISS images of Io s aurora. We can use the laboratory spectra to predict the measured color ratios. The predicted ISS Cassini color ratios are given in Table 4 for the MUV/visible filters (Porco et al., in preparation, 2002). The ratios are given with respect to filter UV3. The abundance of narrow filters, in relation to Galileo, of the imaging system on board Cassini make the spacecraft very sensitive to the detection of the energetics of the Io aurora. The extension of the Cassini UV filter central wavelength of ISS down to 260 nm in comparison to the Galileo SSI value of 400 nm makes Cassini much more sensitive to the SO 2 dissociative process, a revelator of electron energy. The absence of signal in UV1 (diagnostic of MUV1) would indicate the importance of low-energy electrons in the Io auroral atmosphere. 5. Conclusions [19] (1) There are three distinct energy ranges for producing different SO 2 emission spectra: 4 10 ev (MUV2 only), ev (MUV2 + MUV1), and >20 ev (MUV1 + MUV2 + atomic multiplets). (2) The visible tail of the SO 2 fluorescence from electron impact extends to near 600 nm. (3) The measured intensity ratio of MUV2/MUV1 from Io is an electron thermometer. (4) Low-energy electron excitation of SO 2 by 8- to 20-eV electrons can explain the Galileo SSI filter observations with small contributions from SO and O. [20] Acknowledgments. The research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, and was sponsored by NASA Planetary Atmospheres Program Office, NSF and AFOSR. D.L.H., C.A.T., and O.P.M. are each supported by a National Research Council Resident Associateship. [21] Janet G. Luhmann thanks Paul E. Geissler and another referee for their assistance in evaluating this paper. References Ajello, J. M., G. K. James, I. Kanik, and B. O. Franklin, The complete UV spectrum of SO 2 by electron impact, 1, The vacuum ultraviolet spectrum, J. Geophys. Res., 97, 10,473 10,500, 1992a.

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