J. Chem. Sci. Vol. 128, No. 1, January 2016, pp. 159–164. c Indian Academy of Sciences. DOI 10.1007/s12039-015-0996-6

Electron irradiation of dioxide-carbon disulphide ice analog and its implication on the identification of carbon disulphide on Moon

B SIVARAMAN∗ Atomic, Molecular and Optical Physics Division, Physical Research Laboratory, Ahmedabad 380 009, India e-mail: [email protected]

MS received 25 September 2014; revised 19 August 2015; accepted 7 September 2015

Abstract. (CO2) and carbon disulphide (CS2) molecular ice mixture was prepared under low temperature (85 K) astrochemical conditions. The icy mixture irradiated with keV electrons simulates the irradiation environment experienced by icy satellites and Interstellar Icy Mantles (IIM). Upon electron irradi- ation the chemical composition was found to have altered and the new products from irradiation were found to be carbonyl sulphide (OCS), sulphur dioxide (SO2), (O3), carbon trioxide (CO3), sulphur trioxide (SO3), carbon subsulphide (C3S2) and (CO). Results obtained confirm the presence of CS2 molecules in lunar south-pole probed by the Moon Impact Probe (MIP).

Keywords. Interstellar medium; interstellar icy mantles; icy satellites; radiation processing; carbon dioxide; carbon disulphide.

1. Introduction implantation on CO2 ices. However, proton implanta- tiononSO2 could not synthesize molecules containing 6 Icy satellites and Interstellar Icy Mantles (IIM) are H-S bonds. Electron irradiation of CS2 and O2 ices car- known to synthesize and harbour variety of molecules ried out at 12 K7 had shown several products bearing ranging from diatomic to complex molecules that take C-O and S-O bonds. In another experiment3 synthesis part in chemical reactions induced by energetic parti- of C-O and C-S bearing molecules were studied starting cles. The first identification of molecules, such as CO, from CO with SO2/H2S ices using proton irradiation. CS, OCS, SO and SO2 containing C-O, S-O and C-S In most of these experiments CO2 is one another bond in the InterStellar Medium (ISM) was made with- molecule that is largely used as well as a product from in a decade after 1969, when Apollo-11 landed on the synthesis in the study of sulphur bearing molecules. Moon. Since then the formation of such simple molecu- Indeed, in the IIM and icy satellite surfaces CO2 is les and the reactions where they take part in making known to be one of the most abundant C-O bearing the complex molecules were studied in laboratory simu- molecule. Therefore a mixture of CO2 and CS2 molecu- lations in both the gas and ice phase.1 Indeed, in such lar ice may exist on the surface of icy satellites and experimental simulations it is observed that several in the IIM’s, hence, it is imperative to understand the other carbon and sulphur bearing molecules, so far not chemical synthesis in icy mixture containing CO2 and reported to be present in the ISM, were also synthe- CS2. However, so far there is only limited data con- sized. For example, CS2 synthesis in laboratory simula- sidering the CO2-CS2 system and the chemical synthe- tions on carbon and sulphur containing molecular mix- sis induced by energetic processing. In this letter, the 2 tures suggest the presence of CS2 in the IIM of ISM results from electron irradiation of CO2-CS2 ice stud- and this is clearly reflected in the identification of CS2 ied at icy astrochemical conditions reproducing the sur- in comets.3,4 faces of icy satellites and IIM, are reported for the first A few irradiation and implantation experiments were time, with implications. carried out to study the formation of sulphur bearing molecules at astrochemical conditions. Recent experi- ment carried out5 by implanting sulphur ions into CO 2. Experimental and CO2 ices were observed to have altered the chemi- cal composition where molecules containing C-S bonds Electron irradiation experiment was carried out under were synthesized, especially CS2 in the case of sulphur astrochemical ice analog condition in the Low temper- ature astrochemistry experimental facility in Physical ∗For correspondence Research Laboratory (PRL). The Ultra High Vacuum 159 160 B Sivaraman

(UHV) experimental chamber containing a liquid nitro- On the onset of irradiation, new peaks were found in gen cooled cold head simulates the icy astrochemical the recorded IR spectra whilst irradiating the sample. conditions. Details of the experimental chamber can be The peaks at 2142.4 cm−1, 2060 cm−1, 2044 cm−1 and 8 −1 12 7 13 found in our very recent publication. Chemically inert 2035 cm can be assigned to CO, C3S2, CO3 and and mid-Infrared (IR) transparent Selenide (ZnSe) OCS14 molecules, respectively (figure 1(b)). In the fin- window was used as the substrate, which upon cooling ger print region, three new bands were observed with down to 85 K is an analogue of a dust surface for icy peak centers at 1330.2 cm−1, 1148.4 cm−1 and the mantles. shoulder band at 1043.1 cm−1. The first two bands cor- 7 CS2, >98% pure, along with the molecules of CO2, respond to the characteristic vibration from SO2. and 9 purity 99.9%, were premixed before letting into the the third corresponding to O3 molecule (figure 1(c)). A experimental chamber to form the icy layers of mixed set of weak bands were observed at 1396 cm−1 and 1386 −1 7 CS2-CO2. After deposition at 85 K, a spectrum in the cm corresponding to SO3 molecule (figure 1(d)). 4000 cm−1 to 500 cm−1 region, was recorded before By analysing the IR bands observed before irradia- irradiation commenced. Electrons at 2 keV and at ∼9 tion which clearly showed the absence of the morphol- μ A irradiated the sample for about 30 minutes with ogy dependent characteristic band of CO2, suggesting a subsequent recording of IR spectra at regular intervals. crystalline CO2 matrix to be present. Also the presence −1 9 Using the integral absorption coefficient (A) of the cor- of the 2328 cm band corresponding to the CO2 dimer responding molecule and the infrared band intensity, shows the interaction between CO2 molecules. Using the number of molecules of CO2,CS2 and other new equation (1), the number of molecules of CO2 and products can be calculated as follows, CS2 present in the ice phase was calculated using their  bands appearing at 3708.4 cm−1 and 1511 cm−1 along ◦ × −18 N = cos (45 ) τ (ν) dν/A (1) with their corresponding A values of 1.5 10 (cm molecule−1) and9.1x10−17 (cm molecule−1), respec- where N represents the column density (molecules tively (table 1). About ∼2.2 × 1017 molecules cm−2 of − 2 16 −2 cm ), τ(ν) the integrated band area. As the substrate CO2 and ∼8x10 molecules cm of CS2 (figure 2) ◦ ◦ is kept at an angle of 45 to the IR beam, cos(45 )is were found to be present in the ice before irradiation the correction factor for path length. As the molecules suggesting the availability of twice as much as were deposited at 85 K, the characteristic infrared band atoms for every sulphur atom present in the ice phase. strengths CO2 and CS2 were used to calculate the num- The set of new peaks in the spectra recorded at the ber of molecules present in the ice phase rather than time of and after irradiation indicates that new molecu- estimating the number of molecules in the gas phase les have been formed from electron irradiation and are prior to deposition. stable in the CO2-CS2 ice matrix. After assigning the new peaks to their characteristic vibrations correspon- 3. Results and Discussion ding to the specific molecule (table 1) and by applying equation (1), using the A values given in table 1, the IR spectra recorded after deposition showed sev- column densities of new molecules synthesized were eral vibrational bands corresponding to CO2 and CS2 obtained. At the end of irradiation, SO2 and CO were molecules. Fundamental, combination and overtone found to be the dominant products with column densities ∼ × 16 −2 ∼ × 15 bands of CO2 were observed with peak positions at of 1.8 10 molecules cm and 7 10 molecules 3708.3 cm−1, 3599.2 cm−1, 2345.2 cm−1, 660.1 cm−1, cm−2, respectively (figure 2). Whereas column densities −1 −1 14 −2 655.3 cm and 638.8 cm (figure 1(a)) and in good of CO3 and O3 were about ∼2×10 molecules cm agreement with other experimental data reported.9 The and 1 × 1014 molecules cm−2, respectively. However, morphology dependent characteristic vibrational bands the lowest concentrations were found to be for SO3 and ∼ × 13 2ν2 and ν1 of CO2 ice were found to be absent in the OCS with column densities of about 3 10 mole- ice formed at 85 K.9–11 However, the shoulder band at cules cm−2 and ∼4×1013 molecules cm−2, respectively −1 2328 cm was observed which arise due to CO2 dimer, (table 1). At the end of irradiation, number of molecules 9 ∼ × 17 (CO2)2. Vibrational features of CS2 were observed at of CO2 and CS2 left out were 1.6 10 molecules 2146 cm−1, 1511 cm−1 and 1460 cm−1 and in good cm−2 and ∼6×1016 molecules cm−2, where nearly 5 × 7 16 −2 16 agreement with other experimental data reported. In 10 molecules cm of CO2 and 2 × 10 molecules −2 addition, to the bands corresponding to CO2 and CS2 cm of CS2 were dissociated, respectively. a prominent band at with peak center at 1039 cm−1 Necessary reactants for chemical reactions to was also observed where this could be from the S-O synthesize these new products would have come from vibration that can arise from the complex CS2-CO2. the dissociation of CO2 and CS2 molecules as given Electron irradiation of CS2-CO2 ices 161

1.8 0.03

1.6 SO 1.4 2

1.2 0.02

1.0 O 3 0.8

Absorbance 0.6 Absorbance 0.01 CO 0.4 2 CO CS CO 0.2 2 2 2

0.0 0.00 4000 3500 3000 2500 2000 1500 1000 500 1370 1320 1270 1220 1170 1120 1070 1020 Wavenumber, cm-1 Wavenumber, cm-1 (a) (c)

0.025 0.0090

0.020 SO 3

0.015 CO 0.0075

CO 0.010 3 Absorbance C S Absorbance 3 2 OCS 0.0060 0.005

0.000 2230 2200 2170 2140 2110 2080 2050 2020 1990 1410 1405 1400 1395 1390 1385 1380 -1 Wavenumber, cm Wavenumber, cm-1 (b) (d)

Figure 1. IR spectra recorded (a) after deposition of CO2 –CS2 mixture on a ZnSe substrate at 85 K; (b) after irradiation showing the characteristic vibrational bands of new molecules CO, CO3,C3S2 and OCS; (c) SO2,O3; and (d) SO3. Thickness of the film was not determined.

Table 1. IR signatures, column densities of the reactants CO2 and CS2 before/after 2 keV electron irradiation and of the products observed.

Peak position Column density (molecules cm−2) Molecule (cm−1) A (cm molecule−1) Before irradiation After irradiation

−18 (a) 17 17 CO2 3707 1.5 × 10 2.2 × 10 1.64 × 10 CO 2142.4 1.1 × 10−17 (b) 7.14 × 1015 OCS 2035 1.5 × 10−16 (c) 3.77 × 1013 −17 (d) 14 CO3 2044 8.9 × 10 1.88 × 10 −17 (e) 16 16 CS2 1511 9.1 × 10 8 × 10 6.26 × 10 −17 (f) 13 SO3 1386 3 × 10 3.06 × 10 −17 (g) 16 SO2 1330 1.5 × 10 1.8 × 10 −17 (h) 14 O3 1043 1.4 × 10 1.11 × 10

References: (a) Gerakines et al.19, (b) Jiang et al.12, (c) Hudgins et al.14, (d) Bennett et al.13, (e) Pugh and Rao20,(f)Majkowskiet al.21, (g) Garozzo et al.22, (h) Smith et al.23. in equations (2) and (3). However, further dissociation The column density of CO molecules can be regarded as of CO and CS molecules cannot be neglected due to the number of O atoms that are released from CO2 dis- continued irradiation. sociation (equation 2). However, the number of S atoms that could be available after irradiation cannot be cal- → + CO2 CO O(2)culated due to the absence of CS band in the recorded IR spectra. In such a case, by adding the number of S CS2 → CS + S(3)atoms found in the products SO2,SO3 and OCS can be 162 B Sivaraman

an addition of O atom to make O3, as given in equa- tion (8). However, synthesizing OCS could occur via an addition of O to CS or S to CO (equation 10) with a third possibility of C, S, O atoms combining first to make either CS or CO first and then adding O or S, respec- tively, to synthesize OCS. Though the absence of the CS infrared signature may look like CS could be consumed in making OCS, the number of molecules of OCS syn- thesized was only ∼4×1013 molecules which suggest other dominant pathway synthesizing OCS, which can be explored by isotopically labelling the reactants CO2 and CS2. In the case of C3S2, formation pathway via the decomposition (equation 10) of three molecules of CS2 upon electron irradiation yielding C3S2, analogous to 15 C3S2 synthesis in discharge experiments, is proposed. Figure 2. Temporal evolution of the new molecules syn- If this pathway is followed then this could explain the thesized, SO2 and CO, and the number of CO2 and CS2 absence of CS band and additional source for the S molecules dissociated by electron irradiation. atoms for reactions. approximated to the number of S atoms released from CO2 + O → CO3 (7) 16 CS2 and this gives an upper limit of ∼2×10 atoms −2 cm . Considering equation (2) to be the dissociation + → (+O) → O O O2 O3 (8) pathway for CO2 generating O atoms, then an upper limit of O atoms generated would be ∼5×1016 atoms cm−2. CS + O (or) S + CO → OCS (9) It is quite surprising to note the column density of − SO2 molecule which clearly shows the effective use of 3CS( 4S) → C S (10) available S atoms, that are highly mobile at such higher 2 3 2 temperatures, in synthesizing the molecule via reac- tions that can take place in two steps with the combi- 3.1 Implication nation of an S and O atom in the first step followed by the addition of another O atom, or via a reaction of S Results obtained show efficient synthesis of SO2 from atom with a molecule of oxygen, as given in the equa- CO2-CS2 ices, suggesting that origin of SO2 could be tions (4) and (5). Further reaction of the synthesized from such an icy mixture processed by energetic parti- SO2 molecule was found to form SO3 in lesser con- cles. This implies that SO2 on Lunar south pole, could centrations. The formation pathway should be from the have been synthesized by the mixture of CO2-CS2 ices, reaction (given in equation 6) of an O atom with SO2 where SO2 and CO2 molecule in abundances of 3.19% molecule synthesized via reaction given in equation (4) and 2.17%, respectively, relative to H2O molecule was or (5). confirmed by LCROSS.16 Though chemical reactions + ( O) studied experimentally suggest the presence of CS2,so S + O → SO → SO2 (4) far it was not reported. Indeed, the evidence for the presence of CS2 was found in the data sent by the S + O2 → SO2 (5) Chandra’s Altitudinal Composition Explorer (CHACE) instrument on MIP17 where mass of 76 amu correspond- ing to CS was found at ∼9×10−13 Torr (partial pres- SO + O → SO (6) 2 2 3 sure) in the exosphere at the Lunar south pole (figure 3). As the number of oxygen atoms that are available for In fact, the ChACE instrument on MIP was used to con- 18 reaction is larger than the given amount of S atoms, firm minor chemical constituents such as O3, which other reactions involving O atoms do take place synthe- is also a product observed in the present study. Owing sizing CO3,O3 and OCS. Synthesizing CO3 could be to the larger abundances of SO2 molecules, there could via O atom addition to CO2, as given in equation (7), be several other pathways synthesizing SO2 via other whereas for making O3, O atoms have to undergo a sulphur bearing molecules such as H2S, however, the series of two-step reaction in synthesizing O2 first and reaction channels from S atoms originating from the Electron irradiation of CS2-CO2 ices 163

10-12 10-11 10-10

10-13 10-12 10-11

Figure 3. Partial pressure (in Torr) for mass of 76 amu, 64 amu and 44 amu corresponding to CS2,SO2, and CO2, respectively, in the Lunar south pole observed by the CHACE on MIP. Data on CO2 and SO2 are adapted from Sivaraman et al.18

CS2 molecules, in the Lunar south pole, will also con- Mason, Dr. Raja Sekhar, Prof. K P Subramanian and tribute to the chemical synthesis leading to SO2 and Dr. Smitha Thambi for discussion during the prepara- other sulphur bearing molecules. tion of this manuscript. Support from Jasleen Birdi, IIT- Delhi, during the course of this experiment is greatly acknowledged. 4. Conclusions References CO2-CS2 ice kept at 85 K was irradiated by 2 keV elec- trons, simulating the surface ices of icy satellites and the 1. Abbas H 2014 Astrophys. Space Sci. 351 53 IIM. Molecular dissociation of CO2 and CS2 provided 2. Ferrante R H, Moore M H, Spiliotis M M and Hudson R the necessary O and S atoms to take part in reactions L 2008 Astrophys. J. 684 1210 synthesizing SO2,CO,SO3,O3,CO3 and OCS, whereas 3. Garozzo M, Fulvio D, Kanuchova Z, Palumbo M E and a direct dissociation of CS trimer molecules could be Strazzulla G 2010 Astron. Astrophys. 509 A67 2 4. Jackson W M, Scodinu A, Xu D and Cochran A L 2004 yielding C3S2. Column density of molecules synthe- Astrophys. J. Lett. 607 L139 sized clearly showed the dominant product to be SO2 5. Lv X Y, Boduch P, Ding J J, Domaracka A, suggesting most of the S atoms to have been effectively Langlinay T, Palumbo M E, Rothard H and Strazzulla G undergone reaction with the much abundant O atoms in 2014 MNRAS 438 922 the ice matrix. A series of reactions involving O atoms, 6. Strazzulla G 2011 Nucl. Instrum. Meth. B 269 842 7. Maity S and Kaiser R I 2013 Astrophys. J. 773 184 in synthesizing O3/CO3/SO3 /SO2, revealed the dif- 8. Sivaraman B, Mukherjee R, Subramanian K P and ferent pathways undergone by the energetic O atoms; Banerjee S B 2014 Chem. Phys. Lett. 609 167 a limiting reactant in the formation of higher of 9. Sivaraman B, Raja Sekhar B N, Fulvio D, Hunniford carbon and sulphur. The results obtained suggest that A, McCullough B, Palumbo M E and Mason N 2013 J. Chem. Phys. 139 074706 higher oxides of sulphur, up to SO3, can be synthesized 10. Falk M 1987 J. Chem. Phys. 86 560 even in chemically diverse system such as the CO2-CS2 11. Gerakines P A and Hudson R L 2015 Astrophys. J. Lett. used in this experiment. However, the dominant forma- 808 L40 12. Jiang G J, Person W B and Brown K G 1975 J. Chem. tion pathway in synthesizing OCS in the CO2-CS2 ice Phys. 62 1201 matrix could be deduced by isotopically labelling the 13. Bennett C J, Jamieson C, Mebel A M and Kaiser R I reactant partners, which will be carried out in future 2004 PCCP 6 735 experiments. 14. Hudgins D M, Sandford S A, Allamandola L J and TielensAGGM1993 Astrophys. J. Suppl. S 86 98 15. Stadlbauer W and Kappe T 1999 Reports 21 423 Acknowledgements 16. Colaprete A, Schultz P, Heldmann J, Wooden D, Shirley M, Ennico K, Hermalyn B, Marshall W, Ricco A, Elphic BS would like to acknowledge the support from mem- R C, Goldstein D, Summy D, Bart G D, Asphaug E, bers of the MOL-PH group. Thanks to Prof. Sridha- Korycansky D, Landis D and Sollitt L 2010 Science 330 ran, for the data from CHACE on MIP, and Prof. Nigel 463 164 B Sivaraman

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