Radiolysis of H2O:CO2 ices by heavy energetic cosmic ray analogs
S. Pilling, E. Seperuelo Duarte, A. Domaracka, H. Rothard, P. Boduch, E. F. da Silveira
aa r X i v : . [ a s t r o - ph . H E ] A ug Astronomy&Astrophysicsmanuscript no. Pilling˙AeA˙2010b September 6, 2018(DOI: will be inserted by hand later)
Radiolysis of H O:CO ices by heavy energetic cosmic rayanalogs S. Pilling , E. Seperuelo Duarte , A. Domaracka , H. Rothard , P. Boduch and E.F. da Silveira IP&D / UNIVAP, Av. Shishima Hifumi, 2911, S˜ao Jose dos Campos, SP, Brazil. Grupo de F´ısica e Astronomia, CEFET / Qu´ımica de Nil´opolis, Rua L´ucio Tavares, 1052, CEP 2653-060, Nil´opolis, Brazil. Centre de Recherche sur les Ions, les Mat´eriaux et la Photonique (CEA / CNRS / ENSICAEN / Universit´e de Caen-BasseNormandie), CIMAP - CIRIL - GANIL, Boulevard Henri Becquerel, BP 5133, F-14070 Caen Cedex 05, France. Departamento de F´ısica, Pontif´ıcia Universidade Cat´olica do Rio de Janeiro (PUC-Rio), Rua Marquˆes de S˜ao Vicente, 225,CEP 22453-900, Rio de Janeiro, Brazil.Received / Accepted
Abstract.
An experimental study on the interaction of heavy, highly charged, and energetic ions (52 MeV Ni + ) with pureH O, pure CO and mixed H O:CO astrophysical ice analogs is presented. This analysis aims to simulate the chemical and thephysicochemical interactions induced by heavy cosmic rays inside dense and cold astrophysical environments such as molecularclouds or protostellar clouds. The measurements were performed at the heavy ion accelerator GANIL (Grand AccelerateurNational d´Ions Lourds in Caen, France). The gas samples were deposited onto a CsI substrate at 13 K. In-situ analysis wasperformed by a Fourier transform infrared (FTIR) spectrometer at di ff erent fluences. Radiolysis yields of the produced specieswere quantified. The dissociation cross sections of pure H O and CO ices are 1.1 and 1.9 × − cm , respectively. In the caseof mixed H O:CO (10:1) the dissociation cross sections of both species are about 1 × − cm . The measured sputtering yieldof pure CO ice is 2.2 × molec ion − . After a fluence of 2-3 × ions cm − the CO / CO ratio becomes roughly constant( ∼ / H O ratio. A similar behavior is observed for the H O / H O ratio which stabilizes at0.01, independent of the initial H O column density or relative abundance.
Key words. astrochemistry – methods: laboratory – ISM: molecules – Cosmic-rays: ISM – molecular data – molecular pro-cesses
1. Introduction H O and CO are most abundant constituents of icy grain man-tles in the interstellar medium (Whittet et al. 1996; Ehrenfreund& Charnley 2000). Following Gerakines et al. 1999, a signifi-cant fraction of interstellar solid CO exists in mixtures domi-nated by H O in both quiescent cloud and protostellar regions.In those regions the relative abundance of CO with respect toH O ranges from 4% to 25%.Inside the solar system the presence of H O and CO icesis also ubiquitous. For example, CO is widely detected incometary ices. Its abundance, relative to H O, is about 6-7% inthe comets Hale-Bopp (Crovisier et al. 1997; Crovisier 1998)and Hyakutake (Bockel´ee-Morvan 1997). H O and CO icefeatures have been observed also in the IR spectra of the icyGalilean satellites Europa, Ganymede, Callisto (Carlson et al.1999; McCord et al. 1998), at Triton - the largest moon ofNeptune (Quirico et al. 1999) - and at surface of Mars (Herr& Pimentel 1969; Larson & Fink 1972). Send o ff print requests to : S. Pilling,e-mail: [email protected] Deep inside dense molecular clouds and protostellar disks,as well as at the surfaces of solar system bodies surrounded bythick atmospheres, the frozen compounds are protected fromstellar energetic UV photons. However, X-rays and energeticcosmic rays can penetrate deeper and trigger molecular disso-ciation, chemical reactions and evaporation processes. In thecase of solar system ices without significant atmospheres suchas Europa, Enceladus and Oort cloud comets, the material pro-tection to UV photons is supplied by the upper layers (tens nm)of ice and only the energetic particles reach the inner layers(e.g. 52 MeV Ni penetrate roughly 20 µ m in pure water ice).These ices are directly exposed to stellar photons and comicsray, solar wind ( ∼ / amu ionized particles), energetic solarparticles and / or planetary magnetosphere particles.Laboratory studies and astronomical observations indi-cate that photolysis and radiolysis of such ices can createsimple molecules such as CO, CO , O , H CO and H O and others more complex organic compounds such as formicacid, formaldehyde, methanol, etc. (e.g. Gerakines et al. 2000;Moore and Hudson 2000; Brucato et al. 1997, Wu et al. 2003).By comparing laboratory data (production rates, formationcross section and half-lives for these compounds) with astro- Pilling et al.: Radiolysis of water-CO ices by energetic heavy ions nomical observation, we will better understand the physico-chemical processes occurring in the astronomical sources.In this work, we present infrared measurements of two dif-ferent mixtures of H O:CO ices as well as of pure water andCO ices, irradiated by 52 MeV Ni + . In section 2, the ex-perimental setup is briefly described. The infrared spectra offrozen sample measured as a function of ion fluences are thesubject of section 3. In section 4 a discussion is made aboutthe dissociation cross sections and other radiolysis-induced ef-fects, as well as on the formation of new species on ices.Astrophysical implication is discussed in section 5. Final re-marks and conclusions are given in section 6.
2. Experimental
To simulate the e ff ects of heavy and highly ionized cosmic rayson astrophysical cold surfaces, the facilities at the heavy ion ac-celerator GANIL (Grand Accelerateur National d’Ions Lourds,Caen, France) have been used. The gas samples (purity superiorto 99%) were deposited onto a CsI substrate at 13 K througha facing gas inlet. The sample-cryostat system can be rotated180 ◦ and fixed at three di ff erent positions to allow: i) gas depo-sition; ii) FTIR measurement; and iii) perpendicular ion irradi-ation.52 MeV Ni + ion projectiles impinge perpendicularlyonto H O:CO ice (1:1), H O:CO ice (10:1), as well as ontopure H O and CO ices. The target ionizing e ff ects of Ni andFe projectiles, at the same velocity, are almost identical sincethey have almost the same atomic number (Seperuelo Duarte etal. 2010). The incoming charge state 13 + corresponds approx-imately to the equilibrium charge state after several collisionsof 52 MeV Ni atoms (independent of the initial charge state)with matter (e.g. Nastasi et al. 1996). In-situ analysis was performed by Nicolet FTIR spectrom-eter (Magna 550) from 4000 to 600 cm − with resolution of1 cm − . The spectra were collected at di ff erent fluences up to2 × ions cm − . The Ni ion flux was 2 × cm − s − . Duringthe experiment the chamber pressure was below 2 × − mbardue to the cryopumping by the thermal shield. Further detailsare given elsewhere (Seperuelo Duarte et al. 2009; Pilling et al.2010).Assuming an average density for the pure water ice andwater-rich ices samples of 1 g / cm and 1.8 g / cm for the pureCO ice, the thickness and the deposition rate are determinedby using the initial molecular column density of the samples.For H O:CO (1:1) and H O:CO (10:1), the sample thick-nesses were 0.6 and 0.4 µ m. The deposition rates were roughly7 and 3 µ m / h, respectively. In the case of pure H O ice and pureCO ice, the thicknesses were about 0.5 and 0.4 µ m and, the de-position rates were 4 and 3 µ m / h, respectively. The penetrationdepth of 52 MeV Ni ions is much larger than ice thicknessestherefore the ions pass though the target with approximatelythe same velocity (cross sections are constant).The molecular column density of a sample was determinedfrom the relation between optical depth τ ν = ln( I / I ) and theband strength, A (cm molec − ), of the respective sample vibra-tional mode (see d´Hendecourt & Allamandola 1986). In thisexpression, I and I is the intensity of light at a specific fre- quency before and after passing through a sample, respectively.Since the absorbance measured by the FTIR spectrometer was Abs ν = log( I / I ), the molecular column density of ice sampleswas given by N = A Z τ ν d ν = . A Z Abs ν d ν [molec cm − ], (1)where Abs ν = ln( I / I ) / ln(10) = τ ν / . ffi cients (band strengths) used in this work are given inTable 1. Table 1.
Infrared absorption coe ffi cients (band strengths) usedin the column density calculations for the observed molecules. Frequency Wavelength Assignment Band strength (A) Ref.(cm − ) ( µ m) (cm molec − )2342 4.27 CO ( ν ) 7.6 × − [1]3250 3.07 H O ( ν ) 2.0 × − [1] ∼ ∼ O ( ν + ν ) 5.7 × − [3]2139 4.67 CO ( ν ) 1.1 × − [2]2044 4.89 CO ( ν ) 8.9 × − [4]1307 7.65 H CO ( C-OH bend ) 1.0 × − [5] ∼ ∼ ( ν ) 1.4 × − [6][1] Gerakines et al. 1995; [2] Gibb et al. 2000; [3] Loe ffl er et al.2006; [4] Bennett et al. 2004 [5] Gerakines et al. 2000; [6] Smith etal. 1985.
3. Results
Figure 1a presents the infrared spectra of H O:CO ice (1:1) at13 K, before (top trace) and after di ff erent irradiation fluences.Each spectrum has been shifted for clarity. The narrow peak at2342 cm − is the CO stretching mode ( ν ). The broad struc-tures around 3300 and 800 cm − correspond to the vibrationmodes of water, ν and ν L , respectively. The band at 1600 cm − is due to the water ν vibration mode. The presence of the OHdangling band at about 3650 cm − is also observed in the figureindicating a high porosity (Palumbo et al. 2006). The inset fig-ure shows the newly formed H O species due to the radiolysisof water inside the ice.The region between 2200 to 1000 cm − is shown in detailin Fig. 1b. The ion fluence of each spectra is also given. In thisspectral region several peaks grow as a function of fluence: theycorrespond to the appearance of new species formed by the 52MeV Ni atoms bombardment, including CO, CO , O , H CO ,H CO (tentative), HCOOH (tentative). Some IR features orig-inated from unidentified organic species appear around 1350and 1450 cm − .The evolution of the column density as function of fluenceis shown in Fig. 1c. The infrared band strengths used for the de-termination of the column density are listed in Table 1. The col-umn density of water presents constant concentration of about5 × molecules cm − , decreasing very slowly as the fluenceincreases. This fact is attributed to an approximate compen-sation between persisted water condensation (layering) and itsdisappearance by sputtering and dissociation. illing et al.: Radiolysis of water-CO ices by energetic heavy ions 3 Fig. 1. a) Infrared spectra of H O:CO
13 K ice (1:1) be-fore (top dark line) and after di ff erent irradiation fluences. b)Expanded view from 2200 to 1000 cm − . c) Molecular columndensities derived from the infrared spectra during the experi-ment. The lines are to guide the eyes.The initial enhancement in the water column density maybeassociated with the compaction e ff ect produced by ion bom-bardment as discussed elsewhere (Palumbo 2006, Pilling et al.2010). This compaction changes the band strength of some vi-brational modes. Leto and Baratta (2003) showed that the bandstrength of water ν vibrational mode undergoes a 10% increaseduring the first ion impacts (low fluence) on ice in experimentsemploying ions with energy of dozens of keV. Another expla-nation may be a strong water layering just on the begging ofirradiation. The CO abundance reaches its half value at a flu-ence of about 4 × ions cm − . The CO abundance increasesvery rapidly, reaching a maximum around 3-4 × ions cm − and decreasing after that. The same behavior was observedfor the O , H CO and CO , all produced by the radiolysisof CO inside the ice. For O , H CO and CO , the column Fig. 2. a) Infrared spectra of H O:CO
13 K ice (10:1) be-fore (top dark line) and after di ff erent irradiation fluences. b)Expanded view from 2200 to 1000 cm − . c) Molecular columndensities derived from the infrared spectra during the experi-ment. The lines are to guide the eyes.density maximum occurs at 4 × , 3 × and 1 × ionscm − , respectively. In the case of H O the maximum occursat around 1 × ions cm − and remains constant ( ∼ × molec cm − ) until the end of irradiation (up to a fluence of 1 × ions cm − ).The infrared spectra of H O:CO (10:1) ice before and af-ter di ff erent irradiation fluences up to 5 × ions cm − aregiven in Fig. 2a. In contrast to H O:CO (1:1), this ice does notpresent the OH dangling band at about 3650 cm − , may be dueto the lower deposition rate or due to the low amount of CO on ice. As suggested by Bouwman et al. (2007), the presence ofimpurities like CO and CO on amorphous water ices increasesthe ice´s porosity and the presence of the OH db become moreprominent. Pilling et al.: Radiolysis of water-CO ices by energetic heavy ions Fig. 3. a) Infrared spectra of pure water ice at 13 K obtained fordi ff erent 52 MeV Ni fluences. Inset figures present details ofnewly formed species. b) Molecular column densities derivedfrom the infrared spectra during the experiment.Figure 2b shows details of the infrared spectra for regionsbetween 2200 to 1000 cm − . The wavenumber of some uniden-tified IR frequencies are given. The infrared peaks tentativelyattributed to H CO ( ∼ − ) and HCOOH ( ∼ − )seem to be smaller than in the case of H O:CO (10:1) ice. Asin the previous case the appearance of the CO ν line at 2139cm − is due to the CO radiolysis.The variations of the column densities of the most abun-dant molecules observed during the irradiation of H O:CO ice(10:1) by 52 MeV Ni ions as function of fluence are given inFig. 2c. Peaks due to O , H CO or CO molecules were notobserved.The infrared spectra of pure H O and CO ices at 13 K fordi ff erent 52 MeV Ni fluences are shown in Fig. 3a and Fig. 4a,respectively. Upper curves indicate the virgin ice and figure in-sets display peak details of the newly formed species. In thecase of the radiolysis of pure water, only the formation of H O (vibration mode ν + ν ) is observed. In contrast, after the ra-diolysis of CO pure ice, at least three di ff erent compoundsare formed: CO ( ν ), O ( ν ) and CO ( ν ) (see also SeperueloDuarte et al. 2009).Figure 3b shows the evolution of water molecular columndensity derived from di ff erent vibration modes ( ν , ν and ν L )and the column density of H O as a function of ion fluence.The band strengths employed for water vibration modes ν (1650 cm − ) and ν L ( ∼
800 cm − ) were obtained by Gerakineset al (1995) and Gibb et al. (2000), respectively. The evolution Fig. 4. a) Infrared spectra of pure CO ice at 13 K after di ff er-ent 52 MeV Ni fluences. Inset figures present details of newlyformed species. b) Molecular column densities derived fromthe infrared spectra during the experiment.of CO column density, derived from di ff erent vibration modes( ν , ν , ν + ν and 2 ν + ν ), is given in Fig. 4b: the distinct resultare practically the same, as expected. The band strengths em-ployed for CO vibration modes ν (670 cm − ), 2 ν + ν (3599cm − ), and ν + ν (3708 cm − ), were obtained by Gerakines etal. (1995). The column densities of the new species producedby radiolysis are also shown and will be discussed further.The infrared absorption profile of water ( ν ) and carbondioxide ( ν ) in pure and mixed (H O:CO (1:1)) ices as a func-tion of ion fluence are shown in Fig. 5. Upper panels (a andc) indicate pure ices. In the case of mixed ice, the selected IRfeatures are illustrated in the bottom panels (b and d). In allpanels the upper curves indicate the non irradiated spectrum.Each spectrum has a small o ff set for clearer visualization. Thebombardment by heavy ions slightly shifts the water ν vibra-tion mode to low frequencies. Since di ff erent water clusters areresponsible for this IR band (Paul et al. 1997) this suggests thatsmall water clusters within the ice are being converted to largercluster structures. Also large clusters may be less radiation sen-sitive than small clusters. This behavior is enhanced in the caseof mixed ices (Fig. 5b).Figure 5b presents the OH dangling bands ( ∼ − )attributed to water molecules weakly adsorbed into microporesinside the ice (see Palumbo 2006). As the fluence increases,the micropores collapse and the OH dangling vibration modesdecrease. We have shown previously (Pilling et al. 2010) thatthe ice compaction produced by heavy cosmic rays are at least illing et al.: Radiolysis of water-CO ices by energetic heavy ions 5 Fig. 5.
Selected profile of water and CO vibration mode at dif-ference fluences up to 10 ions cm − . Pure water ν vibrationmode (a), water ν mode in mixed ices (b), pure CO ν mode(c) and CO ν mode in mixed ices (d). See details in text.3 orders of magnitude higher than that produced by (0.8 MeV)protons.The profile of the ν vibration mode of carbon dioxideappears to be very sensitive to the presence of water. It be-comes very broad and also presents a shift toward the lowerfrequencies when water is mixed with CO (Fig. 5d). As theion fluence increases, the ν band of CO becomes sharp andthe peak moves toward higher frequencies becoming similar topure CO ice. A detailed discussion on the e ff ects of CO andH O on band profiles observed in mixed H O:CO ices is givenelsewhere ( ¨Oberg et al. 2007).
4. Discussion
As discussed previously by Seperuelo Duarte et al. (2009) andPilling et al. (2010), the complex interaction between an en-ergetic heavy ion with an ice target may be described throughpartial scenarios by considering successive aspects of the phe-nomenon. Since the impact of 52 MeV Ni atoms lies in elec-tronic energy loss domain (projectile velocity & µ s − )most of the deposited energy leads to excitation / ionization oftarget electrons (electronic energy loss regime). In turn, theelectrons liberated from the ion track core (0.3 nm of diame-ter for 52 MeV Ni atoms; Iza et al. 2006), transfer their en-ergy to the surrounding condensed molecules ( ∼ dN i dF = X j , i σ f , i j N j + L i − σ d , i N i − Y i Ω i ( F ) (2) where P j σ f , i j N j represents the total molecular production rateof the i species directly from the j species, L i is the layering, σ d , i is the dissociation cross section, Y i is the sputtering yieldof and Ω i ( F ) is the relative area occupied by the i species onthe ice surface.Considering that the analyzed compounds cannot react inone-step process to form another species originally present inice (i.e., σ f , i j ≈ N i = ( N , i − N ∞ , i ) exp( − σ d , i F ) + N ∞ , i (3)where N ∞ , i = ( L i − Y i ) /σ d , i is the asymptotic value of columndensity at higher fluences due to the presence of layering. N i and N , i are the column density of species i at a given fluenceand at the beginning of experiment, respectively. In the case ofpure CO the layering yield is negligible L CO = Ω i ( F ) → δ i , preventingprogressively sputtering of CO molecules. At the end of theseprocesses, the system of Eq. 2 is decoupled and can be solvedanalytically by: N = ( N − N ∞ ) exp( − σ d F ) + N ∞ for H O (4)and N = N exp( − σ d F ) for CO (5)where N ∞ = ( L − Y ) /σ d is the asymptotic value of columndensity of water due to the presence of layering.As discussed by Loe ffl er et al. (2005), the radiochemicalformation yield ( G f ) of a given compound per 100 eV of de-posited energy, at normal incidence, can be expressed in termsof the formation cross section ( σ f ) and the stopping power (S),in units of eV cm / molec, as: G f = σ f S molecule per 100 eV (6)This definition can be extended for radiochemical dissoci-ation (destruction) yield of a given compound per 100 eV ofdeposited energy ( G d ) by replacing the formation cross sectionin Eq. 6 by the negative value of the dissociation cross section(- σ d ). Therefore, negative G d values indicate that molecules arebeing dissociated or destroyed after energy deposition into theice.Adopting the S values from the Stopping and Ranges mod-ule of the SRIM2003 package (Ziegler 2003), the values of theradiation yield G in the experiments can be determined andcompared with literature values. Table 2 presents the radio-chemical destruction yields of H O and CO per 100 eV ofdeposited energy in each calculated model. The stopping pow-ers of 52 MeV Ni ions for pure water ice and for CO ice are S = . × − eV cm / H O and 2.6 × − eV cm / CO ,respectively. In the case of mixed H O:CO ices the stoppingpower is determined by interpolation between the pure ice val-ues. We considered that the chemical changes during the irra-diation does not a ff ect the stopping power. Pilling et al.: Radiolysis of water-CO ices by energetic heavy ions Table 2.
Dissociation cross section, radiochemical yield and sputtering values of H O and CO obtained from the radiolysis ofpure water ice and mixed water-CO ices by 52 MeV Ni ions. Species Mixture σ d -G d N ∞ Y L ‡ N Model (H O:CO ) (10 − cm ) molec /
100 eV (10 molec cm − ) (10 molec ion − ) (10 molec ion − ) (10 molec cm − )H O (1:0) 1.1 7.48 12 1 a
14 15 1(10:1) ∼ ∼ a ∼ a (0:1) 1.8 6.90 NA † b c ∼ ∼ d ∼ ∼ c ∼ a Taken from Brown et al. (1984). b Estimated to be a half of the value determined from pure CO ice. c No CO sputtering. Assuming that thewater layering is high enough to fully cover the CO molecules on the surface. d Estimated to be a tenth of the value determined from pure CO ice. † NA = Non applied. ‡ For H O: Derived from L = N ∞ σ d + Y . For CO : Assuming no layering due to residual CO . Figure 6 presents the fitting curves for the H O and CO column densities employing Eq. 3 (for pure ices), and Eqs. 4and 5 (for mixed ices). Numeric labels indicate parameterslisted in Table 2. The H O column density data level o ff around N ∞ ≃ . × , 4 × , and 3 × molec cm − for pure wa-ter, H O:CO (10:1) and H O:CO (1:1) ice samples, respec-tively. The average value for the dissociation cross section ofwater, employing Eq. 3, is σ d ∼ × − cm . The sputteringyield for water measured by Brown et al. (1984) was extrapo-lated for the 52 MeV Ni ions impact as discussed in Pilling etal. (2010). The obtained value is Y = × molecules per im-pact. The estimated average water layering of both experimentswas within the 4-14 × molec ion − range, obtained from therelation L = N ∞ σ d + Y . Due to the large value of water layering,CO species in the mixed ices were recovered by a H O filmand the CO ice sputtering yield is considered negligible in thecurrent experiments; therefore, the data are adjusted directly byEq. 5.The fitting parameters (dissociation cross section, radio-chemical yield, sputtering, layering and the initial relativemolecular abundance of each species in the ices) are listed inTable 2. Pure H O and CO ice were fitted by the parametersof models 1 and 4, respectively. For water species the layer-ing yield was determined by L = N inf + σ d + Y . For CO species (model 4 to 6b), it is assumed no layering due to resid-ual CO . Model 5a is the fitting for the CO species in themixed H O:CO ice (1:1) and the sputtered value was consid-ered to be half the value from pure CO ice.In model 5b it is assumed that the water layering is highenough to fully cover the CO on the surface producing a negli-gible sputtering yield of CO molecules. From the comparisonof models 5a and 5b we observe that the error in the dissoci-ation cross section is lower than 30%. Model 6a concerns thefitting for the CO species in the mixed H O:CO (10:1) ice;the sputtering yield is considered to be a tenth of the value de-termined from pure CO ice.From models 5 we observe that the precise determination ofsputtering is possible only for the ices monitored at higher ionfluences ( ∼ ion cm − ) since di ff erent sets between sput-tering and dissociation cross section can be adjusted for the data with low fluences ( < × ions cm − ). Due to the de-generacy of such model for low ion fluence IR data, the errorobserved in the dissociation cross section on models 6 is about40%. Table 2 shows that the dissociation cross section at typi-cal astrophysical ice ([CO ] / [H O] ∼ ∼ × − cm − . Fig. 6.
Variation of the experimental column densities of H Oand CO as a function of fluence. Lines represent the fittingsusing Eq. 3 for pure ices and Eqs. 4 and 5 for mixed ices. Theparameters models are listed in Table 2. The evolution of the column density of the newly formedspecies from H O:CO ices as a function of fluences is shownin Fig. 7a and Fig. 7b. The ratio of H O column density overits parent molecule (H O) initial column density, N H O / N , H O ,as a function of fluence in 3 di ff erent water-concentration ices(pure H O ice, ∼
50% and ∼ , and H CO fromCO -rich ices as a function of fluence is presented in Fig. 7b. N i / N , CO indicates the column density ratio of a given pro-duced species over its parent molecule (CO ) initial column illing et al.: Radiolysis of water-CO ices by energetic heavy ions 7 Table 3.
Formation and dissociation cross section of newly formed species from the radiolysis of H O:CO ices by 52 MeV Niions. Species Parental Mixture σ f σ d G f − G d Modelspecies [H O:CO ] (10 − cm ) (10 − cm ) (molec /
100 eV) (molec /
100 eV)H O H O [1:0] 0.19 10 1.3 68.0 1H O [10:1] 0.13 9.3 0.87 59.1 2H O [1:1] 0.10 6.9 0.48 33.8 3CO CO [0:1] 1.8 11 6.8 42.1 4CO [1:1] 1.2 7.3 5.8 35.8 5CO [10:1] 0.91 4.4 5.8 27.9 6O CO [0:1] 0.27 16 1.1 61.3 7CO [1:1] 0.06 5.0 0.30 24.5 8CO CO [0:1] 0.09 97 0.33 372 9CO [1:1] 0.01 31 0.06 152 10H CO H O and CO [1:1] 0.03 9.9 0.14 48.6 11 density. Three di ff erent CO -rich ices are analyzed (pure CO ice, ∼
50% and ∼ N i / N , p = A i + B i [1 − exp( − σ d , i F )] (7)where N i and N , p represents the column density of a givenproduced species i over its parent molecule p (H O or CO ).A i and B i are constants which indicate the initial and the max-imum amount (after radiolysis) of species i on the ice, σ f , i isthe formation cross section and F is the ion fluence.As the fluence increases, the number of producedmolecules also increases and reaches a maximum. At thismoment, the number of molecules produced by radiolysis ofparental species is equal to the number of new molecules thatdissociate possibly in the reverse reaction set (e.g. p ⇄ i + j + k + ...). At the equilibrium the column density of the parentaland daughter species, N eq , p and N eq , i , respectively, are relatedby the expression: N eq , p σ f , i = N eq , i σ d , i (8)where σ f , i and σ d , i is the formation cross section and dissocia-tion cross section of daughter species i .Since the column densities of daughters species i are muchsmaller than the one observed for parental species p , as a firstapproximation we have the following relation between them: N eq , i N , p + N eq , p N , p ≃ ff ect is not being considered in thismodel. Since not all the parental molecules are converted into agiven species, some could be sputtered from the ice and otherscan react to produce di ff erent daughter species, the amount ofparental molecule always decreases. Consequently, the columndensity of a given daughter species decreases after it reaches amaximum (equilibrium stage). This behavior can be observedin the case of the evolution of CO species as a function offluence (Fig. 7b). Calling the maximum amount of newly formed species i atthe equilibrium as N ∞ , i and using Eq.9, we can be express theconstant B i of Eq. 7 by: B i = N eq , i N , p ≃ − N eq , p N , p (10) Fig. 7.
The evolution of newly formed species from the bom-bardments of H O:CO ices by 52 MeV Ni as a function offluence. a) Production of H O from H O molecules in dif-ferent ices. b) Production of CO, O , CO and H CO fromCO molecules in di ff erent ices. The relative abundance of eachparental species are indicated. The lines indicate the fittings us-ing Eq. 11 and the model parameters are given in Table 3. Pilling et al.: Radiolysis of water-CO ices by energetic heavy ions Moreover, considering that the number of initial daughterspecies i is negligible ( A i =
0) and, after the combination ofEq. 9 with Eq. 10, we rewrite Eq. 7 as: N i / N , p ≃ + σ d , i σ f , i [1 − exp( − σ d , i F )] (11)where σ d , i and σ f , i are the dissociation and formation crosssection of species i .The lines observed in the Fig. 7a and Fig. 7b were ob-tained by fitting the experimental data with Eq.11. The pro-posed model is valid from the beginning of irradiation up tothe fluence in which the column density reaches its maximumvalue. This indicates the equilibrium point between formationand destruction promoted by radiolysis. The formation and dis-sociation cross section as well respective radiolysis yield deter-mined for the newly formed species are given in Table 3. Weobserve that CO presents the highest formation cross sectionamong the studied newly formed species. O In recent years, di ff erent groups have studied the formation ofhydrogen peroxide by ion bombardment of pure water ice andmixed water-CO ices (Moore & Hudson 2000, Strazzulla etal. 2003, 2005a, 2005b; Baragiola et al. 2004; Loe ffl er et al.,2006; Gomis et al. 2004a, 2004b). H O was also observedfrom fast electron bombardment of pure water ices (Baragiola,et al. 2005; Zheng et al. 2006) and also from low energy (3-19eV) electron bombardment (Pan et al. 2004).Following Teolis et al. (2009), the most commonly H O formation mechanisms involves the reaction between two OHradicals coming from the radiolysis of water molecules, asgiven by: H O CR −→ H O ∗ −→ OH + H (12) OH + OH → H O (13)where CR denotes cosmic rays.These OH radicals are strongly hydrogen bonded to watermolecules (Cooper et al., 2003), and it is not until above 80K that they can di ff use within a water-ice lattice (Johnson andQuickenden, 1997). As pointed out by Cooper et al. (2008) en-ergetic OH radicals can di ff use short distances along ion tracksand react at 10 K, but bulk di ff usion probably does not occur.The decrease in the H O production with temperature in-crease was investigated by Zheng et al. (2006) and Loe ffl er etal. (2006). This indicates that electron scavenging may play acritical role in the radiation stability of H O in pure water-ice experiments. Moreover, Gomis et al. (2004a) found that theH O yields were dependent on projectile: ices irradiated withlow energy (30 keV) C + , H + and O + ions produced more H O at 77 K than 16 K, while N + and Ar + had no temperature de-pendence on the H O yield. The authors also observe that theH irradiation produces a much lower quantity of H O than forthe other heavy ions suggesting that the energy deposited byelastic collisions plays an important role in such a process. As discussed by Zheng et al. (2006) and Loe ffl er et al.(2006), there is another reaction sequence to produce hydro-gen peroxide in astrophysical ices from the addition of oxy-gen atom (a product of the by the dissociation of oxidant com-pounds such as O , CO, CO , H O, etc.) to water molecule:O + H O → H O-O ionization −→ H O + + O − → H O (14)This reaction route involves an extra ionization stage triggeredby CR, UV / X-ray photons or fast electrons to produce H O (Loe ffl er et al. 2006), which implies a quadratic dependence onirradiation fluence being a secondary order reaction processes.In the present work we consider negligible the H O formationvia oxygen addition to H O. Future investigations with isotopiclabeling could help to clarify this issue and quantify which frac-tion of O atoms produced from the dissociation of CO or H Omay react with H O to form H O .Table 4 presents the formation and dissociation cross sec-tion, as well the radiochemical yield, of H O obtained fromthe processing of pure water ice and mixed H O:CO ice. Boththe formation and dissociation cross section of H O decreaseas the relative abundance of H O in the ice decreases, a con-sequence of larger averaged distance between parental species.From Fig. 7 we also observe that the presence of CO insidethe ice decreases the H O / H O ratio. This points in the op-posite direction than the results observed from the irradiationof water-CO by light ions (Strazzulla et al. 2005b; Moore andHudson 2000). From Table 4 we observe that the formationcross section of H O from the bombardment of pure water iceby heavy and energetic ions is 10 higher than the value ob-tained by the impact of light or / and slower projectiles.The asymptotic value for the H O / H O ratio presents aslight reduction with the enhancement of initial CO in the ice.This clearly shows that H O formation via O + H O in whichthe oxygen comes from another oxidant compound, such asCO , is indeed a secondary-order process (Loe ffl er et al. 2006).Our value is about 8 times higher than in the case of irradiationwhit 30 keV protons, but very similar to the asymptotic valuesobtained after the impact with other 30 keV ions such as C + , N + and O + (Gomis et al 2004a). A comparison between our radio-chemical yield and cross sections with literature have revealedhigher values for both the formation and destruction of H O during bombardments of ices by heavy and energetic ions. Inother words the chemical processing is very enhanced by theprocessing of ices by heavy and energetic and highly chargedions in comparison with light ions.Hydrogen peroxide has been found on the surface ofEuropa by identifying both an absorption feature at 3.5 µ min the Galileo NIMS spectra and looking at the UV spectrumtaken by the Galileo ultraviolet spectrometer (UVS) (Carlsonet al. 1999). Following the authors, the relative abundance withrespect to water in Europa is H O / H O ∼ = illing et al.: Radiolysis of water-CO ices by energetic heavy ions 9 Table 4.
Comparison between the formation and dissociation cross section of H O from the processing of pure H O ices andH O:CO ices. The formation and dissociation radiochemical yield of H O as well the asymptotic H O / H O ratio in eachexperiment are also given.
Ices Temp. Projectile σ f σ d G f -G d H O / H O † Ref.(K) (Energy) (10 − cm ) (10 − cm ) (molec /
100 eV) (molec /
100 eV) (%)H O 13 Ni + (52 MeV) 0.19 10 1.3 68.0 1.9 [1]H O:CO (10:1) 13 Ni + (52 MeV) 0.13 9.3 0.87 59.1 1.6 [1]H O:CO (1:1) 13 Ni + (52 MeV) 0.10 6.9 0.48 33.8 1.5 [1]H O 16 H + (30 keV) ∼ O 16 C + (30 keV) ∼ O 16 N + (30 keV) ∼ O 16 O + (30 keV) 0.002 0.13 0.34 22.3 1.4 [2]H O 16 Ar + (30 keV) 0.002 0.07 1.34 46.8 2.2 [2]H O 20 H + (100 keV) ∼ O 16 H + (200 keV) ∼ O 16 He + (200 keV) 0.003 0.13 0.37 16.1 2.4 [4]H O 16 Ar + (400 keV) ∼ † Asymptotic value; [1] this study; [2] Gomis et al. 2004a; [3] Loe ffl er et al. 2006; [4] Gomis et al. 2004b. evident on the darker satellites than on Europa. The estimatedratio on Ganymede is H O / H O ∼ = O has also been suggested to be present in icy mantleson grains in dense clouds as pointed out Boudin et al. (1998).Based on observations and laboratory studies, they have esti-mated the relative abundance with respect to water at the densemolecular cloud NGC 7538:IRS 9 of H O / H O ∼ = Ozone has been detected after irradiation of mixed H O:CO (1:1) ices and pure CO ice at 16 K by 1.5-200 keV ions (pro-tons and He + ) (Strazzulla et al. 2005a, 2005b). According tothese authors, the O production from radiolysis is highly atten-uated in the case of CO -poor mixed ices, e.g. for H O / CO > :CO (10:1) ice. The presence of ozone hasalso been observed after the irradiation of CO:O (1:1) ice by60 keV Ar ++ and 3 keV He + (Strazzulla et al 1997). The au-thors suggest that the detection of ozone and the absence ofsuboxides (e.g. C O and C O) in interstellar grains would indi-cate a dominance of molecular oxygen in grain mantles. Thesesuboxides have not been detected in our experiments.Seperuelo Duarte et al. (2010) have bombarded pure COice with 50 MeV Ni + and observe ozone and several subox-ides (C O, C O , C O ) among the radiolysis products. Theydetermined the ozone formation cross section is 3 × − cm .Ozone and suboxides were also observed after irradiation ofpure CO ice at 16 K by 200 keV protons (Palumbo et al. 2008).We have detected ozone after the radiolysis of pure CO ice and mixed H O:CO (1:1) ice by 52 MeV Ni + . The deter-mined formation cross section of ozone is σ O = . × − cm and 0 . × − cm in pure CO ice and H O:CO ice,respectively. These values indicate that the O formation cross section decreases as the relative abundance of the parental CO compound in the ice declines. The value obtained for ozoneformation via pure CO is 2 times higher than the value deter-mined previously in similar radiolysis experiments involvingpure CO irradiated by 46 MeV 58 Ni + (Seperuelo Duarteet al. 2009).In the solar system, ozone has been observed on Ganymede(Noll et al. 1996) and on Saturn’s satellites Rhea and Dione(Noll et al. 1997). Following Strazzulla et al. (2005a) ozonecould be formed where fresh CO rich layers are exposed to ra-diation. Due to the strong silicate absorption band around 1040cm − the observation of ozone in interstellar medium is verydi ffi cult. However, the appearance of ozone seems conceivable,since it is observed in laboratory experiments involving the pro-cessing of astrophysical ices analogs. This compound has been observed after the UV processingof pure CO , CO and O
10 K ices (Gerakines et al. 1996;Gerakines & Moore 2001) and also on radiolysis of pure CO by 0.8 MeV protons (Gerakines & Moore 2001) and 1.5 keVprotons (Brucato et al. 1997). The bombardment of mixedH O:CO ices by 3 keV He + , 1.5 keV and 0.8 MeV protonsalso shows the formation of CO . Furthermore, H CO andpossibly O are produced (Brucato et al. 1997; Moore andKhanna 1991).Seperuelo Duarte et al. (2009) have observed this speciesin the radiolysis of of C O by 46 MeV Ni + up to a fi-nal fluence of 1 . × cm − . In our measurement the forma-tion cross section of CO from radiolysis of pure CO ice is0 . × − cm − , roughly 5 times lower than the value ob-tained previously by Seperuelo Duarte et al. (2009). This valuepresents a decrease in a presence of water in the ice, as ob-served in the case of H O:CO (1:1) ice (see Table 3). The for-mation cross section of CO in our experiments on pure CO is ices by energetic heavy ions roughly the same than the one determined by Seperuelo Duarteet al. (2009).The CO destruction cross section due to radiolysis is verylarge when compared to the other compounds formed into theices. To increase the accuracy of the cross sections next mea-surements should cover large data sets inside the fluence rangebetween 1-10 × ions cm − . On the contrary of UV photoly-sis of pure CO ices (Gerakines et al. 1996) we do not observedthe formation of C O at 2243 cm − from the radiolysis of CO ices.Despite of the prediction of CO among the compoundsin the processed astrophysical ices (e.g. Elsila et al. 1997,Allamandola et al. 1999), this species has not been detectedconclusively in space yet (Elsila et al. 1997, Ferrante et al.2008). CO Carbonic acid (H CO ) has been observed in several experi-ments involving ion bombardment of H O:CO ices (Moore &Khanna, 1991; DelloRusso et al. 1993; Brucato et al. 1997). Itwas also observed after the radiolysis of pure CO ice at 10 Kby 1.5 keV protons (Brucatto et al. 1997), indicating that im-planted hydrogen ions are incorporated in the target to formnew bonds to produce H CO .As discussed by Gerakines et al. (2000) the formation ofH CO from ion bombardment of H O:CO ices is ruled bytwo main reaction schemes. First, the direct dissociation ofH O molecules by incoming projectile:H O CR −→ OH + H + + e − (15)where products such as H , H O ; H O + , and HO are even-tually formed by reactions involving the primary dissociationproducts. The next step is the electron attachment on CO orOH producing reactive compounds which quickly react witheach other to produce bicarbonate (HCO − ):CO − + OH −→ HCO − and / or CO + OH − −→ HCO − (16)Finally, bicarbonate reacts with a proton to produce H CO :HCO − + H + −→ H CO (17)From the evolution of the IR peak at 1307 cm − duringthe radiolysis of H O:CO (1:1) ice by 52 MeV Ni ions wehave determined the formation and the dissociation cross sec-tion of H CO and the values obtained are 3 . × − cm and 9 . × − cm , respectively. This formation cross sectionis 8 orders of magnitude higher than the one derived from theUV photolysis of H O:CO (1:1) ices at 18 K (Gerakines et al.2000) indicating that heavy ion processing of astrophysical iceanalogs is very e ffi cient to form H CO compared to UV pho-tons.H CO is thermally stable at 200 K, higher than the sub-limation temperature of H O. As discussed by Peeters et al.(2008), ices containing both H O and CO have been found ona variety of surfaces such as those of Europa, Callisto, Iapetus,and Mars, where processing by magnetospheric ions or the so-lar wind and energetic solar particles may lead to the formationof H CO . The astrophysical significance of solid carbonic acid havebeen extensively discussed elsewhere (e.g. Khanna et al. 1994,Brucato et al 1997, Strazzullla et al. 1996). H CO has beensuggested to be present at the surface of several solar systembodies with su ffi cient amounts of CO such as Galilean satel-lites of Jupiter (Wayne 1995, McCord et al. 1997, Johnson etal. 2004) and comets (Hage et al. 1998). Europa, Ganymede,and Callisto all exhibit high surface CO abundances and thesesatellites are heavily bombarded by energetic magnetosphericparticles, galactic cosmic rays and solar radiation.It has been suggested that the 3.8 µ m feature in NIMS spec-tra of Ganymede and Callisto arise from H CO (Johnson etal. 2004; Hage et al. 1998). Following Carlson et al. (2005)the relative abundance of carbonic acid with respect to CO inCallisto is H CO / CO ∼ CO column density could be as much as4 . × molec cm − , corresponding roughly 1% of the CO or 0.2% of the total H O in this line of sight (Whittet et al.1998).As discussed by Zheng & Kaiser (2007), in the history ofMars there might be a high concentration of carbonic acid pro-duced by radiolysis of surface H O:CO ices. Without a denseatmosphere and magnetic field, Mars lacks the power to attenu-ate penetrating energetic solar particles, energetic cosmic raysor any type of high energy particles. Following Westall et al.(1998), if available in su ffi cient concentrations, carbonic acidcould potentially dissolve metal ores and catalyze chemicalreactions and its presence on Mars may also lead to the ex-istence of limestone (CaCO ), magnesite (MgCO ), dolomite(CaMg(CO ) ) and siderite (FeCO ).
5. Astrophysical implication
Figure 8 presents the total flux of heavy ions (12 . Z .
29) withenergy between 0.1-10 MeV / u inside solar system as a functionof distance to the Sun. Both Galactic cosmic rays (GCR) andenergetic solar particles are displayed. The integrated flux ofheavy and energetic solar ions (black square) at Earth Orbitwere measured by Mewaldt et al. (2007). Starting from the so-lar photosphere abundance and the asumtion that elements withfirst ionization potential of less than 10 eV are more abundantin the energetic solar particles than in photosphere by a fac-tor of 4.5 (Grevesse et al. 1996) at Earth orbit, the integratedflux of heavy and energetic solar particles ( φ HS W ) with ener-gies around 0.1-10 MeV / u was found to be about 1.4 × − cm − s − (black square in Fig. 8).The heavy ion integrated flux at Earth orbit from galacticsources (heavy cosmic rays) was calculated by using the lunar-GCR particle model of Reedy and Arnold (1972) (see also Fig.3.20 of Vaniman et al. 1991) and taking into an account therelative elemental abundances of 12 < Z <
29 atoms at 1 AU(Simpson 1983; Drury, Meyer, Ellison 1999). The value ob-tained was ∼ × − cm − s − (blue circle in Fig. 8). Assumingthat the fluxes of solar wind and energetic solar particles are in- illing et al.: Radiolysis of water-CO ices by energetic heavy ions 11 -1 -6 -5 -4 -3 -2 -1 Galactic cosmic rays I n t e r s t e ll a r M ed i u m P l u t o M e r c u r y E a r t h J up i t e r S a t u r n O o r t C l oud c o m e t s Distance to the Sun (AU) H e li opau s e H ea vy i on f l u x ( pa r t i c l e s / c m s ) S o l a r w i nd Fig. 8.
Estimated value of the integrated flux of heavy ions(12 . Z .
29) with energy between 0.1-10 MeV / u inside so-lar system and at interstellar medium as a function of distanceto the Sun. Both Galactic cosmic rays and energetic solar par-ticles are displayed. Square: integrated flux of energetic solarparticles. Circle and triangle: integrated flux of cosmic rays.versely proportional to the squared distance, this value can bedetermined as a function of distance up to the heliopause ( ∼ φ HCR ) in interstellar medium was performed by (Pilling et al.2010), which considered the value of φ HCR ∼ × − cm − s − (blue triangle in Fig. 8). This value was rather the same as at theouter border of heliopause. An error region (dashed area) wasintroduced in the Fig. 8 to take into an account the uncertaintyof these estimations.The value of integrated flux of heavy ions with energiesaround 0.1-10 MeV / u from galactic sources and energetic solarparticles are comparable at Mars orbit ( ∼ φ HCR / φ HS W ∼ × , 1 × , 7 × , 3 × and 2 × , respectively. The total flux of heavy particles with energybetween 0.1-10 MeV / u at Oort cloud distance was assumed tobe the same as expected in the interstellar medium.Considering the estimated flux of heavy particles of ener-getic solar particles ( φ HS W ) and of heavy nuclei cosmic rays( φ GCR ) as well the determined dissociation cross section ( σ d ),we can calculate the typical molecular half-lives of frozenmolecules in astrophysical surfaces due the presence of heavyparticles by the expression (see Pilling et al. 2010): t / ≈ ln 2( φ HS W + φ HCR ) × σ d [s − ] (18)It is worth to note that the present experiments were per-formed at low temperature of 13 K, the temperature that is ad-equate to ices in the interstellar medium. Therefore it is usefulto make a comparison between IR spectra of interstellar and Fig. 9.
Comparison between IR spectra of interstellar and lab-oratory ices. The top three curves are infrared spectra of youngstellar sources obtained by the Infrared Space Observatory(ISO). Lower traces indicate di ff erent laboratory spectra of irra-diated H O:CO ices: a and c (this work); b (Hudson & Moore2001); d (Gerakines et al. 2000); e (Strazzulla et al. 2005b); f(UV photons, Gerakines et al. 2000). Vertical dashed lines in-dicate the location of vibrational modes of frozen H CO .laboratory ices is shown in Figure 9. The top three curves areIR spectra of young stellar sources obtained by the InfraredSpace Observatory (ISO). The two bold (blue) curves are ourdata. The six bottom curves indicate di ff erent laboratory spec-tra of processed H O:CO ices: a) mixture (10:1) irradiated by52 MeV Ni ions (this work); b) mixture (10:1) irradiated by 0.8MeV protons (Hudson & Moore 2001); c) mixture (1:1) irradi-ated by 52 MeV Ni ions (this work); d) mixture (1:1) irradiatedby 0.8 MeV protons (Gerakines et al. 2000); e) mixture (1:1)irradiated by 0.8 MeV protons (Strazzulla et al. 2005b) and f)mixture (1:1) irradiated by UV photons (Gerakines et al. 2000).Vertical dashed lines indicate the location of vibrational modesof frozen H CO (Moore & Khanna 1991). The presence ofH CO seems to be independent of the ionization source.The three bumps observed on the IR spectra of the youngstelar objects at about 1600 cm − , 1400 cm − and 1350 cm − (see arrows on Fig 9) present a good similarity with the featurespresent in the IR spectra of H O:CO (10:1) irradiated by 52MeV ions (curve a). Nevertheless, up to know, its molecularassignment remains unknown. Although very attenuated, thesefeatures are still observed in the IR spectra of H O:CO (1:1)(curve c). This suggests that heavy cosmic rays can be a good ices by energetic heavy ions candidate to explain some features observed in the IR spectraof some interstellar sources.The comparison between the current results with the pre-vious one (Pilling et al. 2010) reveals that, independently ofice constitution (involving H O, CO , CO and NH ), the dis-sociation cross section due to heavy and energetic cosmic rayanalogs are in the same range, σ d = × − cm . This sup-ports the extension of our previous estimative for the half-lifeof ammonia-containing ice, τ / = × years, to all kindinterstellar grains inside of dense interstellar environments inthe presence of a constant galactic heavy cosmic ray flux.The temperature of solar system ices is about 80 K, about5-8 times higher than observed in interstellar ices, thereforesome molecular species (the most volatile as O and CO) arenot e ffi ciently trapped / adsorbed in / on the ices. This issue canmakes an enormous di ff erence on surface and bulk chemistry.Future experiments employing H O:CO ices at 80 K will beperformed to investigate this issue.
6. Summary and conclusions
The interaction of heavy, highly charged and energetic ions(52 MeV Ni + ) with pure H O and CO ices and mixed(H O:CO ) ices was studied experimentally. The aim was tounderstand the chemical and the physicochemical processes in-duced by heavy cosmic rays inside dense and cool astrophysi-cal environments such as molecular clouds, protostellar cloudsas well at the surfaces solar system ices. Our main results andconclusions are the following:1. In all experiments containing CO (pure ice and mixtures)after a fluence of about 2-3 × ions cm − , the CO / COratio became roughly constant ( ∼ / H O. The experiments suggest that the abun-dances of CO , O , H CO in typical astrophysical ices([CO ] / [H O] ∼ .2. After a fluence of about 2-3 × ions cm − the H O / H Oratio stabilizes at ∼ O rel-ative abundance and column density.3. A comparison between our radiochemical yield and crosssections with literature have revealed higher values for boththe formation and destruction of H O during bombard-ments of ices by heavy and energetic ions. In other wordsthe chemical processing is very enhanced by the processingof ices by heavy and energetic and highly charged ions incomparison with light ions.4. The dissociation cross section of pure H O and CO icesare 1.1 and 1.9 × − cm , respectively. In the case ofmixed H O:CO (10:1) the dissociation cross section ofboth species is roughly 1 × − cm .5. Some IR features observed in ices after the bombardmentby heavy ions present great similarity with those observedat molecular clouds, suggesting that heavy cosmic rays playan important role in the processing of frozen compounds ininterstellar environments. Acknowledgements.
The authors acknowledge the agenciesCOFECUB (France) as well as CAPES, CNPq and FAPERJ(Brazil) for partial support. We thank Th. Been, I. Monnet, Y.Ngono-Ravache and J.M. Ramillon for technical support.
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