Formation of unsaturated hydrocarbons by cosmic ray analogs in interstellar ices

Mon. Not. R. Astron. Soc. 423, 2209–2221 (2012) doi:10.1111/j.1365-2966.2012.21031.x

Formation of unsaturated hydrocarbons in interstellar ice analoguesby cosmic rays

S. Pilling,1� D. P. P. Andrade,1 E. F. da Silveira,2 H. Rothard,3 A. Domaracka3

and P. Boduch3

1Instituto de Pesquisa e Desenvolvimento (IP&D), Universidade do Vale do Paraıba (UNIVAP), Sao Jose dos Campos, SP 12244-000, Brazil2Instituto de Fisica, Pontifıcia Universidade Catolica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, RJ 22451-000, Brazil3Centre de Recherche sur les Ions, les Materiaux et la Photonique CIMAP (GANIL/CEA/CNRS/ENSICAEN/Universite de Caen Basse-Normandie),F-14070 Caen Cedex 05, France

Accepted 2012 April 2. Received 2012 April 1; in original form 2012 January 14

ABSTRACTThe formation of C=C and C≡C bonds from the processing of pure c-C6H12 (cyclohex-ane) and mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ices by highly charged and energetic ions(219-MeV 16O7 + and 632-MeV 58Ni24 +) is studied. The experiments simulate the physicalchemistry induced by medium-mass and heavy-ion cosmic rays in interstellar ice analogues.The measurements were performed inside a high vacuum chamber at the heavy-ion acceler-ator Grand Accelerateur National d’Ions Lourds (GANIL) in Caen, France. The gas sampleswere deposited on to a polished CsI substrate previously cooled to 13 K. In situ analysis wasperformed by a Fourier transform infrared spectrometry at different ion fluences. Dissociationcross-section of cyclohexane and its half-life in astrophysical environments were determined.A comparison between spectra of bombarded ices and young stellar sources indicates that theinitial composition of grains in these environments should contain a mixture of H2O, NH3,CO (or CO2), simple alkanes and CH3OH. Several species containing double or triple boundswere identified in the radiochemical products, such as hexene, cyclohexene, benzene, OCN−,CO, CO2, as well as several aliphatic and aromatic alkenes and alkynes. The results suggestan alternative scenario for the production of unsaturated hydrocarbons and possibly aromaticrings (via dehydrogenation processes) in interstellar ices induced by cosmic ray bombardment.

Key words: astrobiology – astrochemistry – molecular data – methods: laboratory – cosmicrays – ISM: molecules.

1 IN T RO D U C T I O N

A large number of gas-phase molecules containing up to 13 atomshave been discovered in interstellar space (see review in Herbst &van Dishoeck 2009 and Ehrenfreund & Charnley 2000). These in-terstellar molecules are mainly organic compounds involving unsat-urated carbon-chain species such as simple alkenes (CnH2n), cumu-lene carbenes [H2C(=C)n] and alkynes (CnH2n−2) (e.g. Winnewisser& Herbst 1987; Herbst 1995; Thaddeus & McCarthy 2011). Cumu-lene carbenes and other unsaturated carbon-chain species have alsobeen suggested in the literature as possible carriers of diffuse inter-stellar bands (Apponi et al. 2000).

As discussed by Cernicharo et al. (2001a,b) small unsaturatedcompounds such C4H2, C6H2, methylpolyynes (e.g. CH3C2H andCH3C4H) and also ring compounds such as benzene have alsobeen detected in space environments such as in dust shells of

�E-mail: [email protected]

late stars, protoplanetary nebulae (PPNe). Long aliphatic unsatu-rated chains containing nitrogen atoms, such as polyacetylenes orpolyynes H–(C≡C)n–H, and cyanopolyynes [H–(C≡C)n–CN], mayalso be present in interstellar medium (ISM; Fukuzawa et al. 1998).The polyacetylenes are also considered the missing link betweensmall gas-phase molecules; among them acetylene (C2H2), carbonmonoxide (CO), hydrogen cyanide (HCN) and carbonaceous grainparticles formed in the outflow of carbon-rich stars (Duley, Grishko& Pinho 2005; Cordiner & Millar 2009). Aromatic compoundssuch as naphthalene and anthracene cation have been detected incold molecular clouds (Iglesias-Groth et al. 2008, 2010). In addi-tion, Duley et al. (2005) suggested the presence of cyclic alkanesand alkenes, such as cyclohexene, in an attempt to justify the appear-ance of spectral features near 3.4 µm in the infrared (IR) spectra ofinterstellar clouds (IC) and in diffuse interstellar medium (DISM).For example, they found that the average CH2/CH3 ratio in a num-ber of cyclohexene and cyclohexadiene compounds with spectrathat match the 3.4-µm band in the PPN CRL 618 is ≈1.15. Singlering systems, such as c-C3H and c-C3H2, have also been detectedin space (Smith 1992).

C© 2012 The AuthorsMonthly Notices of the Royal Astronomical Society C© 2012 RAS

2210 S. Pilling et al.

Several mechanisms have been proposed to explain the presenceof these unsaturated compounds in space including gas-phase ion–molecule reactions and neutral–neutral reactions (e.g. Cherchneff,Barker & Tielens 1992; Taylor & Duley 1997; Fukuzawa et al. 1998;Duley et al. 2005, and references therein). The present experimentalradiochemical study suggests an alternative scenario for the produc-tion of unsaturated carbon chain species (and dehydrogenation) ininterstellar ices induced by cosmic ray bombardment. The experi-ments were performed bombarding cyclohexane (c-C6H12) contain-ing ices at 13 K with fast ions to simulate the physical chemistryinduced by cosmic rays in interstellar unsaturated ice analoguesinside dense regions of ISM, such as molecular clouds and proto-stellar discs. Although no large saturated cyclic compound in theISM, such as cyclohexane, has been directly detected, the presenceof this compound, as well as of other large saturated hydrocarbonshas been suggested in these regions (e.g. Pendleton et al. 1994;Duley et al. 2005). In this paper, this compound is just a prototypefor saturated hydrocarbons. We expect that the unsaturation pro-cesses described here also occur with other saturated hydrocarbonsalready detected in space (e.g. Pendleton & Allamandola 2002;Duley et al. 2006). In addition, saturated hydrocarbons can beformed on the surface of frozen grains by H-atom reactions ongraphite grains (Bar-Num, Litman & Rappaport 1980) and directhydrogenation (catalytic hydrogenation) of alkenes/alkynes (e.g.Olah & Molnar 2003; Marcelino et al. 2007).

Section 2 describes briefly the experimental set-up. The resultson the radiolysis of saturated hydrocarbon containing ices, includ-ing formation and dissociation cross-sections, are presented anddiscussed in Section 3. Some astrophysical implications, as well asan estimate for half-lives of these molecules at possible astrophysi-cal environments, are provided and discussed in Section 4. Finally,Section 5 contains the final remarks and conclusions.

2 EXPERIMENTA L

To simulate the physicochemical changes induced by medium-massand heavy-ion cosmic rays in interstellar ice analogues, we used thefacilities of the heavy-ion accelerator Grand Accelerateur Nationald’Ions Lourds (GANIL) in Caen, France. The measurements wereperformed inside a high vacuum chamber which could be coupledto different ion beam lines: IRRadiation Sud (IRRSUD) (for projec-tiles with 219-MeV 16O7 +) and Sortie Moyenne Energie (SME) (for632-MeV 58Ni24 +). The beam flux was φ = 2 × 109 ions cm−2 s−1.The gas samples were deposited on to a polished CsI substratepreviously cooled to 13 K. The ion projectiles impinged perpendic-ularly on to the ice target. In situ analysis was performed by Fouriertransform infrared (FTIR) spectrometry at different ion fluences.

In this paper, results for pure c-C6H12 ice and mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ice, both at 13 K, are presented. Pure c-C6H12

ice was bombarded by 219-MeV 16O7 + (13.7 MeV a.m.u.−1). Forthe mixed ice, the projectile employed was 632-MeV 58Ni24 +

(10.9 MeV a.m.u.−1). The equilibrium charge states of O and Niatoms (independent of the initial charge state) after several colli-sions with matter (ice) were around 7 and 13, respectively (e.g.Nastasi, Mayer & Hirvonen 1996).

In situ IR spectra were recorded for ices irradiated at differentfluences, up to 6 × 1013 ions cm−2 using a Nicolet FTIR spectrom-eter (Magna 550) from 4000 to 600 cm−1 with 1 cm−1 resolution. Abackground allowing the absorbance measurements was collectedbefore gas deposition. Experimental details are given elsewhere(Seperuelo Duarte et al. 2009, 2010; Pilling et al. 2010a,b).

Figure 1. Schematic diagram of the experimental set-up. The ion beamimpinges perpendicularly on the thin ice film deposited on a CsI crystal(taken from Pilling et al. 2010a).

The sample cryostat system can be rotated over 180◦ and fixedat three different positions to allow (i) gas deposition, (ii) FTIRmeasurement and (iii) perpendicular irradiation as shown in Fig. 1(taken from Pilling et al. 2010a). The thin ice film was prepared

by condensation of gases (purity superior to 99 per cent) on to aCsI substrate attached to a closed-cycle helium cryostat, cooled to12–13 K. Experiments without sample bombardment did not showany chemical alteration of ice sample due to vacuum with the ex-ception of a small water layering as a function of time. Duringthe experiment the chamber pressure was around 2 × 10−8 mbar.After the irradiation the samples were heated (∼2 K min−1) to roomtemperature. During the slow heating phase IR spectra were takento monitor sample changes. Future experiments will be performedto increase the amount of produced organic residues which will beanalysed further by ex-situ chromatographic techniques.

The molecular column densities of samples were determinedfrom the relation between optical depth τ ν = ln (I0/I) and bandstrength, A(cm molecule−1), of the respective sample vibrationalmode. In this expression, I0 and I are the intensity of light at aspecific wavenumber, before and after passing through a sample,respectively. Because the absorbance measured by the FTIR spec-trometer is Aν = log (I0/I), the molecular column density of icesamples is given by

N = 1

A

∫τνdν = 2.3

A

∫aνdν (molecules cm−2), (1)

where aν = ln (I0/I)/ln (10) = τ ν /2.3.From these measurements and assuming an average density for

the ice samples of about 1 g cm−3, the thickness, the depositionrate and the sample mass were determined (see the equations inPilling et al. 2011). For the pure c-C6H12, the thickness was about1.8 µm, the deposition rate about 11 µm h−1 and the mass about100 µg. For the mixed H2O:NH3:c-C6H12 (1:0.3:0.7), the estimatedthickness was 4.5 µm, the deposition rate ∼27 µm h−1 and the mass∼250 µg.

Knowing the ice thickness and the initial column densityand considering that each monolayer has roughly NML ∼1015 molecules cm−2, the thickness of a single monolayer, dML, wasestimated by the expression

dML = 104 d

N0/NML= 1019 d

N0(Å), (2)

where d is the ice thickness (µm) and N0 is the initial molecularcolumn density (molecules cm−2). The monolayer thickness of pure

C© 2012 The Authors, MNRAS 423, 2209–2221Monthly Notices of the Royal Astronomical Society C© 2012 RAS

Unsaturation induced by cosmic rays 2211

Figure 2. FTIR spectra from 4000 to 600 cm−1 of non-irradiated ices at13 K: mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ice (top) and pure c-C6H12 ice(bottom). For comparison, spectra of pure H2O ice and pure NH3 ice arealso shown.

cyclohexane ice and mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ice werearound 15 and 8 Å, respectively.

The analysed ice layers were thin enough (i) to avoid saturation ofthe FTIR signal in transmission mode and (ii) to be fully crossed byan ion beam with velocity approximately constant. This latter pointis important because the relatively low total kinetic energy loss ofthe projectile in the film guarantees that the studied cross-sectionsremain constant.

The dissociation cross-sections determined from such experi-ments employing ions have a good reproducibility as we observefor the two H2O:CO2 (1:1) 13-K ices irradiated by 52-MeV Ni ionswith the same instrumentation (Pilling et al. 2011). For pure ices,the temperature is the main parameter that can affect experimentalreproducibility. However, for mixed ices, besides the temperature,changes in the initial molecular abundance ratio may affect the en-tire system, resulting in variations in the dissociation cross-sections,formation cross-section of newly produced species as well as in thesputtering yield (e.g. Pilling et al. 2010b).

For comparison, the IR spectra of non-bombarded mixedH2O:NH3:c-C6H12 (1:0.3:0.7) ice are presented together with spec-tra of non-bombarded pure H2O, NH3 and c-C6H12 ices in Fig. 2.Despite the similarities in the IR spectra corresponding to mixedand pure ices, the spectra of mixed ice cannot be obtained by the

simple sum of the spectra of pure ices. The presence of differentchemical environments in the ice (in the case of mixed ices) changessome IR band profiles and also may change the band strength ofsome molecular vibration modes. Except for the sharp ammoniapeak at 3400 cm−1 (–NH stretching mode), all other sharp peaks inthe mixed ices are assigned to c-C6H12. The NH3 umbrella vibra-tion mode is easily observed in the mixed ice. However, this bandis slightly shifted to higher frequencies (1095 cm−1) in comparisonwith the band’s location observed in pure NH3 ice (1070 cm−1).The broad water bands at ∼3300 cm−1 (–OH stretching mode) andat ∼800 cm−1 (–OH libration mode) are also easily observed in theIR spectra of mixed ices.

The vibrational band positions and IR absorption coefficients(band strengths) used in this work to derive the column densitiesof the selected species are given in Table 1. Because of the diffi-culty in determining the area of NH3 and H2O bands due to theconvolution with other peaks, their column density values shouldbe employed with caution. In an attempt to quantify the amountof produced alkenes, we considered the cleanest and well-definedalkene peak in the IR spectra, which occurred at 719 cm−1 (13.9 µm)(=CH out-of-plane bending). For this IR transition, an average bandstrength value of ∼8 × 10−18 cm molecule−1 was adopted. Thisvalue is only an approximation for the amount of alkenes, sinceit represents an average value for this band in simple alkenes,such as hexene, cyclohexene and benzene (e.g. d’Hendecourt& Allamandola 1986; Ruiterkamp et al. 2005; Pavia et al.2009).

3 R ESULTS AND DI SCUSSI ON

Figs 3(a)–(b) present the evolution of the IR spectra of pure c-C6H12

ice at 13 K as a function of ion fluence. In both figures, the upper-most curve shows the IR spectra of pure c-C6H12 ice at 13 K beforeirradiation. Other curves show the IR spectra at different irradiationfluences employing 219-MeV O ions. Each spectrum is offset forclearer visualization. The figure inset shows the IR spectrum of thepure c-C6H12 ice as a whole. The IR absorption band employed todetermine the molecular column density of cyclohexane is indicatedby the arrow. The lowest curve was obtained after a fluence of 6 ×1013 O ions cm−2. The residual gas inside the chamber (mainly wa-ter) causes the condensation of water (broad bands around 3300 and800 cm−1), better seen at high fluences, as well as some radiolysisproducts such as CO (2133 cm−1) and CO2 (2341 cm−1). In bothfigures, the newly formed products from the radiolysis of c-C6H12

Table 1. IR absorption coefficients (band strengths) used in the column density calculations for theobserved molecules.

Wavenumber Wavelength Assignment Band strength Ref.(cm−1) (µm) (cm molecule−1)

2340 4.27 CO2 (ν3) 7.6× 10−17 (1)∼2165 4.62 OCN− (ν3) ∼4× 10−17 (2)2135 4.67 CO (ν1) 1.1× 10−17 (3)1299 7.69 CH4 (ν4) 7× 10−18 (4)1095 9.13 NH3 (ν2; umbrella mode) 1.2× 10−17 (5)861 11.6 c-C6H12 (–C–C– stretch) 8× 10−19 (6)∼800 12.5 H2O (νL; libration mode) 2.6× 10−17 (6)719 13.9 Alkenes (–C–H out-of-plane bending) ∼8× 10−18 (7)

Note. (1) Gerakines et al. (1995); (2) average value from d’Hendecourt et al. (1986) and Demyket al. (1998); (3) Jiang, Pearson & Brown (1975); (4) Gerakines et al. (2005); (5) Kerkhof, Schutte &Ehrenfreund (1999); (6) d’Hendecourt & Allamandola (1986); (7) tentative average value for simplealkenes, such as cyclohexene (see details in text).



Figure 3. (a) Expanded view of the IR spectra of pure c-C6H12 ice at 13 Kbefore (highest curve) and after different irradiation fluences employing219-MeV O ions (medium-mass cosmic ray analogue) from 3200 to1500 cm−1. Inset figure shows IR spectrum of non-bombarded c-C6H12

ice from 4000 to 600 cm−1. (b) Expanded view from 1600 to 600 cm−1. Ionfluences are indicated. Asterisks indicate the location of newly produced IRbands.

are indicated by asterisks and their possible assignments are listedin Table A1.

Figs 4(a)–(b) present the IR spectra of mixed H2O:NH3:c-C6H12

(1:0.3:0.7) ice at 13 K before (uppermost curve) and after differentirradiation fluences employing 632-MeV Ni ions. Each spectrumis offset for clearer visualization. As in the previous figure, theIR absorption band employed to determine the molecular columndensity of cyclohexane is indicated by the arrow. The lowest curvewas obtained after a fluence of 3 × 1013 Ni ions cm−2. Inset figureshows IR spectrum of non-bombarded c-C6H12 ice from 4000 to600 cm−1. The broad structure from 3100 to 3500 cm−1 representsa combination of vibration modes of water (ν1) and ammonia (ν1).The narrow peak at 2100 cm−1 is the CO stretching mode (ν1).The band at 1600 cm−1 consists of two lines corresponding to thewater ν2 vibration mode (1650 cm−1) and ammonia ν4 vibrationmode (1630 cm−1). The feature around 1100 cm−1 is the umbrellavibration (ν2) mode of ammonia and the one at 800 cm−1 is thelibration mode (νL) of water molecules. The newly formed productsfrom the radiolysis of mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ice at13 K are indicated by asterisks and their assignments are listed inTable A1.

The formation of CO2 (2341 cm−1) from the bombardment ofpure c-C6H12 is sensitively triggered after employing 1013 ions cm−1

Figure 4. (a) Expanded view of the IR spectra of mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ice at 13 K before (highest curve) and after differentirradiation fluences employing 632-MeV Ni ions (heavy cosmic ray ana-logue) from 3200 to 1500 cm−1. Inset figure shows IR spectrum of non-bombarded mixed ice from 4000 to 600 cm−1. (b) Expanded view from1600 to 600 cm−1. Ion fluences are indicated. Asterisks indicate the loca-tion of newly produced IR bands.

in the ice. In addition, for higher fluences, this CO2 peak has ashoulder in the lower wavenumber region (2334 cm−1) which maybe attributed to the –C≡C– symmetric stretch of non-symmetricalkynes (Pavia et al. 2009).

Figs 5(a)–(b) present a comparison of the IR spectra of thetwo irradiated ices (at highest fluences) in this study with the IRspectra of different non-irradiated cyclic and aliphatic hydrocar-bons from National Institute of Standards and Technology (NIST;Linstrom & Mallard 2005) database1 (liquid-phase). Skeletal for-mulae of each species (cyclohexene, 1,3-cyclohexadiene, benzene,hexane, 1-hexene and 1-hexyne) are also shown. Asterisks indicatethe peaks that have possible identification in the spectra of irradi-ated ices. Fig. 5(a) shows the IR spectra from 3200 to 1500 cm−1,and Fig. 5(b) presents the 1500–600 cm−1 wavenumber range. Thepresence of a broad structure at 1600 cm−1 indicates the presenceof the rings of aromatic systems in the ice after bombardment asresult of dehydrogenation of cyclohexane molecules. A theoreticalstudy about the energies and the transient states involved duringthe ring opening of cyclohexane to produce 1-hexene (c-C6H12 →1-C6H12) was performed by Sirjean et al. (2006). The authors also

1 http://webbook.nist.gov/chemistry/



Figure 5. Comparison between the IR spectra of the two irradiated ices at highest fluences (this work) with IR spectra of different non-irradiated cyclic andaliphatic hydrocarbons from the NIST database (Lindstrom & Mallard 2005). Skeletal formulae of each species (cyclohexene, 1,3-cyclohexadiene, benzene,hexane, 1-hexene and 1-hexyne) are shown. Asterisks indicate the peaks that have possible identification in the spectra of irradiated ices. (a) From 3200 to1500 cm−1. (b) From 1600 to 600 cm−1.

suggested that species such as biradicals (·C6H12· and ·C4H8·) andethylene (C2H4) are present in such ring-opening mechanisms. Thislast species is required in the production of aromatic compounds ininterstellar environments (e.g. Frenklach & Feigelson 1989).

Table A1 lists the new IR bands formed after ion bombardment ofthe cyclohexane containing ices at 13 K. A large number of vibra-tional modes are attributed to alkenes and possibly to ring/aromaticsystems. For mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ice, the forma-tion of CO, CO2 and OCN− is observed. Due to the local heat andenergy delivered by projectiles in the sample, the bombardment isexpected to promote desorption of molecular hydrogen; however,no measurement of this species has been performed.

3.1 Cross-section and radiolysis yield

The column density variation as a function of ion fluence for eachcondensed molecular species (parental or daughter) during radiol-ysis can be written as

dNi

dF=

∑k �=i

σf,ikNk + Li − σd,iNi − Yi�i(F ), (3)

where∑

kσ f,ikNk represents the total molecular production rate ofthe i species directly from the k species, Li is the layering, σ d,i

the dissociation cross-section, Yi the sputtering yield and �i(F) therelative area covered by the i species on the ice surface. For moredetails, see equations in Pilling et al. (2010a) and de Barros et al.(2011).

For pure cyclohexane ice, the solution of equation (3), consider-ing no extra production (σ f,ik = 0), full covering [�i(F) = 1] and

no layering (L = 0) is given analytically by

N = (N0 + Y

σd) exp(−σdF ) − Y

σd, (4)

where N0, Y and σd are the initial column density, sputtering anddissociation cross-section of cyclohexane species, respectively.

In the case of mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ice, after therapid ice compaction phase at the beginning of the irradiation, waterlayering tends to recover the ice surface [�i(F) → 0], progressivelypreventing the sputtering of other species. Therefore, under theassumption that both sputtering and layering are negligible (Y =L = 0) the solution of equation (3) for the column density of c-C6H12 as a function of fluence is obtained analytically by

N = N0 exp(−σdF ), (5)

where N0 and σ d are, respectively, the initial column density anddissociation cross-section of cyclohexane species in the mixed ice.

For experiments employing water (and viscous liquids), an ad-ditional layering may occur due to the molecular retention in thechamber walls. In this case, and considering no other formationsource (σf,ik = 0), the column density evolution with the fluence isobtained directly by solving equation (3), which gives

N = (N0 − N∞) exp(−σdF ) + N∞, (6)

where N0 and σ d are, respectively, the initial column density anddissociation cross-section of H2O. In this equation, N∞ = (L −Y)/σ d is the asymptotic value of column density of water due to thepresence of layering. In experiments where high layering values areobserved, the sputtering may be considered negligible.

As discussed by de Barros et al. (2011), for low fluences (up to∼5 × 1012 ions cm−2), the column density evolution of daughter



Figure 6. (a) Variation of the column density of cyclohexane and selecteddaughter species (CH4) in pure ice experiment as function of ion (219-MeV O) fluence. (b) Variation of the column density of cyclohexane, waterand ammonia and selected daughter species (CO, CO2, CH4 and OCN−)in mixed ice experiment as a function of ion (632-MeV Ni) fluence. Thelines indicate the fittings using equation (4) (pure cyclohexane), equation (5)(cyclohexane and ammonia in mixed ice), equation (6) (water in mixed ice)and equation (7) (products). The model parameters are given in Table 3.

species produced by the radiolysis of both pure and mixed ices is

Nk ≈ N0σf,k

(F − σd + σd,k

2F 2

), (7)

where Nk is the column density of daughter species k at a givenfluence. σ f,k and σ d,k are, respectively, the formation cross-sectionand dissociation cross-section of daughter species k. N0 and σ d

are, respectively, the initial column density and dissociation cross-section of parental species. This equation assumes a negligible valuefor the sputtering of daughter species (Yk = 0).

Fig. 6 shows the evolution of the column density of cyclohexanein pure and mixed studied ices as a function of ion fluences. Thecolumn densities of four radiolysis products (CO, CO2, CH4 andOCN−) observed in the radiolysis of the mixed ice are also shown.The decreasing of the cyclohexane column density is related to theformation of other species and to the sputtering induced by heavyions (Seperuelo Duarte et al. 2009, 2010; Pilling et al. 2010a,b).

Table 2 shows the stopping power and penetration depth val-ues for 219-MeV (13.6 MeV a.m.u.−1) 16O ions and 632-MeV(10.8 MeV a.m.u.−1) 58Ni ions in the studied ices, calculated bythe Stopping and Range of Ions in Matter (SRIM) code.2 The SRIM

code is a collection of software packages that calculate many fea-tures of the transport of ions in matter (Ziegler, Biersack & Ziegler2008). For comparison, values for 632-MeV (10.8 MeV a.m.u.−1)

2 http://www.srim.org/

58Ni, 1- and 12-MeV protons, all in pure cyclohexane ice, and1-MeV protons in pure water ice are also shown. The energy deliv-ered per micron by a single 632-MeV Ni ion is roughly 11 timeshigher than for a single 219-MeV O ion, and about 110 times higherthan for a single 1-MeV proton.

The radiochemical formation yield (Gf ) of a given compound per100 eV of deposited energy, at normal incidence, is written by

Gf = 100σf

Smolecules per 100 eV, (8)

where σ f is the formation cross-section and S is the stopping power,in units of eV (molecules cm−2)−1 (Loeffler et al. (2005).

This definition can be extended to the radiochemical dissociation(destruction) yield of a given compound per 100 eV of depositedenergy (Gd) by replacing the formation cross-section in equation (8)with the negative value of the dissociation cross-section (σ d). There-fore, negative Gd values indicate that molecules are being dissoci-ated or destroyed after energy deposition into the ice. By adoptingthe S values from the stopping and ranges module of the SRIM code,the values of the radiation yield G in the experiments can be deter-mined and compared with values in the literature.

Figs 6(a)–(b) present the column density evolution of speciesinitially present in pure and mixed ice experiments, as well somenewly formed products due to the radiolysis, as a function of ionfluences. Fig. 6(a) shows the evolution of cyclohexane, the newlyformed CH4, as well as the possible alkenes produced from thebombardment of the pure cyclohexane ice by 219-MeV O ions. Theevolution of cyclohexane, water and ammonia, four newly formedspecies (CO, CO2, CH4 and OCN−) and possible alkenes observedin the radiolysis of the mixed ice by 632-MeV Ni ions are given inFig. 6(b). The lines indicate the fittings using equation (4) (pure cy-clohexane), equation (5) (cyclohexane and ammonia in mixed ice),equation (6) (water in mixed ice) and equation (7) (products). Nu-meric labels indicate the models employed in which the parametersare listed in Table 3.

Table 3 lists the values obtained by the best-fitting curve em-ploying equations (4)–(7) to the column density evolution data ofbombarded ices. First columns show the selected species, followedby the formation cross-section (only for daughter species), dissocia-tion cross-section and radiolysis yields (formation and destruction).For the radiolysis yields we employed the stopping power valueslisted in Table 2, 3.49 × 10−13 eV (molecules cm−2)−1 (for pureice) and 1.27 × 10−12 eV (molecules cm−2)−1 (for mixed ice). Inaddition, for mixed ice it was considered that the projectile energywas absorbed by the whole bulk sample and not only for a singlespecies. The last three columns in Table 3 are the sputtering yield(only for pure ice), the layering and the initial column density. Thesputtering value for water of 104 molecular per impact was takenfrom Brown et al. (1984). The water layering was derived by theequation L = N∞σ d + Y .

The destruction cross-section obtained in this study (around10−13 cm2) for the frozen species bombarded with 632-MeV Niions are in the same order of magnitude of the values obtainedpreviously also employing swift heavy ions (Pilling et al. 2010a,b,2011). However, the formation cross-sections and also the forma-tion yields of new species decrease with the projectile energy. Thisissue reinforces that an optimal energy associated with the forma-tion of new (organic) species during the processing of astrophysicalices could exist (see also Pilling et al. 2011).

The formation cross-section of alkenes, considering theC–H out-of-plane bending mode at 719 cm−1 (A ∼ 8 ×10−18 cm molecule−1), in the pure c-C6H12 ice and mixed



Table 2. Stopping power and penetration depth values for 219-MeV (13.6 MeV a.m.u.−1) 16Oions and 632-MeV (10.8 MeV a.m.u.−1) 58Ni ions in the studied ices, calculated by the SRIM code.For comparison, values for 632-MeV (10.8 MeV a.m.u.−1) 58Ni, 1- and 12-MeV protons in purecyclohexane ice and 1-MeV protons in pure water ice are also shown.

Projectile Stopping power Penetration depthElectronic Nuclear Total (µm)

(keV µm−1) (keV µm−1) [eV (molecules cm−2)−1]

219-MeV Oa 250.4 0.13 3.49 × 10−13 509.7632-MeV Nib 2787 1.70 1.27 × 10−12 172.5632-MeV Nia 2802 1.72 3.88 × 10−12 169.81-MeV pa 28.89 0.023 3.12 × 10−15 21.41-MeV pc 25.11 0.021 7.51 × 10−13 25.112-MeV pa 4.296 0.002 4.03 × 10−14 1540

Note. Employed ice density = 1.0 g cm−3.aIn pure c-C6H12 ice (∼7 × 1021 molecules cm−3).bIn mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ice (∼2 × 1022 molecules cm−3).cIn pure H2O ice (∼3 × 1022 molecules cm−3).

H2O:NH3:c-C6H12 ice were about 1 × 10−15 and 1 × 10−14 cm2,respectively. These can be compared with similar experimentsemploying other saturated hydrocarbons such as ethane, propaneand cyclopropane. Future experiments will compare the formationcross-section of alkenes (–C=C–), induced by cosmic rays in in-terstellar ice analogues, with respect to the number of carbons ofparental saturated hydrocarbon present initially in the ice.

4 A STRO PHYSICAL IMPLICATIONS

Despite the ion flux employed in laboratory experiments being sev-eral orders of magnitude higher than the flux of similar ions inspace and the thickness of laboratory ice also being higher, theenergy delivered and the damage induced by similar ions in bothscenarios have great similarity. For example, let us quantify the roleof ice size in the experiments. As discussed in Section 2, the thick-ness of pure cyclohexane ice was about 1.5 µm and the stoppingpower of the employed projectile in this ice was 250 keV µm−1

(see Table 2). Thus, the energy’s variation of projectile when it goesthrough the ice (since its first molecular layer up to the last one) was0.375 MeV (∼0.2 per cent of projectile energy). For the mixed ice,the thickness was around 4.5 µm and the projectile stopping powerwas 2788 keV µm−1, resulting in the projectile energy difference,between top and bottom ice layers, of about 12.5 MeV (∼2 per centprojectile energy). Therefore, in both cases we consider a constantprojectile energy within the sample and also a linear energy deposit.These assumptions are in good agreement with cosmic ray impactin submicron ices in interstellar or interplanetary regions.

In a similar experiment performed by Pilling et al. (2010a), theauthors discussed that each projectile induces significant changes inthe sample only in a region with roughly 3 nm of diameter. Consider-ing a constant and homogeneous ion flux of 2 × 109 ions cm−2 s−1,the average distance between two nearby impacts is roughly 300 nm(about 100 times higher than the length of processed sample by asingle projectile hit). From these values, we estimate that the prob-ability of an ion to hit a given area of 3 nm in diameter each secondis about P ∼ 1.4 × 10−4. Therefore, only after about 2 h of con-tinuous bombardment can we expect a second projectile hit in thesame nanometric region. This is enough time to consider that eachprojectile hit always a thermalized region in the laboratory. Sucha scenario is very similar to interstellar or interplanetary regionswhich have very low ion flux as well as ion fluence (even integratedover a very extended period of time).

The equivalent cosmic ray exposure time (T ISM) in ISM withrespect to the experimental ion fluence can be obtained by theequation

TISM 3.2 × 10−8 F

φ(yr), (9)

where F is the ion fluence employed in the experiments, in units ofions cm−2 and φ indicates the flux of Galactic cosmic rays (e.g. formedium-mass cosmic rays φMCR or for heavy cosmic rays φHCR)in units of ions cm−2 s−1. Pilling et al. (2010a) estimated the inte-grated cosmic ray flux for heavy-ion (12 � Z � 29) componentswith energies between 0.1 and 10 MeV a.m.u.−1 in ISM. The valuefound was φHCR ∼ 5 × 10−2 cm−2 s−1. Pilling et al. (2011), em-ploying a similar methodology, estimated the medium-mass (3 �Z � 11) component of Galactic cosmic rays in ISM with the sameenergy range. The value obtained by those authors was φMCR ∼4 × 10−1 cm−2 s−1. Following those authors, both values are also agood estimation for the heavy-ion and medium-mass componentsof Galactic cosmic rays at the outer border of the heliopause.

The chemical evolution of the studied ices extrapolated to spaceconditions (dust grains in ISM) is shown in Figs 7(a)–(b). In thesefigures, lines are only to guide the eyes. The column density ratios ofcyclohexane and some of its radiolysis products over initial cyclo-hexane column density, as a function of equivalent exposure time inISM by cosmic rays, are shown in Fig. 7(a) for pure c-C6H12 ice at13 K irradiated by 219-MeV O ions (medium-mass cosmic ray ana-logue) and in Fig. 7(b) for H2O:NH3:c-C6H12 (1:0.3:0.7) ice at 13 Kirradiated by 632-MeV Ni ions (heavy cosmic ray analogue). Thetotal column density of produced alkenes was obtained by using thearea of the IR feature at 719 cm−1 (–CH out-of-plane bending) con-sidering, as a first hypothesis, that this band can be representativeof all the alkenes in the ice. This value represents an intermediatevalue between the strongest and weakest IR features in the case ofcyclohexane (e.g. d’Hendecourt & Allamandola 1986). The errorbars in the figure reflect the uncertainty in the methodology em-ployed to derive the column density of alkenes from IR spectra.The maximum production of alkenes is obtained after about 3–5 ×106 yr, independently of the projectile type. Its maximum value wasroughly the same in both experiments, ∼10−2 alkenes per cyclo-hexane molecule. However, this value must be adopted with cautionsince the employed assumption for the column density of alkenesmay not be representative for all alkenes. Better determination of


2216 S. Pilling et al.Ta

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Figure 7. Column density ratios of cyclohexane and some radiolysis prod-ucts over initial cyclohexane column density as function of equivalent cosmicray exposure time at ISM. (a) Pure c-C6H12 ice at 13 K irradiated by 219-MeV O ions (medium-mass cosmic ray analogue). (b) Mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ice at 13 K irradiated by 632-MeV Ni ions (heavy cosmicray analogue). In each figure, the half-life of cyclohexane (t1/2) in ISM as aresult of cosmic ray bombardment is indicated. Lines are only to guide theeyes.

the band strengths of alkenes for the 719 cm−1 IR feature in icephase is needed to make this methodology more accurate.

Employing the estimated cosmic ray ion flux and the determineddissociation cross-section, the half-life of cyclohexane (t1/2) in ISMas a result of cosmic ray bombardment for pure c-C6H12 ice is about5 × 106 yr (considering only medium-mass cosmic rays), and formixed ice is about 1 × 106 yr (considering only heavy-ion cosmicrays). These values are shown in Fig. 7. The results also show thatfor the mixed ice (H2O:NH3:c-C6H12), after 20 × 106 yr in ISM,almost 20 per cent of the initial cyclohexane was converted intoCO by heavy cosmic rays, 3 per cent was transformed into OCN−

and 1 per cent into CO2. This suggests that highly hydrogenatedhydrocarbons in water-rich grain mantles inside ICs can be largelyconverted into CO during the lifetime of the cloud.

Fig. 7(b) also suggests that some daughter species such as CH4

(1300 cm−1) and OCN− (2165 cm−1) can be used to estimate theintegrated dose of incoming radiation and, assuming a constantcosmic ray flux over the time, the exposure time of interstellar icesto cosmic rays. At ion fluences higher than 3 × 1012 ions cm−2

(∼1 × 1016 yr in ISM), the abundance of these species increasesalmost linearly with the fluence.

4.1 3.4-µm band

The 3.4-µm (∼2925 cm−1) band observed in the IR spectra of sev-eral astrophysical sources, including protostellar ices, hot grainsin DISM, dust grains in planetary nebulae (PN) and meteoriteextractions, is frequently associated with hydrocarbons (e.g



Allamandola et al. 1992, and references therein). As pointed outby Pendleton & Allamandola (2002), although the 3.4-µm bandprobes only the CH stretching modes of the carrier, the analysis ofthe peak position and the band profile has revealed that this portionof the refractory material contains aliphatic hydrocarbons (openchain –CH2– and –CH3 groups) that incorporate at least 3 per centof the available interstellar carbon. In addition, the other IR fea-tures near this band, for example around 3.2–3.3 µm, also indicatethe presence of alkenes and aromatic rings (=CH2 sp2).

Fig. 8 presents a comparison of the 3.4-µm band observed be-tween the FTIR spectra of the studied mixed-ice with the spectra ofsome astronomical objects such as the young stellar object (YSO)NGC 7538 IRS9 (Allamandola et al. 1992), PN IRAS 05341+0852(Joblin et al. 1996), PPN CRL 618 (Chiar et al. 1998) and DISM atGalactic Centre through IRS 6E (Pendleton et al. 1994). A compar-ison involving the IR spectrum of Murchison meteorite extract (deVries et al. 1993) centered in this band is also shown. Each set inFig. 8(a) shows the 3.4-µm band of bombarded H2O:NH3:c-C6H12

ice at three different ion fluences (0, 1.3 × 1013 and 3 × 1013 ionscm−2). Each set of Fig. 8(b) illustrates the evolution of the 3.4-µmband of bombarded H2O:NH3:c-C6H12 ice after 3 × 1013 ions cm−2

and at some temperatures during the heating from 13 K to roomtemperature. Temperature values are indicated in the figure.

The 3.46-µm (∼2880 cm−1) sharp feature observed in the YSONGC 7538 IRS9 (squares in the upper set of Figs 8a–b) indicates thatcyclohexane (and possibly other aliphatic hydrocarbons) was notpresent initially in the ices of this protostellar object. As discussedby Allamandola et al. (1992), the peak at 3.54 µm (∼2850 cm−1)is attributed to methanol and this species is expected to be in thechemical inventory of protostellar grains.

The comparison between the 3.4-µm band in the PN IRAS05341+0852 (the IR spectrum was inverted to appear like anabsorption-type spectrum in Fig. 8) and the H2O:NH3:c-C6H12 ice isremarkably good. The –CH3 stretching mode of aliphatic hydrocar-bons at 3.37 and 3.48 µm is a good indicator of the amount of energythat was delivered to the grains. The same is also true for the =CH2

(sp2) vibration mode at 3.33 µm (∼3010 cm−1), which becomesvery strong only at higher fluences (e.g. 3 × 1013 ions cm−2). Asillustrated in Fig. 8(b), this band is very sensitive to the temperatureof the ice, suggesting that the carrier must be a volatile alkene. Thisband may also be used as a thermometer for bombarded interstel-lar ices. In addition, since this band is not observed in Murchisonmeteorite extract, this suggests that possible volatile alkene speciesthat were present at one time are now completely lost. Looking atthe temperature effect on the 3.33-µm band (fourth graph set inFig. 8b), the interstellar grains in the DISM through IRS 6E shouldhave experienced temperatures higher than 50 K.

In general, the 3.4-µm bands in PPN CRL 618, DISM IRS 6E andMurchison meteorite extract are very similar to each other, and byexcepting the sharp absorption peak at 3.37 µm (∼2960 cm−1), theyare also very similar to the IR spectra of bombarded H2O:NH3:c-C6H12 ice at a fluence of 3 × 1013 ions cm−2. The enhancement ofthe 3.37-µm band as a function of fluence suggests that these in-terstellar grains (and the grains in the Murchison meteorite extract)were probably exposed to a heavy cosmic ray fluence higher than3× 1013 ions cm−2 or to some kind of ionizing agent which deliv-ered an energy dose3 higher than ∼2 × 108 J kg−1 ≈ 1015 eV ng−1

(1013 ions cm−2 ∼ 6 × 107 J kg−1; Pilling et al. 2011).

3 Considering a typical density of 1 g cm−3 and a size of 0.1 µm, the typicalmass of interstellar grains is about 1 ng.

Fig. 9 presents a comparison of the 3.4-µm band observed af-ter heavy-ion bombardment of two simulated interstellar ices at13 K and after warming [H2O:NH3:CO (1:0.6:0.4) bombarded with46-MeV 46Ni13 + (Pilling et al. 2010a) and H2O:NH3:c-C6H12

(1:0.3:0.7) bombarded with 632-MeV 58Ni24 + (this work)]. Bothexperiments were performed with the same instrumentation andmethodology. The ion fluence and ice temperature employed ineach experiment are indicated in the figure. In each set of this figurewe also present a comparison of the laboratory spectra with an as-tronomical IR spectrum (YSO NGC 7538 IRS9 (Allamandola et al.1992); PN IRAS 05341+0852 (Joblin et al. 1996)) and the Murchi-son meteorite extract (de Vries et al. 1993). The 3.46-µm featureobserved in the YSO NGC 7538 IRS9 is in good agreement with theband detected in the radiolysis product from H2O:NH3:CO ice at13 K. However, small features at 3.7 and 3.43 µm seem to match theIR features of the 300-K residue for bombarded H2O:NH3:c-C6H12

ices. Furthermore, an additional abundance of methanol is neededto get a good fitting of the observation at 3.53 µm. This scenariosuggests that the initial abundance of this protostellar disc may con-tain species such as NH3, CO and simple aliphatic hydrocarbonsincluding cyclohexane and methanol as pointed out by Allamandolaet al. (1992). The data also indicate that warmer ices (closer to thestar) may be rich in simple aliphatic hydrocarbon and colder icesmay be rich in CO.

For the PN IRAS 05341+0852 (IR spectrum was inverted toappear like an absorption-type spectrum in Fig. 9), the best matchoccurred with the residue (∼130 K) obtained after the ion bom-bardment of H2O:NH3:c-C6H12 ice. This puts a constraint on thetemperature of dust grains in the region and indicates that a negli-gible amount of CO was present in the grains initially. The organicresidues for both bombarded ices (H2O:NH3:CO and H2O:NH3:c-C6H12) at temperatures hotter than 250 K have some similarities inthis wavelength region with peaks around 3.53 and 3.42µm asso-ciated with aliphatic hydrocarbons. However, the IR spectrum ofMurchison meteorite extract around 3.4 µm cannot be fitted by anyspectrum taken from these two simple interstellar ice analogues.

4.2 5–10 µm range

Fig. 10 presents a comparison between IR spectra from 5 to 10 µm(2000–1000 cm−1) of interstellar and bombarded laboratory ices.The top three curves are IR spectra of YSOs (W33, AFGL 989and NGC7538 IRS9) obtained by the Infrared Space Observatory(ISO). The four bottom curves in this figure indicate different labo-ratory spectra of two ammonia-containing ices irradiated by heavyand energetic ions: H2O:NH3:CO (1:0.6:0.4) at fluence of 2× 1013

ion cm2 (Pilling et al. 2010a) and H2O:NH3:c-C6H12 (1:0.3:0.7)at fluence of 3× 1013 ion cm2 (this work). For comparison, theorganic residues (after heating) of these two simulated ices afterbombardment are also shown.

The 7.7-µm (1300 cm−1) feature in YSOs, which is attributed toCH4 (–CH deformation mode; Boogert et al. 1997), is best repre-sented by the products coming from the radiolysis of ices containingalkanes initially, rather than by the ices where the primary source ofcarbon comes from CO. This suggests that saturated hydrocarbons,such as cyclohexane, may be a part of the initial chemical inven-tory of interstellar ices. However, the presence of broad features at1450 and 1680 cm−1 in the astronomical observations, similar to thefeatures observed in the IR spectra of the irradiated H2O:NH3:COice (Pilling et al. 2010a), requires the presence of CO (or/and CO2)as one of the parental species in the interstellar grain ice mantles.Based on these results and the fact that the 3.4-µm band in these



Figure 8. Comparison between the 3.4-µm band of four different astronomical observations (YSO NGC 7538 IRS9: Allamandola et al. 1992; PN IRAS05341+0852: Joblin et al. 1996; PPN CRL 618: Chiar et al. 1998; and DISM at Galactic Centre IRS 6E: Pendleton et al. 1994) and the Murchison meteoriteextract (de Vries et al. 1993) with the FTIR spectra of H2O:NH3:c-C6H12 ice (this work) at (a) three different ion fluences and (b) during sample heating from13 K up to room temperature.



Figure 9. Comparison between the radiolysis products from two simu-lated interstellar ices at 13 K and after warming [H2O:NH3:CO (1:0.6:0.4)bombarded with 46-MeV 46Ni13 + (Pilling et al. 2010a) and H2O:NH3:c-C6H12 (1:0.3:0.7) bombarded with 632-MeV 58Ni24 + (this work)]. Eachset in this figure also compares the laboratory spectra with one astronomicalIR spectrum (YSO NGC 7538 IRS9 (Allamandola et al. 1992); PN IRAS05341+0852 (Joblin et al. 1996)) and the Murchison meteorite extract (deVries et al. 1993).

YSOs requires the presence of methanol (e.g. Allamandola et al.1992), we propose that the initial composition of grains in the discof these young stellar sources should contain a mixture of H2O,NH3, CO (or CO2), alkanes and CH3OH. A future work will focuson the initial abundance of these simple compounds in young stellarenvironments.

5 C O N C L U S I O N

We performed an experimental study of the interaction of medium-mass ions (219-MeV 16O7 +) and heavy-ions ions (632-MeV58Ni13 +) with cyclohexane-containing ices at 13 K [pure c-C6H12

and H2O:NH3:c-C6H12 (1:0.3:0.7)] to simulate the physicochemi-cal changes (e.g. molecular unsaturation) induced by cosmic raysin saturated hydrocarbon-rich ice analogues inside dense regions ofthe ISM, such as dense molecular clouds or protoplanetary discs,as well on the surfaces of Solar system ices. Our main results andconclusions are the following.

(i) For the experiment with medium-mass cosmic ray analogues(219-MeV O7 +) on pure c-C6H12 ice at 13 K, the determined dis-sociation cross-section is 1 × 10−14 cm2, and the sputtering yieldis 103 c-C6H12 molecules per ion impact. For the experiment inwhich mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ice at 13 K was bom-barded by heavy-ion cosmic ray analogues (632-MeV Ni24 +), theobtained dissociation cross-sections of H2O, NH3 and c-C6H12

are ∼3 × 10−13, ∼3 × 10−13 and ∼2 × 10−13 cm2, respectively. Theresults indicate that the dissociation cross-section of cyclohexaneis about one order of magnitude higher when swift heavy cosmicray analogues were employed in comparison with the experimentemploying medium-mass cosmic rays.

(ii) The half-life of cyclohexane (t1/2) in ISM as a result of cosmicray bombardment for pure c-C6H12 ice is found to be about 5 × 106

yr (considering only medium-mass cosmic rays); for mixed ice it isabout 1 × 106 yr (considering only heavy-ion cosmic rays).

(iii) For the mixed ice (H2O:NH3:c-C6H12), after 20 × 106 yr inISM, almost 20 per cent of the initial cyclohexane was convertedinto CO by heavy cosmic rays, 3 per cent was transformed intoOCN− and 1 per cent into CO2. Such a result suggests that highlyhydrogenated hydrocarbons in water-rich grain mantles inside ICs(or other astrophysical ices exposed to medium-mass and heavycosmic rays) can mostly be converted into CO.

(iv) The results of the radiolysis of simple alkanes such as cy-clohexane in astrophysical ice analogues are consistent with theproduction of unsaturated molecules containing double bond(s),triple bond(s) (e.g. 2334 cm−1) and/or ring(s) (e.g. 3085 cm−1). Ex-trapolating to an astrophysical scenario, the maximum productionof alkenes is obtained after about 3–5 × 106 yr independent ofthe projectile type and the presence of polar species in the ice(e.g. water and ammonia). The formation cross-sections of alkenes,considering the C–H out-of-plane bending mode at 719 cm−1

(A ∼ 8 × 10−18 cm molecule−1), in the pure c-C6H12 ice and mixedH2O:NH3:c-C6H12 ice were about 1 × 10−15 and 1 × 10−14 cm2,respectively. The maximum value was roughly the same for bothexperiments, ∼10−2 alkenes per cyclohexane molecule irradiatedby cosmic ray analogue. Another consequence of this induced un-saturation is the production of molecular hydrogen in the ice, whichcan be released to the gas phase (desorbed) depending on its energyor ice temperature.

(v) The 3.4-µm band of PN IRAS 05341+0852 is very similar tothe band observed for the organic residue (∼130 K) obtained afterthe ion bombardment of H2O:NH3:c-C6H12 ice. This suggests twodifferent issues: first, it puts a constraint in the temperature of dustgrains in the region, and secondly, it also indicates that a negligibleamount of CO was present in the grains initially.

(vi) Considering the sharp 3.37-µm absorption peak (C–H asym-metric stretching mode in –CH3), we suggest that grains in PPNCRL 618, DISM GC IRS 6E and Murchison carbonaceous meteoritewere probably exposed to heavy cosmic rays with fluence higher



Figure 10. Comparison between IR spectra in the 2000 to 1000 cm−1 range (6–10 µm), of ices observed in the interstellar medium and produced in thelaboratory. The top three curves are IR spectra of young stellar sources obtained by the ISO. Lower traces indicate different laboratory spectra of two ammonia-containing ices irradiated by heavy-ions at 13 K [H2O:NH3:CO (1:0.6:0.4) bombarded with 46-MeV 46Ni13 + (Pilling et al. 2010a) and H2O:NH3:c-C6H12

(1:0.3:0.7) bombarded with 632-MeV 58Ni24 + (this work)].

than 3 × 1013 ions cm−2 or to some kind of ionizing agent that de-livered an energy dose higher than ∼2 × 108 J kg−1 ≈ 1015 eV ng−1.

(vii) A comparison between IR spectra from the laboratory andYSOs suggests that the initial composition of grains in young stellardiscs may include a mixture of H2O, NH3, CO (or CO2), alkanesand CH3OH.

Finally, the results indicated that cosmic ray bombardment of sat-urated alkanes can be an alternative scenario for the production ofunsaturated hydrocarbons and possibly aromatic rings (via dehydro-genation processes) in interstellar and protostellar ices. Moreover,a comparison between the laboratory spectra and IR observationsof protostellar ices indicates that saturated hydrocarbons such ascyclohexane may be a part of the initial chemical inventory of in-terstellar ices.

AC K N OW L E D G M E N T S

The authors acknowledge the agencies COFECUB (France),CAPES, CNPq and FAPERJ (Brazil) for financial support. Wealso thank Ms Alene Alder Rangel for the English revision of thismanuscript.

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Table A1. Possible assignment of IR absorption features produced by the radiolysis of the cyclohexane containing ices at 13 K.

Wavenumber (cm−1) Attribution and comments Referencesc-C6H12 H2O:NH3:c-C6H12

– 3091 =C–H (sp2) str. benzene; cyclohexene; alkenes (1,6)3021 3021a =C–H (sp2) str. benzene; aromatic ? (1,6)

– 3008 =C–H (sp2) str. alkenes (1)– 2971 ?

2341 2337 CO2 (2)2334 – –C≡C– sym. str of non sym. alkynes (1)

– 2258 CH2–C≡N? (C≡N str.) (7)– 2225 –C≡C–H (C≡C str. of internal alkynes at 2220 cm−1)? (1)– 2166 OCN− (3)

2133 2137 CO; -C≡C–H (C≡C str. of terminal alkynes at 2120 cm−1)? (1)– 2081 HCN– 1951 ?– 1713 ?

1688 – Broad. C=C str. aromatic?, ring? (1)1642 1641 Broad. C=C str. aromatic?, ring? (1)1584 1584 Broad. NH3CH2COO−? (4)

– 1436 NH3CH2COO−? (4)1392 – t-butyl; benzene? (1)1376 1375 –CH3 bending sym. t-butyl or dimethyl (1)1300 1302 CH4 (C–H ν4 deformation mode) (5)1137 1136a ?993 994 R

HC=CHH (C–H bending out plane); cyclohexene; aromatic? (1)

967 – ?– 954 ?

948 – ?916 916 R

HC=CHH (C–H bending out plane); terminal vinyl at 910 cm−1? (1)

877 877a ?– 845 R

RC=CRH (C–H bending out plane) (1)

818 – ? weak810 – ? weak751 754 ? broad and strong; aromatic C–H bending out of plane; (1)

CH2 rocking aliphatic alkanes (present in molecules with less than four carbonand very strong in molecules with less than three carbon)

720 719a RHC=CR

H bending out of plane aliphatic alkenes; cyclohexene (1)680 683 Benzene? (C–H bending in plane) (1,6)661 – CO2 bending? (2)643 644 R

HC=CRH (C–H bending out of plane); cyclohexene (1)

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APPENDI X A : N EW I R FEATURES A FTE RR A D I O LY S I S

Table A1 lists the new IR features formed after ion bombardmentof the cyclohexane containing ices at 13 K.

This paper has been typeset from a TEX/LATEX file prepared by the author.


Formation of unsaturated hydrocarbons by cosmic ray analogs in interstellar ices

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