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Infrared spectra of SO2 and SO2:H2O ices at low temperature
Journal of Molecular Structure 644 (2003) 151–164 www.elsevier.com/locate/molstruc
Infrared spectra of SO2 and SO2:H2O ices at low temperature L. Schriver-Mazzuolia,b,*, H. Chaabounia, A. Schrivera a
Laboratoire de Physique Mole´culaire et Applications, UMR 7092, Universite´ Pierre et Marie Curie, Tour 13, case 76, 4 place Jussieu, 75252 Paris Cedex 05, France b Laboratoire d’Etude des Nuisances Atmosphe´riques et de leurs effets, Universite´ Paris Nord, IUP Ville et Sante´, rue de la Convention, Bobigny 93017 France Received 3 July 2002; revised 16 September 2002; accepted 16 September 2002
Abstract Fourier Transform infrared reflection spectroscopy (incidence angle of 58) was used to characterize thin films of sulfur dioxide and thin films containing water and sulfur dioxide in various ratios between 10 and 200 K under a pressure of 1027 mbar. Pure solid sulfur dioxide begins to sublimate above 108 K with an sublimation enthalpy of 29 ^ 3 kJ mol21.When sulfur dioxide is deposited on a water ice surface or trapped in the ice bulk, no reaction is observed even after annealing. Gaseous mixtures of sulfur dioxide containing water traces deposited at 30 K leads to condensation of pure sulfur dioxide and small clusters (SO2)x(H2O)y. States of mixed H2O/SO2 films with excess of water are dependent upon deposition conditions. With He as carrier gas, a metastable (H2O)m(SO2)n phase is observed. Deposition without carrier gas leads to a solid solution of dioxide sulfur in water ice. Above 120 K a large part of sulfur dioxide diluted in water ice escapes, and one observes changes in the frequency and the profile of sulfur dioxide absorptions which probe the gradual transformation of water ice. Sulfur dioxide which remains in the lattice escapes at 170 K when water ice sublimates. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Fourier Transform infrared reflection spectroscopy; Sulfur dioxide; Spectro-imagers
1. Introduction Recent infrared spectroscopic observations furnished by in orbit telescopes (ISO, HST, SIRTF…) and spectro-imagers or spectrophotometers aboard space craft (Galileo, Cassini, Rosetta…) have provided information about the composition and relative abundances of species present on planetary surfaces. * Corresponding author. Address: Lab de Phys Molecilaire et Applications, Unite´ propre du CNRS, Universite Pierre et Marie Curie, Tour 13, case 76, 4 Place Jussieu, F-75252 Paris Cedex 05, France. Tel.: þ33-1-4427-4959; fax: þ 33-1-4427-7033. E-mail address: [email protected] (L. SchriverMazzuoli).
However, the analysis of such observations requires systematic laboratory experiments. The present work focus upon the interaction of sulfur dioxide with water ice between 10 and 200 K. Condensed phases of sulfur dioxide exists naturally on Io, a satellite of Jupiter, and have been identified in the near and midinfrared by astronomical observations associated with laboratory experiments [1 –4]. However, an extra band at about 3590 cm21 observed first by the Kuiper Airborne observatory [5] is still very much discussed. This band was earlier identified as the (2n1 þ n3) band of solid 32S16O2 [6] then later it was suggested that it originates from traces of H2O frozen in solid SO2 on Io, based on laboratory infrared transmission spectra
0022-2860/03/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 8 6 0 ( 0 2 ) 0 0 4 7 7 - 5
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of thin films of H2O/H2S/SO2 mixtures reported by Salama et al. [7,8]. More recent spectra of Io given by the Infrared Spatial Observatory (ISO) with high resolution confirmed the existence of a narrow band at 3584 cm21 (2.79 mm). Modelling of the spectrum by B. Smith with software Spectrimag suggested again its assignment at the 2n1 þ n3 combination [9]. Minor amounts of sulfur dioxide was also identified upon the surface of Europa, an other galileen satellite [10,11]. This surface consists mainly of water ice and hydrated sulphuric acid [12]. However, laboratory studies of films containing sulfur dioxide in excess of water have not been reported and one of aims of this paper is to fill this gap. This paper contains four parts. In the first part, spectra of pure SO2 in the mid-infrared are described and the sublimation enthalpy is determined. In the second part, the nature of the interaction between SO2 with the ice surface and the possibility of migration of SO2 in the bulk with an increase in temperature are investigated. In the third part, a study of H2O (D2O) diluted in solid SO2 is presented and discussed in light of previous studies and finally, the fourth part is devoted to the spectral characteristics of SO2 trapped in water ice at different temperatures.
2. Experimental Sulfur dioxide (Air liquide N50) was dried over P2O5 and distilled at low temperature. Water was deionised, triply distilled, and then degassed by freeze-thaw cycles under vacuum before use. The experiments were performed using a closed cycle helium refrigerator (Air Product, Displex 202 A), which was pumped continuously (1027 mbar background pressure range) and equipped with a rotatable sample holder. Thin films of SO2 or of H2O – SO2 mixtures were supported inside upon the thermostated gold side of a substrate cube. The temperature of the metal substrate (10 – 170 K) was controlled by a silicon diode within ^ 0.1 K (Scientific Instruments 9600-1). SO2 films or mixed water/sulfur dioxide films were condensed at 11 or 30 K from a gaseous species mixture prepared in a 1 l glass bulb on a glass vacuum line. In most cases, Helium as carrier gas was used with a typical dilution in He of 1/25 with a total
pressure of Helium of 150 Torr. Gas was deposited via a stainless steel capillary with a rate of 3 mmol h21. The deposition nozzle parameters were 1 mm inner diameter and a distance of 20 mm from the cold substrate. At 30 K, He does not condense. Typical ice film thicknesses were less than 500 nm. The infrared measurements (from 5000 to 400 cm21) were made in a reflection absorption geometry with an incidence angle of 58 to the surface normal using a Bruker 113v spectrometer. Spectra were recorded at 0.5 or 1 cm21 nominal resolution from co-addition of 100 interferograms.
3. Results and discussion 3.1. Neat solid SO2 films 3.1.1. Fundamental vibrational absorptions A single crystal of sulphur dioxide is orthorhombic with space group Aba2 with two molecules per primitive cell on symmetry sites C2 [13]. From the correlation diagram between the gas and crystal symmetries [14] four active IR absorptions are expected: two of A1 symmetry corresponding to n1 (symmetric stretch) and n2 (bending mode) and two of symmetry B1 and B2 corresponding to n3 (asymmetric stretch). The components A2 for n1 and n2 are IR inactive. Several studies of the infrared and Raman spectra of amorphous SO2 (T , 70 K) and crystalline SO2 (T . 70 K) have been reported [14 – 19]. Extra bands were also observed and assigned to LO – TO splitting and to various isotopic forms of SO2 [18,20]. Infrared absorptions of isotopic SO2 species at 77 K were described and assigned by Barbe et al. [21]. Querico et al. studied the near infrared spectra of crystalline SO2 and most of the combination and harmonic modes were identified [22]. In the present work, spectra of solid SO2 were recorded between 4000– 500 cm21 under different deposition conditions and at various temperatures. Our spectra were in good agreement with previous studies. Fig. 1 presents some typical spectra of thin films of SO2 deposited at 30 K then annealed at 60, 70, 90 and 110 K in the n1 and n3 regions. From 108 K, SO2 begins to sublimate and the sublimation is total at 120 K after 10 min. As observed in Fig. 1, at 70 K, solid SO2 transforms
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Fig. 1. FTIR spectra in the n3 and n1 regions of condensed films of sulfur dioxide recorded at different temperatures T, K: 30 (a), 60 (b), 70 (c), 80 (d), 110 (e).
from the amorphous state to the crystalline state and important spectral changes occur. At 30 K the amorphous phase is characterized by large and asymmetric bands measured at 1315.5 cm21 for the n3 mode and at 1145.8 cm21 for the n1 mode. Between 30 and 60 K the profiles of these bands remain unchanged but their maxima are slightly shifted toward the red. When going from amorphous phase to crystalline phase, their full width at half maximum (FWHM) decreases strongly and structure appears. At 90 K, the n3 region is characterized by three components partially overlapped located at 1321.6, 1310.8 and at 1304.5 cm21 with a shoulder at 1302.8 cm21 corresponding from literature data to B1 (TO), B2(TO) of 32S16O2 and at 34S16O2 and 32 1618 S O2, respectively. Indeed, due to intermolecular coupling between isotopic SO2 species diluted in natural sulfur dioxide, the position and intensities of bands due to isotopic species differ from those observed for species isolated in matrices or in the gas phase; their intensities are often relatively strong and not correlated to the small abundance of those species in the natural SO2 (4.2 and 0.8% for 34SO2 and
S18O2, respectively) [20]. At the same temperature (90 K), the n1 region is characterized by a doublet at 1142.8 cm21 and at 1140.1 cm21 assigned to the A1 (TO) mode of 32S16O2 and of 34S16O2, respectively. From 80 and 110 K, profiles of the n1 and n3 bands do not change but the position of the two components of the n3 mode shifts towards the blue with a Dn/DT ratio of 0.027 ^ 0.002 cm21K21 for the lower component and of 0.060 ^ 0.004 cm21K21 for the higher component. Schmitt et al. [20] noted also than the combinations bands including the n3 mode are very sensitive to temperature. It was found a Dn/DT ratio of 0.075 cm21K21 for the bands at 3933 (3n3) and at 5047 cm21 (n1 þ 3n3). Knowledge of the temperature dependence of band positions can help to evaluate the thermal gradient at Io’s surface. From their results, Schmitt et al., estimated that the temperature on Io’s surface is about 110 ^ 10 K [9] The absorption corresponding to the n2 mode of solid SO2, not shown in Fig. 1, was slightly asymmetric towards the high frequencies with a maximum at 520.1 cm21 at 30 K and at 520.8 cm21 at 70 K. A narrowing of a factor 5 was observed when
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Fig. 2. Comparison of the spectra, in the n3 and n1 regions, of condensed films of sulfur dioxide after a) deposition at 90 K, b) deposition at 30 K and annealing at 90 K.
going from the amorphous phase to the crystalline phase. The intensity ratio I n3 =I n2 =I n1 was found to be of about 100/3.8/22 for the two phases. Several spectra were recorded after deposition of a SO2/He mixture at 90 K in order to determine if the bands profiles of n1 and n3 were sensitive to deposition conditions. Fig. 2 compares SO2 spectra obtained after deposition at 90 K and after annealing at 90 K of a film deposited at 30 K, in the two regions n1 and n3. As can be seen, a narrowing of the bands is observed in spectrum of SO2 directly deposited at 90 K and components belonging at 34S16O2 and 16OS18O are now distinct. The broadening of the absorptions in the film obtained after annealing, are due either to a small amount of amorphous phase which subsists in the crystalline phase or to crystalline defects. 3.1.2. Overtone and combination absorptions In the 4000– 600 cm21 range, the combinations (n1 þ n2), (n1 þ n3), (2n1 þ n3) and the overtone 2n1 are expected. However, in the films of Fig. 1 only the combination (n1 þ n3) and the overtone 2n1 were observed with an optical density at 80 K of 0.02 and 0.003, respectively. Consequently, spectra of thicker films deposited without He were recorded. Fig. 3
illustrates the spectra obtained at 11 K and 75 K for the regions of interest. As reported by Schmitt et al. [20] the most intense band characterizing the (n1 þ n3) combination at 2456.1 cm21 (75 K) narrows by a factor 3.5 when going from the amorphous to crystalline phase. It is accompanied of two weak satellites at 2433.5 and 2412.9 cm21 corresponding to the isotopic species 34 16 S O2 and 16OS18O. Its position is sensitive to the temperature and D n/DT was found to be þ 0.019 ^ 0.001 cm 21K 21. The overtone 2n1 measured at 2287.7 cm21 at 75 K also narrows when SO2 transforms from the amorphous to the crystalline phase. Two satellites at 2274.1 cm21 and 2237.9 cm21 were assigned by Barbe at 34S16O2 and 16 OS18O. The combination (n1 þ n2) has not been observed by Barbe [21]. It was assigned probably in a typographical error by Nash to an absorption at 1681.0 cm21 [19]. In fact it appears in our spectra at 1611.7 cm21 at 75 K. The identification of the combination (2n1 þ n3) at 3584.5 cm21 at 75 K is of interest because it is close to the band observed on the Io surface. In our spectra it is very weak relative to the (n1 þ n3) combination in agreement with results reported by
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Fig. 3. FTIR spectra, in the 3650–3560 cm21, 2600–2350 cm21, 2400–2200 cm21, 1700–1500 cm21,spectral regions, of condensed films of sulfur dioxide recorded at (a)11 K (b)75 K after annealing of spectrum (a).
Schmitt et al., which found an intensity ratio (n1 þ n3)/(2n1 þ n3) of 31 [3]. Table 1 summarizes frequencies of all observed bands. They are compared with literature data. The assignment is that reported in Refs. [15,18,21] 3.1.3. Binding energy of SO2 The knowledge of the surface binding energy of molecules in the pure solid is important information. Forty years ago, Honig and Hook [23] indicated a value approximately of 32 kJ mol21 for the vaporisation of solid SO2. More recently, Sandford and Allamendola [24] found a value of 29 ^ 3 kJ mol21 for the vaporisation of SO2 –SO2 ice system. In a series of experiments we have measured the variation in intensity of the (n1 þ n3) combination band versus
time at different temperatures 108, 110, 115 and 118 K for which SO2 sublimates. Fig. 4 presents three curves obtained from measurements of the 2456.2 cm21 band intensity. For presentation, curves have been translated to the same initial intensity origin. As can be seen, curves are linear indicating a zero order for the sublimation of SO2. In fact the slope of the curves do not give the absolute value of the rate constant k which is dependent upon the absorption coefficient of the chosen band. However, from Arrhe´nius law, it is possible to obtain the sublimation energy DHs. The logarithm of the sublimarion rates varied linearly with reciprocal temperature (Fig. 4B) and DHs was deduced from such a plot. We find DHs to be 29 ^ 3 kJ mol21, a value in perfect agreement with that of reference [24].
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Table 1 Frequencies in cm21 of absorptions of SO2 at 30 K and 90 K in the 3600– 500 cm21 spectral region Modes
n2
n1
Amorphous SO2n, (cm21)
Crystalline SO2n, (cm21)
20 K [15]
90 K [17]
517.0 525.0 546.0 1140.5 1147.0 1160.0
30 K this work
n1 þ n2 2n1
90 K this work 34 16
520.1
1145.8
n3 1304.0 1313.0 1327.0 1358.0
Assignment
1315.5
520.9– 522.7 530.0 1140.0 1143.0 1150.0 1302.8 1304.3 1310.1 1323.3 1345.0
1607.6 2288.2
2288.4
2457.4
2455.8
n1 þ n3 2n1 þ n3
3.2. Interaction of SO2 with ice surface Three sets of experiments were carried out. In the first set, spectra were recorded after exposing a water film at 72 K to a constant pressure of SO2 for different exposure times. At this temperature, sulfur dioxide is in crystalline phase and the narrowing of the bands is favourable to detect any changes. In the second set,
2273.9 2287.4 2433.7 2456.2
520.8 1140.1 1142.8 1302.8 1304.5 1310.8 1321.6 1611.3 2273.9 2287.4 2433.7 2456.2 3584.5
S O2 S O2 A1 (TO) 32 16 S O2 A1 (LO) 34 16 S O2 32 16 S O2 A1 (TO) 32 16 S O2 A1 (LO) 16 32 18 O S O 34 16 S O2 32 16 S O2 B1 (TO) 32 16 S O2 B2 (TO) 32 16 S O2 B2 (LO) 32 16 S O2 34 16 S O2 32 16 S O2 34 16 S O2 32 16 S O2 32 16 S O2 32 16
SO2, condensed on the ice surface, was allowed to slowly sublimate and spectra were recorded at different times. The third set of experiments consisted in a study of the evolution of amorphous water sandwich layers (ice/SO2/ice) with temperature. Fig. 5 shows typical spectra obtained by exposing a thin ice film to increasing amounts of SO2. As can seen, resultant spectra are simply composites of pure
Fig. 4. (A) Kinetic curves for solid SO2 sublimation at 108, 115, 118 K. The normalized integrated intensity of the (n1 þ n3) combination band at 2456 cm21 is plotted versus time at each temperature. (B) Plots of Ln (k ) versus reciprocal temperatures. The apparent rate constant k is extracted from the slopes of the curves in (A).
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Fig. 5. FTIR spectra at 72 K in the nOH region of water and in the n3 region of sulfur dioxide obtained after exposing a water film to a constant pressure of SO2 during different times (a) 0 min (b) 3 min, (c) 8 min, (d) 11 min.
water and pure solid SO2. However, when multilayers of SO2 are condensed on the ice surface, a weak band is observed at 3626 cm21. This band is not due to interaction of the first layer of SO2 with the ice surface because it does not appear at low coverage. It originates, as discussed below, from traces of water complexed to sulfur dioxide. Thus on an amorphous ice surface, SO2 is molecularly physisorbed by weak intermolecular forces between the water oxygen atom and sulfur atom as one might expect from the structure of the one to one complex between H2O and SO2 isolated in argon and nitrogen matrices [25]. This result is in agreement with the small uptake coefficient of SO2 on ice at 191 K determined by Liang Chu et al. [26] In the second set of experiments, when SO2 was totally desorbed, no weak bands remained in the spectra indicating that dioxide sulfur does not diffuse in the water bulk. This behaviour was confirmed by the study of a sandwich H2O/SO2/H2O condensed at 30 K then kept at 80 K during 12 h then annealed from 80 K to 100 K. No change in the spectrum was observed after depositing the second water film on top of the dioxide layer. At 70 K, amorphous SO2 phase transforms in crystalline phase as expected and at 120 K, when amorphous ice transforms in more
ordering phase, SO2 escapes through the resulting opening channels. 3.3. H2O containing SO2 films Fig. 6 compares spectra obtained by condensation at 30 K of a gaseous mixture SO2/H2O/He (20/2/150) at different deposition times in the 3800– 2800 cm21, 1800– 1400 cm21 and 1380 –1260 cm21 regions, the first spectrum recorded after 5 min (trace a), second after 13 min (trace b). In the nOH region, one observes principally two absorptions at 3627.6 cm21 and 3553.0 cm21of FWHM of 38.4 and 32.8 cm21, respectively. These bands are characteristic of a water OH oscillator complexed by SO2. At lower frequency a broad weak band with two sub-maxima at 3345.0 and 3230.0 cm21 is also observed. The n3 absorption of solid SO2 measured at 1315.4 cm21 is comparable to that of pure amorphous SO2. The absorption at 1605.5 cm21 close to the (n1 þ n2) combination of SO2 is probably partially due to the water molecule because its relative intensity in regard to the SO2 n3 band is stronger than that expected for the (n1 þ n2) combination. Comparison of spectra obtained after 5 and 13 min of deposition time is of interest. Indeed, the intensity of the two absorptions at
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Fig. 6. FTIR spectra of H2O containing SO2 films (initial gaseous ratio SO2 =H2 O ¼ 10Þ at 30 K after different deposition times: (a) 5 min, (b) 13 min.
3627.6 and 3553.0 cm21 in Fig. 6 are nearly similar in the two spectra while, in the spectrum obtained after 13 min deposition time, the n3 SO2 band is enhanced by a factor 6 as compared to the spectrum recorded after 5 min deposition time. Thus this observation suggests that the species responsible for the doublet at 3627.6 and 3553.0 cm21 are aggregates (SO2)x (H2O)y not trapped in the SO2 lattice. Their origin could be due to the substrate surface that can induce heterogeneous freezing of the gaseous mixture. Evolution of the spectrum with annealing from 30 to 125 K was studied. In the nOH region the two absorptions at 3627.6 and 3553.0 cm21 diminish in intensity with temperature increase, the first one from 60 K and the second one from 50 K. At 75 K the band at 3553.0 cm21 disappears totally when sulfur dioxide is going from the amorphous phase to the crystalline phase while the band at 3627.6 cm21 disappears only at 120 K. The different behaviour observed for the two bands with annealing is an indication that they belong to two different aggregates labelled hereafter A and B. At 75 K, the disappearance of the band at 3553.0 cm21 assigned to species B and the decrease in intensity of the band at 3627.6 cm21 assigned to species A permits the observation of weak narrow bands at 3667.7, 3584.2 and 3564.6 cm21 which were previously overlapped by absorptions of A and B. The weak feature at 3584.2 cm21 corresponds to
the (2n1 þ n3) band of 32SO2 and the feature at 3564.6 cm21 probably to the combination (2n1 þ n3) of 34SO2. The weak absorption at 3667.7 cm21 has been already observed in pure water ice and assigned to the free OH of water aggregate [27]. In parallel with the progressive disappearance of the bands at 3627.6 and 3553.0 cm21 one observes an increase in intensity of the two overlapped bands at 3345.0 and 3230.0 cm 21 then the formation of a band at 3250 cm21 characteristic of amorphous low density ice, Ial [28] . In the dOH region of water, the band at 1605.5 cm 21 diminishes in intensity and disappears at 120 K. In the n3 SO2 region, at 75 K, as expected, the transformation of multi-layers of SO2 from the amorphous to the crystalline phase occurs then, above 120 K, dioxide sulfur begins to sublimate and disappears at 125 K. Difference spectra showed that a small amount of SO2 disappeared with the 3627.6 and 3553.0 cm21 bands probed by two weak negative absorptions at 1310.8 and 1320.2 cm21 (FWHM of 3.5 and 17.7 cm 21, respectively) corresponding probably to SO2 in interaction with water in the A and B clusters. The same experiments were performed with SO2 films containing traces of D2O instead of H2O. Fig. 7 presents the obtained spectra in the OH and OD stretching regions. The simultaneous presence
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Fig. 7. Spectral changes with temperature in the 3800– 2000 cm21 region after deposition at 30 K of a SO2/D2O/He gaseous mixture (initial pressures ratio SO2 =D2 O ¼ 10Þ (a) 30 K, (b) 75 K, (c) 80 K, (d) 110 K, (e) 130 K.
of HOD and D2O in the water traces leads to the observation of a new band at 3595.3 cm21 with two shoulders at about 3633 and 3565 cm21 in the OH stretching region and of three lines partially overlapped but clearly identified at 2693, 2646, 2598 cm21 in the OD stretching region on the higher frequency side of the (n1 þ n3) combination band of SO2. When the sample is annealed to 60 K, the features at 2693 and 2598 cm21 and the two shoulders 3633 and 3565 cm21 decrease and disappear at 80 K when the transformation of amorphous SO2 into crystalline SO2 is achieved, this permits the observation of a very weak feature at 2604 cm21 due to 2n3 of SO2. With annealing, the features at 3595 and at 2646 cm21 shift progressively toward the blue (from 3595 to 3605 cm21 and from 2646 to 2665 cm21). Above 85 K, they diminish in intensity and disappear at 130 K. In parallel, broad bands at about 3300 cm21 and at 2410 cm21, which characterize OH and OD stretching modes of water ice, grow above 60 K. From temperature effects and by comparison with previous results, the two bands at 2693 and 2598 cm21 with their counterparts at 3633 and 3565 cm21 in the OH region can be assigned to
the B species whenever bands at 3595 and 2646 cm 21 can be assigned to the species A. Identification of species A and B bands is not straightforward. In matrices two aggregates (SO2)2(H2O) and (SO2)(H2O)2 were spectroscopically identified. In the first one, characterized by a perturbed OH stretching band at 3598 cm21, the water molecule was engaged in the 2:1 complex through formation of one hydrogen bond with one SO2 molecule and of one charge transfer interaction O· · ·S with the second SO2 molecule. The second one, the 1:2 complex, was characterized by two perturbed OH stretching bands at 3600 and 3480 cm21 and was interpreted as occurring from a water dimer engaged in a cyclic structure through one hydrogen bond and one charge transfer interaction with SO2. Thus it could be suggested that species A characterized by one perturbed OH (OD) oscillator could be identified as the 2:1 complex, (SO2)2(H2O), and that species B characterized by two perturbed OH (OD) oscillators could be identified as the 1:2 aggregate, (SO2)(H2O)2. In the H2O experiment, species B was characterized by one band at 3553 cm21 probably because the second absorption was overlapped by the band
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at 3627 cm21 for which a narrowing is observed with temperature increase. The broad absorptions with natural water observed at lower frequency at 3345 and 3230 cm21 could characterize either aggregates containing more than two water molecules or H2O clusters trapped in the SO2 lattice. However, such an assumption is speculative because their frequencies are close to frequencies of pure water aggregates. Comparison with literature data is of interest. Laboratory infrared transmission spectra of thin films of H2O/H2S/SO2 mixtures (from 1/3/100 to 1/30/1000 relative concentration ranges) reported by Salama et al. [7,8] revealed two bands at 3623 and 3559 cm21 and an absorption at 3590 cm21, this latter one being only detectable in the most diluted sample and probably due to the combination (2n1 þ n3) of SO2. A pair of weak broad bands at 3355 and 3205 cm21 were also observed. Bands at 3623 and 3559 cm21 which are close to bands of A and B aggregates observed in our spectra were assigned to the n3 and n1 OH stretching modes of H2O molecules isolated in a cage of sulfur dioxide producing a complex (H2O)nSO2 with n in the range 1 –3. In fact, as previously shown these bands belong to two different aggregates not formed in the SO2 lattice. The two
other absorptions at 3355 and 3205 cm21 were associated with the n3 and n1 OH stretching modes of higher H2O multimers complexed with the SO2 matrix. Such bands were also observed in our samples but as reported above no spectroscopic argument exists for their assignment. 3.4. SO2 containing water ice films Two different sets of films were produced and studied. In the first set, mixed films were obtained by condensation of SO2 and H2O gas mixtures (initial pressure H2 O=SO2 ¼ 5=1Þ with Helium as the carrier gas (H2O/SO2/He ¼ 25/5/150) while in the second set, water vapour and gaseous sulfur dioxide in the same initial pressure were condensed without carrier gas. As shown in Fig. 8, which compares the spectra recorded at 30 K after deposition during 10 min with He and during 60 min without He, no new band due to reaction appears, but the relative proportions of water ice and sulfur dioxide are very different in the two systems. Furthermore evolution of the spectra with annealing was not similar, as described below, indicating two different structures of the films for the solid state.
Fig. 8. Comparison at 30 K of spectra in the nOH region of water and in the n3 region of sulfur dioxide of a mixed H2O/SO2 film rich in water (a) after depositon at 30 K with He as carrier gas, (b) after deposition at 15 K without carrier gas.
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Fig. 9. FTIR spectra of mixed H2O/SO2 films rich in water (initial gaseous ratio SO2 =H2 O ¼ 1=5Þ deposited at 30 K with He during different times: (a) 10 min, (b) 15 min, (c) 20 min.
3.4.1. Films obtained with carrier gas Fig. 9 presents several spectra obtained in the n(OH) and d(OH) regions of water and in the n3 region of sulfur dioxide, after different deposition times (10, 15, 20 min at 30 K. The stretching region of water is characterized by a relative narrow band at 3613.0 cm21 with a shoulder at 3555.0 cm21 ðFWHM ¼ 57 cm21 Þ and by a broad band ðFWHM ¼ 392 cm21 Þ with two sub-maxima at 3340 and 3228 cm21. In the water bending region two absorptions are located at 1636.7 and 1615.0 cm21. The n3 absorption profile of sulfur dioxide is similar to that of pure SO2 at 30 K with a frequency maximum at 1314.0 cm21. An important observation can be made
from the spectra presented in Fig. 9. With an increase in deposition time, all the bands grow with the same relative intensity ratio namely: I3613:0 =I3500 – 3000 =I1675 – 1575 =I1314:0 ¼ 1:3=6=0:15=1: This behaviour not observed for other species as for example ozone [29], suggests the formation of a homogeneous phase (SO2)m(H2O)n having a single composition with m (SO2) molecules linked to n (H2O) molecules through hydrogen bonds between SO2 and H2O identified by the feature at 3613.0 cm21. Existence of this phase appears to be confirmed by a photolysis experiment which consisted of irradiating a film containing ozone trapped in this (SO2)m(H2O)n phase at 266 nm. As shown in Fig. 10, in parallel with
Fig. 10. Spectral changes with irradiation at 266 nm in the nOH region of water and in the n3 regions of sulfur dioxide and ozone of a mixed H2O/SO2/O3 film rich in water deposited at 30 K with He through two inlets. Irradiation times: (a) 0 min, (b) 18 min, (c) 90 min, (d) 130 min.
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Fig. 11. Spectral changes with temperature in the nOH region of water and in the n3 region of sulfur dioxide of a H2O/SO2 mixture rich in water deposited without carrier gas (a) 50 K, (b) 80 K, (c) 100 K, (d) 120 K, (e) 130 K, (f) 150 K.
ozone dissociation, bands at 3626, 3340, 3230, 1636, 1613 of water ice and stretching bands of SO2 at 1322.4 cm21 which are weakly shifted by interaction with ozone, diminish in concert and one observes the formation of a weak yield of H2O2 and of SO3 [30]. Temperature effects of the (SO2)m(H2O)n phase showed that this phase was metastable. At 90 K, the absorptions at 3613.1, 1615.0 cm21 decreased in intensity, the stretching mode of SO2 slightly shifted toward the blue and its intensity was reduced by 5%, the broad band at about 3300 cm21 changed its structure and increased in intensity. At 115 K, the crystalline phase of SO2 appeared as well as the characteristic n(OH) absorption of low density amorphous ice at 3250 cm21. At 125 K, SO2 sublimated but at 140 K it remained some weak bands at about 1340 and at 1146 cm21 characterizing a weak yield of SO2 in the water lattice. Indeed, a temperature increase to between 90– 115 K results in the breaking of the hydrogen bonds between water molecules and sulfur dioxide molecules and pure water ice appears in addition to pure sulfur dioxide ice. 3.4.2. Films obtained without carrier gas Deposition at 15 K without carrier gas leads to a film containing a weak yield of SO2 in relation to water ice and the film is characterized by
a n3 absorption profile different from that observed in a SO2 film, containing water traces or in the (SO2)m(H2O)n phase; the nearly symmetrical absorption is measured at 1321.4 cm21 with a FWHM of 31.8 cm21. In the n(OH) region, one observes the characteristic band of amorphous ice with an absorption at 3609.7 cm21 ðFWHM ¼ 14 cm21 Þ on the frequency high side indicating the formation of hydrogen bonds between water and sulfur dioxide molecules. The intensity ratio I3609 =I3300 =I1321 was found to be 2.5/26/1 after a deposition time of 60 min at 15 K. Fig. 11 shows some typical spectra of an H2O/SO2 film deposited at 15 K and gradually annealed at 170 K, and Fig. 12 presents the evolution of the integrated intensities of water perturbed OH (labelled OHa) and of the SO2 asymmetric stretching as a function of temperature as well as the evolution of their frequencies. Above 70 K, the OH hydrogen bonded feature begins to diminish in intensity while the OH stretching absorption of water weakly increases. Above 120 K, the OH hydrogen bonded feature disappears and SO2 decreases with temperature till 150 K. The small amount of sulfur dioxide which is trapped in the cubic ice starts to disappear above 150 K when water itself evaporates. No partial crystallisation of SO2 is observed indicating that SO2 is well diluted in ice. As can be seen in Fig. 12, the n3
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Fig. 12. Integrated band intensities as a function of temperature for the associated OH band (OHa) about 3609 cm21 and for the sulfur dioxide n3 band from spectra partially presented in Fig. 11 (A). Temperature dependence of frequencies of the associated OH band (B) and of the n3 sulfur dioxide Band (C) from spectra partially presented in Fig. 11.
frequency of SO2 changes with temperature. From 80 to 160 K, the initial band at 1321.4 cm21 shifts gradually at 1340.5 cm21. At 150 K it becomes symmetrical and narrows (12.2 cm21) and a weak satellite at 1315.3 appears on the lower frequency wing; this later band which was overlapped at lower temperature by the monomer band of SO2 diluted in water is perhaps due to some SO2 dimers formed in the gas phase and diluted in the water ice. Frequencies of the hydrogen bonded OH band are also sensitive to the temperature change. From 30 to 40 K, it shifts from 3609.7 to 3308.0 cm21 then from 70 to 120 K, it is again red shifted from 3608.0 to 3606.0 cm21. All these changes probe the structural transformation of water ice with temperature as evidenced for other species trapped in water ice [31 –34]. Between 30 and
70 K, high density amorphous ice transforms into low density amorphous ice and pores are collapsed. From 120 to 140 K, low density amorphous ice transforms into crystalline ice and the rearrangement of water molecules opens some blocked channels [35] and results in the loss of SO2. Absence of hydrogen bonded OH oscillator above 120 K, is the indication that the configuration of sulfur dioxide differs from the amorphous ice lattice to cubic ice lattice.
4. Summary Systematic studies of the evolution of spectra of thin films of SO2 /H 2O in various ratios with the temperature in the 10 – 200 K has allowed us
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1. to identify two small aggregates (SO2)x(H2O)y, one of them disappearing when sulfur dioxide transforms from amorphous phase to crystalline phase between 75-80 K and the other one at 125 K when water ice transforms in a more organized phase 2. to characterize a metastable hydrated sulfur dioxide phase, (H2O)m(SO2)n, when SO2 in excess of water vapour is deposited with carrier gas. 3. to examine thermal processes upon a solid solution of SO2 in water ice showing that the profile (shape, width, and peak position) of the SO2 absorptions depends on the structure of the water matrix. The present laboratory study can help in the IR identification of SO2 embedded in water ice upon the surface of Europa, and the present results will be useful for future studies of the reactivity in water ice of sulfur dioxide under UV irradiation. The narrow band ðFWHM ¼ 6 cm21 Þ observed recently by ISO at 3584 cm21 in IR spectra of the Io surface cannot be assigned to water/SO2 aggregates from our results because their bands in this region are broader by a factor of 5 than the Io absorption at 3584 cm21. The 3584 cm21 feature originates from the (2n1 þ n3) of SO2 as reported recently by Schmitt [9]. The very weak features not yet identified but observed in the Io spectrum at about 3320 – 3300 cm 21 and close to OH bands of the (H2O)m(SO2)n phase seem also too narrow to be assigned to water trapped in SO2 ice.
Acknowledgements This work was performed with support from the Programme National de Plane´tologie (PNP).
References [1] B. Hapke, Icarus 79 (1989) 56. [2] D.P. Cruikshank, R.R. Howell, T.R. Howell, T.R. Geballe, F.P. Fanale, J. Klinger (Eds.), Ices in Solar System, D. Reidel Publishing Company, 1985, pp. 805 –815. [3] B. Schmitt, C. de Bergh, E. Lellouch, J.P. Maillard, A. Barbe, S. Doute, Icarus 111 (1994) 79. [4] E. Quirico, B. Schmitt, R. Bini, P.R. Salvia, Planet Space Sci. 44 (1996) 973.
[5] J.D. Bergman, F. Salama, L.J. Allamendola, S.A. Sandford, F.C. Witteborn, D.P. Cruikshank, Bull. Am. Astron. Soc. 25 (1993) 851. [6] D.B. Nash, Icarus 107 (1994) 418. [7] F. Salama, L.J. Allamendola, F.C. Witteborn, D.P. Cruikshank, S.A. Sandford, J.D. Bergman, Icarus 83 (1990) 66. [8] F. Salama, L.J. Allamendola, S.A. Sandford, J.D. Bergman, F.C. Witteborn, D.P. Cruikshank, Icarus 107 (1994) 413. [9] B. Schmitt, S. Doute, E. Lellouch, H. Feuchtgruber, C. de Bergh, J. Crovisier, in: W. Kofman (Ed.), Acte du 2 e`me Colloque de Plane´tologie de l’INSU, vol. 1, 1998, pp. 1–22. [10] A.L. Lane, R.M. Nelson, D.L. Matson, Nature 192 (1981) 38. [11] K.S. Noll, H.A. Weaver, A.M. Gonella, J. Geophys. Res. 100 (1995) 19057. [12] R.N. Carlson, R.E. Johnson, M.S. Anderson, Science 289 (1999) 97. [13] B. Post, R.S. Swartz, J. Franuchen, Acta Crystallogr. 5 (1952) 372. [14] A. Anderson, R. Savoie, Can. J. Chem. 43 (1965) 2271. [15] A. Anderson, M.C.W. Campbell, J. Chem. Phys. 67 (1977) 4300. [16] M.H. Brooker, J. Mol. Struct. 112 (1984) 221. [17] R.K. Khanna, G. Zhao, M.J. Aspina, J.C. Pearl, Spectrochim. Acta 44A (1988) 581. [18] M.H. Brooker, J. Chen, Spectrochim. Acta 47A (1991) 315. [19] D.B. Nash, B.H. Betts, Icarus 117 (1995) 402. [20] B. Schmitt, C. de Bergh, M. Fesjou (Eds.), Solar System Ices, Astrophysics and Space Sciences Library, vol. 227, Kluwer, Dordecht, 1995. [21] A. Barbe, A. Delahaigue, R. Jouve, Spectrochim. Acta 27A (1971) 1439. [22] E. Quirico, B. Schmitt, R. Bini, P.R. Salvi, Planet Space Sci. 44 (1996) 973. [23] R.E. Honig, H.O. Hook, RCA Reviews (1960) 360. [24] S.A. Sandford, L.J. Allamandola, Icarus 106 (1993) 478. [25] A. Schriver, L. Schriver-Mazzuoli, J.P. Perchard, J. Mol. Spectrosc. 127 (1988) 125. [26] L. Chu, G. Diao, L.T. Chu, J. Phys. Chem. A104 (2000) 7565. [27] L. Schriver-Mazzuoli, A. Schriver, A. Hallou, J. Mol. Struct. 554 (2000) 289. [28] M.S. Bergren, D. Schuh, M.G. Seeats, S.A. Rice, J. Chem. Phys. 69 (1978) 3477. [29] H. Chaabouni, L. Schriver-Mazzuoli, A. Schriver, J. Phys. Chem. A104 (2000) 6962. [30] L. Schriver-Mazzuoli, H. Chaabouni, A. Schriver, Can. J. Phys., submitted for publication. [31] D. Laufer, E. Kochavi, A. Bar-Nun, Phys. Rev B36 (1987) 9219. [32] S.A. Sandford, L.J. Allamendola, Icarus 76 (1988) 201. [33] A. Givan, A. Loewenschuss, C.J. Nielsen, Vib. Spectrosc. 12 (1996) 1. [34] H. Chaabouni, L. Schriver-Mazzuoli, A. Schriver, Low Temp. Phys. 26 (2000) 963. [35] P. Jenniskens, D.F. Blake, Science 265 (1994) 753.
Infrared spectra of SO2 and SO2:H2O ices at low temperature L. Schriver-Mazzuolia,b,*, H. Chaabounia, A. Schrivera a
Laboratoire de Physique Mole´culaire et Applications, UMR 7092, Universite´ Pierre et Marie Curie, Tour 13, case 76, 4 place Jussieu, 75252 Paris Cedex 05, France b Laboratoire d’Etude des Nuisances Atmosphe´riques et de leurs effets, Universite´ Paris Nord, IUP Ville et Sante´, rue de la Convention, Bobigny 93017 France Received 3 July 2002; revised 16 September 2002; accepted 16 September 2002
Abstract Fourier Transform infrared reflection spectroscopy (incidence angle of 58) was used to characterize thin films of sulfur dioxide and thin films containing water and sulfur dioxide in various ratios between 10 and 200 K under a pressure of 1027 mbar. Pure solid sulfur dioxide begins to sublimate above 108 K with an sublimation enthalpy of 29 ^ 3 kJ mol21.When sulfur dioxide is deposited on a water ice surface or trapped in the ice bulk, no reaction is observed even after annealing. Gaseous mixtures of sulfur dioxide containing water traces deposited at 30 K leads to condensation of pure sulfur dioxide and small clusters (SO2)x(H2O)y. States of mixed H2O/SO2 films with excess of water are dependent upon deposition conditions. With He as carrier gas, a metastable (H2O)m(SO2)n phase is observed. Deposition without carrier gas leads to a solid solution of dioxide sulfur in water ice. Above 120 K a large part of sulfur dioxide diluted in water ice escapes, and one observes changes in the frequency and the profile of sulfur dioxide absorptions which probe the gradual transformation of water ice. Sulfur dioxide which remains in the lattice escapes at 170 K when water ice sublimates. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Fourier Transform infrared reflection spectroscopy; Sulfur dioxide; Spectro-imagers
1. Introduction Recent infrared spectroscopic observations furnished by in orbit telescopes (ISO, HST, SIRTF…) and spectro-imagers or spectrophotometers aboard space craft (Galileo, Cassini, Rosetta…) have provided information about the composition and relative abundances of species present on planetary surfaces. * Corresponding author. Address: Lab de Phys Molecilaire et Applications, Unite´ propre du CNRS, Universite Pierre et Marie Curie, Tour 13, case 76, 4 Place Jussieu, F-75252 Paris Cedex 05, France. Tel.: þ33-1-4427-4959; fax: þ 33-1-4427-7033. E-mail address: [email protected] (L. SchriverMazzuoli).
However, the analysis of such observations requires systematic laboratory experiments. The present work focus upon the interaction of sulfur dioxide with water ice between 10 and 200 K. Condensed phases of sulfur dioxide exists naturally on Io, a satellite of Jupiter, and have been identified in the near and midinfrared by astronomical observations associated with laboratory experiments [1 –4]. However, an extra band at about 3590 cm21 observed first by the Kuiper Airborne observatory [5] is still very much discussed. This band was earlier identified as the (2n1 þ n3) band of solid 32S16O2 [6] then later it was suggested that it originates from traces of H2O frozen in solid SO2 on Io, based on laboratory infrared transmission spectra
0022-2860/03/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 8 6 0 ( 0 2 ) 0 0 4 7 7 - 5
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of thin films of H2O/H2S/SO2 mixtures reported by Salama et al. [7,8]. More recent spectra of Io given by the Infrared Spatial Observatory (ISO) with high resolution confirmed the existence of a narrow band at 3584 cm21 (2.79 mm). Modelling of the spectrum by B. Smith with software Spectrimag suggested again its assignment at the 2n1 þ n3 combination [9]. Minor amounts of sulfur dioxide was also identified upon the surface of Europa, an other galileen satellite [10,11]. This surface consists mainly of water ice and hydrated sulphuric acid [12]. However, laboratory studies of films containing sulfur dioxide in excess of water have not been reported and one of aims of this paper is to fill this gap. This paper contains four parts. In the first part, spectra of pure SO2 in the mid-infrared are described and the sublimation enthalpy is determined. In the second part, the nature of the interaction between SO2 with the ice surface and the possibility of migration of SO2 in the bulk with an increase in temperature are investigated. In the third part, a study of H2O (D2O) diluted in solid SO2 is presented and discussed in light of previous studies and finally, the fourth part is devoted to the spectral characteristics of SO2 trapped in water ice at different temperatures.
2. Experimental Sulfur dioxide (Air liquide N50) was dried over P2O5 and distilled at low temperature. Water was deionised, triply distilled, and then degassed by freeze-thaw cycles under vacuum before use. The experiments were performed using a closed cycle helium refrigerator (Air Product, Displex 202 A), which was pumped continuously (1027 mbar background pressure range) and equipped with a rotatable sample holder. Thin films of SO2 or of H2O – SO2 mixtures were supported inside upon the thermostated gold side of a substrate cube. The temperature of the metal substrate (10 – 170 K) was controlled by a silicon diode within ^ 0.1 K (Scientific Instruments 9600-1). SO2 films or mixed water/sulfur dioxide films were condensed at 11 or 30 K from a gaseous species mixture prepared in a 1 l glass bulb on a glass vacuum line. In most cases, Helium as carrier gas was used with a typical dilution in He of 1/25 with a total
pressure of Helium of 150 Torr. Gas was deposited via a stainless steel capillary with a rate of 3 mmol h21. The deposition nozzle parameters were 1 mm inner diameter and a distance of 20 mm from the cold substrate. At 30 K, He does not condense. Typical ice film thicknesses were less than 500 nm. The infrared measurements (from 5000 to 400 cm21) were made in a reflection absorption geometry with an incidence angle of 58 to the surface normal using a Bruker 113v spectrometer. Spectra were recorded at 0.5 or 1 cm21 nominal resolution from co-addition of 100 interferograms.
3. Results and discussion 3.1. Neat solid SO2 films 3.1.1. Fundamental vibrational absorptions A single crystal of sulphur dioxide is orthorhombic with space group Aba2 with two molecules per primitive cell on symmetry sites C2 [13]. From the correlation diagram between the gas and crystal symmetries [14] four active IR absorptions are expected: two of A1 symmetry corresponding to n1 (symmetric stretch) and n2 (bending mode) and two of symmetry B1 and B2 corresponding to n3 (asymmetric stretch). The components A2 for n1 and n2 are IR inactive. Several studies of the infrared and Raman spectra of amorphous SO2 (T , 70 K) and crystalline SO2 (T . 70 K) have been reported [14 – 19]. Extra bands were also observed and assigned to LO – TO splitting and to various isotopic forms of SO2 [18,20]. Infrared absorptions of isotopic SO2 species at 77 K were described and assigned by Barbe et al. [21]. Querico et al. studied the near infrared spectra of crystalline SO2 and most of the combination and harmonic modes were identified [22]. In the present work, spectra of solid SO2 were recorded between 4000– 500 cm21 under different deposition conditions and at various temperatures. Our spectra were in good agreement with previous studies. Fig. 1 presents some typical spectra of thin films of SO2 deposited at 30 K then annealed at 60, 70, 90 and 110 K in the n1 and n3 regions. From 108 K, SO2 begins to sublimate and the sublimation is total at 120 K after 10 min. As observed in Fig. 1, at 70 K, solid SO2 transforms
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Fig. 1. FTIR spectra in the n3 and n1 regions of condensed films of sulfur dioxide recorded at different temperatures T, K: 30 (a), 60 (b), 70 (c), 80 (d), 110 (e).
from the amorphous state to the crystalline state and important spectral changes occur. At 30 K the amorphous phase is characterized by large and asymmetric bands measured at 1315.5 cm21 for the n3 mode and at 1145.8 cm21 for the n1 mode. Between 30 and 60 K the profiles of these bands remain unchanged but their maxima are slightly shifted toward the red. When going from amorphous phase to crystalline phase, their full width at half maximum (FWHM) decreases strongly and structure appears. At 90 K, the n3 region is characterized by three components partially overlapped located at 1321.6, 1310.8 and at 1304.5 cm21 with a shoulder at 1302.8 cm21 corresponding from literature data to B1 (TO), B2(TO) of 32S16O2 and at 34S16O2 and 32 1618 S O2, respectively. Indeed, due to intermolecular coupling between isotopic SO2 species diluted in natural sulfur dioxide, the position and intensities of bands due to isotopic species differ from those observed for species isolated in matrices or in the gas phase; their intensities are often relatively strong and not correlated to the small abundance of those species in the natural SO2 (4.2 and 0.8% for 34SO2 and
S18O2, respectively) [20]. At the same temperature (90 K), the n1 region is characterized by a doublet at 1142.8 cm21 and at 1140.1 cm21 assigned to the A1 (TO) mode of 32S16O2 and of 34S16O2, respectively. From 80 and 110 K, profiles of the n1 and n3 bands do not change but the position of the two components of the n3 mode shifts towards the blue with a Dn/DT ratio of 0.027 ^ 0.002 cm21K21 for the lower component and of 0.060 ^ 0.004 cm21K21 for the higher component. Schmitt et al. [20] noted also than the combinations bands including the n3 mode are very sensitive to temperature. It was found a Dn/DT ratio of 0.075 cm21K21 for the bands at 3933 (3n3) and at 5047 cm21 (n1 þ 3n3). Knowledge of the temperature dependence of band positions can help to evaluate the thermal gradient at Io’s surface. From their results, Schmitt et al., estimated that the temperature on Io’s surface is about 110 ^ 10 K [9] The absorption corresponding to the n2 mode of solid SO2, not shown in Fig. 1, was slightly asymmetric towards the high frequencies with a maximum at 520.1 cm21 at 30 K and at 520.8 cm21 at 70 K. A narrowing of a factor 5 was observed when
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Fig. 2. Comparison of the spectra, in the n3 and n1 regions, of condensed films of sulfur dioxide after a) deposition at 90 K, b) deposition at 30 K and annealing at 90 K.
going from the amorphous phase to the crystalline phase. The intensity ratio I n3 =I n2 =I n1 was found to be of about 100/3.8/22 for the two phases. Several spectra were recorded after deposition of a SO2/He mixture at 90 K in order to determine if the bands profiles of n1 and n3 were sensitive to deposition conditions. Fig. 2 compares SO2 spectra obtained after deposition at 90 K and after annealing at 90 K of a film deposited at 30 K, in the two regions n1 and n3. As can be seen, a narrowing of the bands is observed in spectrum of SO2 directly deposited at 90 K and components belonging at 34S16O2 and 16OS18O are now distinct. The broadening of the absorptions in the film obtained after annealing, are due either to a small amount of amorphous phase which subsists in the crystalline phase or to crystalline defects. 3.1.2. Overtone and combination absorptions In the 4000– 600 cm21 range, the combinations (n1 þ n2), (n1 þ n3), (2n1 þ n3) and the overtone 2n1 are expected. However, in the films of Fig. 1 only the combination (n1 þ n3) and the overtone 2n1 were observed with an optical density at 80 K of 0.02 and 0.003, respectively. Consequently, spectra of thicker films deposited without He were recorded. Fig. 3
illustrates the spectra obtained at 11 K and 75 K for the regions of interest. As reported by Schmitt et al. [20] the most intense band characterizing the (n1 þ n3) combination at 2456.1 cm21 (75 K) narrows by a factor 3.5 when going from the amorphous to crystalline phase. It is accompanied of two weak satellites at 2433.5 and 2412.9 cm21 corresponding to the isotopic species 34 16 S O2 and 16OS18O. Its position is sensitive to the temperature and D n/DT was found to be þ 0.019 ^ 0.001 cm 21K 21. The overtone 2n1 measured at 2287.7 cm21 at 75 K also narrows when SO2 transforms from the amorphous to the crystalline phase. Two satellites at 2274.1 cm21 and 2237.9 cm21 were assigned by Barbe at 34S16O2 and 16 OS18O. The combination (n1 þ n2) has not been observed by Barbe [21]. It was assigned probably in a typographical error by Nash to an absorption at 1681.0 cm21 [19]. In fact it appears in our spectra at 1611.7 cm21 at 75 K. The identification of the combination (2n1 þ n3) at 3584.5 cm21 at 75 K is of interest because it is close to the band observed on the Io surface. In our spectra it is very weak relative to the (n1 þ n3) combination in agreement with results reported by
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Fig. 3. FTIR spectra, in the 3650–3560 cm21, 2600–2350 cm21, 2400–2200 cm21, 1700–1500 cm21,spectral regions, of condensed films of sulfur dioxide recorded at (a)11 K (b)75 K after annealing of spectrum (a).
Schmitt et al., which found an intensity ratio (n1 þ n3)/(2n1 þ n3) of 31 [3]. Table 1 summarizes frequencies of all observed bands. They are compared with literature data. The assignment is that reported in Refs. [15,18,21] 3.1.3. Binding energy of SO2 The knowledge of the surface binding energy of molecules in the pure solid is important information. Forty years ago, Honig and Hook [23] indicated a value approximately of 32 kJ mol21 for the vaporisation of solid SO2. More recently, Sandford and Allamendola [24] found a value of 29 ^ 3 kJ mol21 for the vaporisation of SO2 –SO2 ice system. In a series of experiments we have measured the variation in intensity of the (n1 þ n3) combination band versus
time at different temperatures 108, 110, 115 and 118 K for which SO2 sublimates. Fig. 4 presents three curves obtained from measurements of the 2456.2 cm21 band intensity. For presentation, curves have been translated to the same initial intensity origin. As can be seen, curves are linear indicating a zero order for the sublimation of SO2. In fact the slope of the curves do not give the absolute value of the rate constant k which is dependent upon the absorption coefficient of the chosen band. However, from Arrhe´nius law, it is possible to obtain the sublimation energy DHs. The logarithm of the sublimarion rates varied linearly with reciprocal temperature (Fig. 4B) and DHs was deduced from such a plot. We find DHs to be 29 ^ 3 kJ mol21, a value in perfect agreement with that of reference [24].
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Table 1 Frequencies in cm21 of absorptions of SO2 at 30 K and 90 K in the 3600– 500 cm21 spectral region Modes
n2
n1
Amorphous SO2n, (cm21)
Crystalline SO2n, (cm21)
20 K [15]
90 K [17]
517.0 525.0 546.0 1140.5 1147.0 1160.0
30 K this work
n1 þ n2 2n1
90 K this work 34 16
520.1
1145.8
n3 1304.0 1313.0 1327.0 1358.0
Assignment
1315.5
520.9– 522.7 530.0 1140.0 1143.0 1150.0 1302.8 1304.3 1310.1 1323.3 1345.0
1607.6 2288.2
2288.4
2457.4
2455.8
n1 þ n3 2n1 þ n3
3.2. Interaction of SO2 with ice surface Three sets of experiments were carried out. In the first set, spectra were recorded after exposing a water film at 72 K to a constant pressure of SO2 for different exposure times. At this temperature, sulfur dioxide is in crystalline phase and the narrowing of the bands is favourable to detect any changes. In the second set,
2273.9 2287.4 2433.7 2456.2
520.8 1140.1 1142.8 1302.8 1304.5 1310.8 1321.6 1611.3 2273.9 2287.4 2433.7 2456.2 3584.5
S O2 S O2 A1 (TO) 32 16 S O2 A1 (LO) 34 16 S O2 32 16 S O2 A1 (TO) 32 16 S O2 A1 (LO) 16 32 18 O S O 34 16 S O2 32 16 S O2 B1 (TO) 32 16 S O2 B2 (TO) 32 16 S O2 B2 (LO) 32 16 S O2 34 16 S O2 32 16 S O2 34 16 S O2 32 16 S O2 32 16 S O2 32 16
SO2, condensed on the ice surface, was allowed to slowly sublimate and spectra were recorded at different times. The third set of experiments consisted in a study of the evolution of amorphous water sandwich layers (ice/SO2/ice) with temperature. Fig. 5 shows typical spectra obtained by exposing a thin ice film to increasing amounts of SO2. As can seen, resultant spectra are simply composites of pure
Fig. 4. (A) Kinetic curves for solid SO2 sublimation at 108, 115, 118 K. The normalized integrated intensity of the (n1 þ n3) combination band at 2456 cm21 is plotted versus time at each temperature. (B) Plots of Ln (k ) versus reciprocal temperatures. The apparent rate constant k is extracted from the slopes of the curves in (A).
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Fig. 5. FTIR spectra at 72 K in the nOH region of water and in the n3 region of sulfur dioxide obtained after exposing a water film to a constant pressure of SO2 during different times (a) 0 min (b) 3 min, (c) 8 min, (d) 11 min.
water and pure solid SO2. However, when multilayers of SO2 are condensed on the ice surface, a weak band is observed at 3626 cm21. This band is not due to interaction of the first layer of SO2 with the ice surface because it does not appear at low coverage. It originates, as discussed below, from traces of water complexed to sulfur dioxide. Thus on an amorphous ice surface, SO2 is molecularly physisorbed by weak intermolecular forces between the water oxygen atom and sulfur atom as one might expect from the structure of the one to one complex between H2O and SO2 isolated in argon and nitrogen matrices [25]. This result is in agreement with the small uptake coefficient of SO2 on ice at 191 K determined by Liang Chu et al. [26] In the second set of experiments, when SO2 was totally desorbed, no weak bands remained in the spectra indicating that dioxide sulfur does not diffuse in the water bulk. This behaviour was confirmed by the study of a sandwich H2O/SO2/H2O condensed at 30 K then kept at 80 K during 12 h then annealed from 80 K to 100 K. No change in the spectrum was observed after depositing the second water film on top of the dioxide layer. At 70 K, amorphous SO2 phase transforms in crystalline phase as expected and at 120 K, when amorphous ice transforms in more
ordering phase, SO2 escapes through the resulting opening channels. 3.3. H2O containing SO2 films Fig. 6 compares spectra obtained by condensation at 30 K of a gaseous mixture SO2/H2O/He (20/2/150) at different deposition times in the 3800– 2800 cm21, 1800– 1400 cm21 and 1380 –1260 cm21 regions, the first spectrum recorded after 5 min (trace a), second after 13 min (trace b). In the nOH region, one observes principally two absorptions at 3627.6 cm21 and 3553.0 cm21of FWHM of 38.4 and 32.8 cm21, respectively. These bands are characteristic of a water OH oscillator complexed by SO2. At lower frequency a broad weak band with two sub-maxima at 3345.0 and 3230.0 cm21 is also observed. The n3 absorption of solid SO2 measured at 1315.4 cm21 is comparable to that of pure amorphous SO2. The absorption at 1605.5 cm21 close to the (n1 þ n2) combination of SO2 is probably partially due to the water molecule because its relative intensity in regard to the SO2 n3 band is stronger than that expected for the (n1 þ n2) combination. Comparison of spectra obtained after 5 and 13 min of deposition time is of interest. Indeed, the intensity of the two absorptions at
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Fig. 6. FTIR spectra of H2O containing SO2 films (initial gaseous ratio SO2 =H2 O ¼ 10Þ at 30 K after different deposition times: (a) 5 min, (b) 13 min.
3627.6 and 3553.0 cm21 in Fig. 6 are nearly similar in the two spectra while, in the spectrum obtained after 13 min deposition time, the n3 SO2 band is enhanced by a factor 6 as compared to the spectrum recorded after 5 min deposition time. Thus this observation suggests that the species responsible for the doublet at 3627.6 and 3553.0 cm21 are aggregates (SO2)x (H2O)y not trapped in the SO2 lattice. Their origin could be due to the substrate surface that can induce heterogeneous freezing of the gaseous mixture. Evolution of the spectrum with annealing from 30 to 125 K was studied. In the nOH region the two absorptions at 3627.6 and 3553.0 cm21 diminish in intensity with temperature increase, the first one from 60 K and the second one from 50 K. At 75 K the band at 3553.0 cm21 disappears totally when sulfur dioxide is going from the amorphous phase to the crystalline phase while the band at 3627.6 cm21 disappears only at 120 K. The different behaviour observed for the two bands with annealing is an indication that they belong to two different aggregates labelled hereafter A and B. At 75 K, the disappearance of the band at 3553.0 cm21 assigned to species B and the decrease in intensity of the band at 3627.6 cm21 assigned to species A permits the observation of weak narrow bands at 3667.7, 3584.2 and 3564.6 cm21 which were previously overlapped by absorptions of A and B. The weak feature at 3584.2 cm21 corresponds to
the (2n1 þ n3) band of 32SO2 and the feature at 3564.6 cm21 probably to the combination (2n1 þ n3) of 34SO2. The weak absorption at 3667.7 cm21 has been already observed in pure water ice and assigned to the free OH of water aggregate [27]. In parallel with the progressive disappearance of the bands at 3627.6 and 3553.0 cm21 one observes an increase in intensity of the two overlapped bands at 3345.0 and 3230.0 cm 21 then the formation of a band at 3250 cm21 characteristic of amorphous low density ice, Ial [28] . In the dOH region of water, the band at 1605.5 cm 21 diminishes in intensity and disappears at 120 K. In the n3 SO2 region, at 75 K, as expected, the transformation of multi-layers of SO2 from the amorphous to the crystalline phase occurs then, above 120 K, dioxide sulfur begins to sublimate and disappears at 125 K. Difference spectra showed that a small amount of SO2 disappeared with the 3627.6 and 3553.0 cm21 bands probed by two weak negative absorptions at 1310.8 and 1320.2 cm21 (FWHM of 3.5 and 17.7 cm 21, respectively) corresponding probably to SO2 in interaction with water in the A and B clusters. The same experiments were performed with SO2 films containing traces of D2O instead of H2O. Fig. 7 presents the obtained spectra in the OH and OD stretching regions. The simultaneous presence
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Fig. 7. Spectral changes with temperature in the 3800– 2000 cm21 region after deposition at 30 K of a SO2/D2O/He gaseous mixture (initial pressures ratio SO2 =D2 O ¼ 10Þ (a) 30 K, (b) 75 K, (c) 80 K, (d) 110 K, (e) 130 K.
of HOD and D2O in the water traces leads to the observation of a new band at 3595.3 cm21 with two shoulders at about 3633 and 3565 cm21 in the OH stretching region and of three lines partially overlapped but clearly identified at 2693, 2646, 2598 cm21 in the OD stretching region on the higher frequency side of the (n1 þ n3) combination band of SO2. When the sample is annealed to 60 K, the features at 2693 and 2598 cm21 and the two shoulders 3633 and 3565 cm21 decrease and disappear at 80 K when the transformation of amorphous SO2 into crystalline SO2 is achieved, this permits the observation of a very weak feature at 2604 cm21 due to 2n3 of SO2. With annealing, the features at 3595 and at 2646 cm21 shift progressively toward the blue (from 3595 to 3605 cm21 and from 2646 to 2665 cm21). Above 85 K, they diminish in intensity and disappear at 130 K. In parallel, broad bands at about 3300 cm21 and at 2410 cm21, which characterize OH and OD stretching modes of water ice, grow above 60 K. From temperature effects and by comparison with previous results, the two bands at 2693 and 2598 cm21 with their counterparts at 3633 and 3565 cm21 in the OH region can be assigned to
the B species whenever bands at 3595 and 2646 cm 21 can be assigned to the species A. Identification of species A and B bands is not straightforward. In matrices two aggregates (SO2)2(H2O) and (SO2)(H2O)2 were spectroscopically identified. In the first one, characterized by a perturbed OH stretching band at 3598 cm21, the water molecule was engaged in the 2:1 complex through formation of one hydrogen bond with one SO2 molecule and of one charge transfer interaction O· · ·S with the second SO2 molecule. The second one, the 1:2 complex, was characterized by two perturbed OH stretching bands at 3600 and 3480 cm21 and was interpreted as occurring from a water dimer engaged in a cyclic structure through one hydrogen bond and one charge transfer interaction with SO2. Thus it could be suggested that species A characterized by one perturbed OH (OD) oscillator could be identified as the 2:1 complex, (SO2)2(H2O), and that species B characterized by two perturbed OH (OD) oscillators could be identified as the 1:2 aggregate, (SO2)(H2O)2. In the H2O experiment, species B was characterized by one band at 3553 cm21 probably because the second absorption was overlapped by the band
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at 3627 cm21 for which a narrowing is observed with temperature increase. The broad absorptions with natural water observed at lower frequency at 3345 and 3230 cm21 could characterize either aggregates containing more than two water molecules or H2O clusters trapped in the SO2 lattice. However, such an assumption is speculative because their frequencies are close to frequencies of pure water aggregates. Comparison with literature data is of interest. Laboratory infrared transmission spectra of thin films of H2O/H2S/SO2 mixtures (from 1/3/100 to 1/30/1000 relative concentration ranges) reported by Salama et al. [7,8] revealed two bands at 3623 and 3559 cm21 and an absorption at 3590 cm21, this latter one being only detectable in the most diluted sample and probably due to the combination (2n1 þ n3) of SO2. A pair of weak broad bands at 3355 and 3205 cm21 were also observed. Bands at 3623 and 3559 cm21 which are close to bands of A and B aggregates observed in our spectra were assigned to the n3 and n1 OH stretching modes of H2O molecules isolated in a cage of sulfur dioxide producing a complex (H2O)nSO2 with n in the range 1 –3. In fact, as previously shown these bands belong to two different aggregates not formed in the SO2 lattice. The two
other absorptions at 3355 and 3205 cm21 were associated with the n3 and n1 OH stretching modes of higher H2O multimers complexed with the SO2 matrix. Such bands were also observed in our samples but as reported above no spectroscopic argument exists for their assignment. 3.4. SO2 containing water ice films Two different sets of films were produced and studied. In the first set, mixed films were obtained by condensation of SO2 and H2O gas mixtures (initial pressure H2 O=SO2 ¼ 5=1Þ with Helium as the carrier gas (H2O/SO2/He ¼ 25/5/150) while in the second set, water vapour and gaseous sulfur dioxide in the same initial pressure were condensed without carrier gas. As shown in Fig. 8, which compares the spectra recorded at 30 K after deposition during 10 min with He and during 60 min without He, no new band due to reaction appears, but the relative proportions of water ice and sulfur dioxide are very different in the two systems. Furthermore evolution of the spectra with annealing was not similar, as described below, indicating two different structures of the films for the solid state.
Fig. 8. Comparison at 30 K of spectra in the nOH region of water and in the n3 region of sulfur dioxide of a mixed H2O/SO2 film rich in water (a) after depositon at 30 K with He as carrier gas, (b) after deposition at 15 K without carrier gas.
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Fig. 9. FTIR spectra of mixed H2O/SO2 films rich in water (initial gaseous ratio SO2 =H2 O ¼ 1=5Þ deposited at 30 K with He during different times: (a) 10 min, (b) 15 min, (c) 20 min.
3.4.1. Films obtained with carrier gas Fig. 9 presents several spectra obtained in the n(OH) and d(OH) regions of water and in the n3 region of sulfur dioxide, after different deposition times (10, 15, 20 min at 30 K. The stretching region of water is characterized by a relative narrow band at 3613.0 cm21 with a shoulder at 3555.0 cm21 ðFWHM ¼ 57 cm21 Þ and by a broad band ðFWHM ¼ 392 cm21 Þ with two sub-maxima at 3340 and 3228 cm21. In the water bending region two absorptions are located at 1636.7 and 1615.0 cm21. The n3 absorption profile of sulfur dioxide is similar to that of pure SO2 at 30 K with a frequency maximum at 1314.0 cm21. An important observation can be made
from the spectra presented in Fig. 9. With an increase in deposition time, all the bands grow with the same relative intensity ratio namely: I3613:0 =I3500 – 3000 =I1675 – 1575 =I1314:0 ¼ 1:3=6=0:15=1: This behaviour not observed for other species as for example ozone [29], suggests the formation of a homogeneous phase (SO2)m(H2O)n having a single composition with m (SO2) molecules linked to n (H2O) molecules through hydrogen bonds between SO2 and H2O identified by the feature at 3613.0 cm21. Existence of this phase appears to be confirmed by a photolysis experiment which consisted of irradiating a film containing ozone trapped in this (SO2)m(H2O)n phase at 266 nm. As shown in Fig. 10, in parallel with
Fig. 10. Spectral changes with irradiation at 266 nm in the nOH region of water and in the n3 regions of sulfur dioxide and ozone of a mixed H2O/SO2/O3 film rich in water deposited at 30 K with He through two inlets. Irradiation times: (a) 0 min, (b) 18 min, (c) 90 min, (d) 130 min.
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Fig. 11. Spectral changes with temperature in the nOH region of water and in the n3 region of sulfur dioxide of a H2O/SO2 mixture rich in water deposited without carrier gas (a) 50 K, (b) 80 K, (c) 100 K, (d) 120 K, (e) 130 K, (f) 150 K.
ozone dissociation, bands at 3626, 3340, 3230, 1636, 1613 of water ice and stretching bands of SO2 at 1322.4 cm21 which are weakly shifted by interaction with ozone, diminish in concert and one observes the formation of a weak yield of H2O2 and of SO3 [30]. Temperature effects of the (SO2)m(H2O)n phase showed that this phase was metastable. At 90 K, the absorptions at 3613.1, 1615.0 cm21 decreased in intensity, the stretching mode of SO2 slightly shifted toward the blue and its intensity was reduced by 5%, the broad band at about 3300 cm21 changed its structure and increased in intensity. At 115 K, the crystalline phase of SO2 appeared as well as the characteristic n(OH) absorption of low density amorphous ice at 3250 cm21. At 125 K, SO2 sublimated but at 140 K it remained some weak bands at about 1340 and at 1146 cm21 characterizing a weak yield of SO2 in the water lattice. Indeed, a temperature increase to between 90– 115 K results in the breaking of the hydrogen bonds between water molecules and sulfur dioxide molecules and pure water ice appears in addition to pure sulfur dioxide ice. 3.4.2. Films obtained without carrier gas Deposition at 15 K without carrier gas leads to a film containing a weak yield of SO2 in relation to water ice and the film is characterized by
a n3 absorption profile different from that observed in a SO2 film, containing water traces or in the (SO2)m(H2O)n phase; the nearly symmetrical absorption is measured at 1321.4 cm21 with a FWHM of 31.8 cm21. In the n(OH) region, one observes the characteristic band of amorphous ice with an absorption at 3609.7 cm21 ðFWHM ¼ 14 cm21 Þ on the frequency high side indicating the formation of hydrogen bonds between water and sulfur dioxide molecules. The intensity ratio I3609 =I3300 =I1321 was found to be 2.5/26/1 after a deposition time of 60 min at 15 K. Fig. 11 shows some typical spectra of an H2O/SO2 film deposited at 15 K and gradually annealed at 170 K, and Fig. 12 presents the evolution of the integrated intensities of water perturbed OH (labelled OHa) and of the SO2 asymmetric stretching as a function of temperature as well as the evolution of their frequencies. Above 70 K, the OH hydrogen bonded feature begins to diminish in intensity while the OH stretching absorption of water weakly increases. Above 120 K, the OH hydrogen bonded feature disappears and SO2 decreases with temperature till 150 K. The small amount of sulfur dioxide which is trapped in the cubic ice starts to disappear above 150 K when water itself evaporates. No partial crystallisation of SO2 is observed indicating that SO2 is well diluted in ice. As can be seen in Fig. 12, the n3
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Fig. 12. Integrated band intensities as a function of temperature for the associated OH band (OHa) about 3609 cm21 and for the sulfur dioxide n3 band from spectra partially presented in Fig. 11 (A). Temperature dependence of frequencies of the associated OH band (B) and of the n3 sulfur dioxide Band (C) from spectra partially presented in Fig. 11.
frequency of SO2 changes with temperature. From 80 to 160 K, the initial band at 1321.4 cm21 shifts gradually at 1340.5 cm21. At 150 K it becomes symmetrical and narrows (12.2 cm21) and a weak satellite at 1315.3 appears on the lower frequency wing; this later band which was overlapped at lower temperature by the monomer band of SO2 diluted in water is perhaps due to some SO2 dimers formed in the gas phase and diluted in the water ice. Frequencies of the hydrogen bonded OH band are also sensitive to the temperature change. From 30 to 40 K, it shifts from 3609.7 to 3308.0 cm21 then from 70 to 120 K, it is again red shifted from 3608.0 to 3606.0 cm21. All these changes probe the structural transformation of water ice with temperature as evidenced for other species trapped in water ice [31 –34]. Between 30 and
70 K, high density amorphous ice transforms into low density amorphous ice and pores are collapsed. From 120 to 140 K, low density amorphous ice transforms into crystalline ice and the rearrangement of water molecules opens some blocked channels [35] and results in the loss of SO2. Absence of hydrogen bonded OH oscillator above 120 K, is the indication that the configuration of sulfur dioxide differs from the amorphous ice lattice to cubic ice lattice.
4. Summary Systematic studies of the evolution of spectra of thin films of SO2 /H 2O in various ratios with the temperature in the 10 – 200 K has allowed us
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1. to identify two small aggregates (SO2)x(H2O)y, one of them disappearing when sulfur dioxide transforms from amorphous phase to crystalline phase between 75-80 K and the other one at 125 K when water ice transforms in a more organized phase 2. to characterize a metastable hydrated sulfur dioxide phase, (H2O)m(SO2)n, when SO2 in excess of water vapour is deposited with carrier gas. 3. to examine thermal processes upon a solid solution of SO2 in water ice showing that the profile (shape, width, and peak position) of the SO2 absorptions depends on the structure of the water matrix. The present laboratory study can help in the IR identification of SO2 embedded in water ice upon the surface of Europa, and the present results will be useful for future studies of the reactivity in water ice of sulfur dioxide under UV irradiation. The narrow band ðFWHM ¼ 6 cm21 Þ observed recently by ISO at 3584 cm21 in IR spectra of the Io surface cannot be assigned to water/SO2 aggregates from our results because their bands in this region are broader by a factor of 5 than the Io absorption at 3584 cm21. The 3584 cm21 feature originates from the (2n1 þ n3) of SO2 as reported recently by Schmitt [9]. The very weak features not yet identified but observed in the Io spectrum at about 3320 – 3300 cm 21 and close to OH bands of the (H2O)m(SO2)n phase seem also too narrow to be assigned to water trapped in SO2 ice.
Acknowledgements This work was performed with support from the Programme National de Plane´tologie (PNP).
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