Destruction of CO ice and formation of new molecules by irradiation with 28 keV O6+ ions

Nuclear Instruments and Methods in Physics Research B 269 (2011) 852–855

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Destruction of CO ice and formation of new molecules by irradiation with 28 keV O6+ ions A.L.F. de Barros a,e,⇑, E. Seperuelo Duarte b,d, L.S. Farenzena c, E.F. da Silveira d, A. Domaracka e, H. Rothard e, P. Boduch e a

Departamento de Disciplinas Básicas e Gerais, CEFET-RJ, Av. Maracanã 229, 20271-110 Rio de Janeiro, RJ, Brazil Grupo de Física e Astronomia, Instituto Federal do Rio de Janeiro (IFRJ), Rua Lucio Tavares 1045, 26530-060 Nilópolis, RJ, Brazil Departamento de Física, Universidade Federal de Santa Catarina, Campus Trindade, 88010-970 Florianópolis, SC, Brazil d Departamento de Física, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente 225, 22453-900 Rio de Janeiro, RJ, Brazil e Centre de Recherche sur les Ions, les Matériaux et la Photonique CIMAP-GANIL (CEA-CNRS-ENSICAEN-UCBN), BP 5133, Boulevard Henri Becquerel, F-14070 Caen Cedex 05, France b c

a r t i c l e

i n f o

Article history: Received 31 July 2010 Received in revised form 14 December 2010 Available online 21 December 2010 Keywords: Astrochemistry Methods laboratory Solar system, ices Solar wind

a b s t r a c t The effect of solar wind on cometary ice was studied by using oxygen ions with energy near to that corresponding to their maximum abundance in space for bombarding CO ice. This gas was condensed on a CsI substrate at 14 K and irradiated by 28 keV 18O6+ ions up to a final fluence of 1.3  1016 cm2. We have used a methodology in which the sputtering yields, the destruction rate of CO, and the rate of formation of new molecular species are determined by Fourier transform infrared spectroscopy (FTIR). In the current experiment, the condensation of a thin water ice film has prevented the CO sputtering. Quantities such as the dissociation yield, Yd (the number of ice molecules destroyed or dissociated per projectile impact), and the formation yield, Yf (the number of daughter molecules of a given species formed per projectile) are found to be more appropriate and useful than using an integrated or average cross section, since the projectiles are slowing down in the ice from their initial energy until zero velocity (implantation). Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction The interaction of cosmic rays with interplanetary or interstellar ices induces sputtering and material modification. Observations show that condensed H2O, CO and CO2 molecules are three of the most abundant constituents of these astrophysical ices, as determined for instance for the Hale Bopp comet [1]. In space, the ices are exposed to ion irradiation (stellar winds and galactic cosmic rays) ranging from protons up to heavy ions such as Fe, with kinetic energies from keV to TeV. The 28 keV oxygen irradiation corresponds to solar wind ions. However, it should be noted that the sample temperature (14 K) is more appropriate for interstellar ice, since temperatures in the solar system ices are higher (rather of the order of 80 K). Such bombardment leads to radiolysis, sputtering and formation of new molecules. As part of a systematical study on this subject, our group has previously reported results on CO, CO2 and H2O:CO:NH3 ices bombarded with MeV ions to investigate electronic-regime effects produced by heavy ions traversing

⇑ Corresponding author at: Departamento de Disciplinas Básicas e Gerais, CEFET-RJ, Av. Maracanã 229, 20271-110 Rio de Janeiro, RJ, Brazil. E-mail address: [email protected] (A.L.F. de Barros). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.12.055

interstellar grain ice mantles [2–4]. In this work, our first measurements with keV oxygen ions implantation on CO ice are described.

2. Experimental setup The experimental apparatus is constituted by the analysis chamber, the beam line and the deposition system [2]. At the center of the chamber, a CsI substrate is installed in thermal contact with a cold finger cooled by a closed-cycle helium cryostat. Typical residual gas pressure was about 2  108 mbar. The deposition system consists essentially of a pre-chamber, where the gas for analysis is prepared, and a micro-valve to control the deposition flux, around 1  1011 cm2 s1. The samples of CO ice 4000 Å thick were condensed at 14 K and bombarded by 18O6+ ion beam of 28 keV, i.e., of 1.75 keV/u. The beam projectiles are implanted inside the ice according to a distribution centered at 1600 Å of range in a 4000 Å ice thickness. During the ion bombardment (4 h), around 21 monolayers of water ice had recovered the ice preventing the CO sputtering. The water layer thickness was determined during radiation from the measured column density. In spite of the relatively low energy of the beam, the energy loss of the oxygen ions is about 4 keV at the end of irradiation for water (estimation was done by using the code SRIM [5]). We should

A.L.F. de Barros et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 852–855

Fig. 1. Infrared spectrum of CO ice: before (below) and after 28 keV irradiation (above) with a fluence of 1.7  1016 cm2.

18

O6+

853

Fig. 2. Dependence of the column density of new species on beam fluence.

mention that potential sputtering was indeed observed for different materials [6]. Charge equilibration at such low energies occurs in the first monolayer [7]. A Fourier transform infrared spectrometer (FTIR) was used to obtain the spectrum in the 5000–600 cm1 range with spectral resolution of 1 cm1. Fig. 1 displays the FTIR spectra of CO before and after irradiation.

3. Results 3.1. FTIR analysis Fig. 1 displays the spectra before and after oxygen ion irradiation (1.17  1013 ions/cm2). The virgin spectrum is characteristic of pure carbon monoxide. Six optical absorptions are identified: (i) the fundamental stretching peak (m1) centered at 2136 cm1, (ii) the fundamental plus lattice vibration combination band (m1 + mL) at 2208 cm1 peak, (iii) the first overtone of the fundamental (2m1) at 4251 cm1, (iv) the 13C peak of the CO fundamental (m1) at 2091 cm1, (v) the C18O peak at 2088 cm1, and (vi) the band at 2112 cm1, which is associated with C17O. We have identified eight molecular species that were formed after irradiation of the CO ice. The carbon dioxide – CO2 has been identified by means of its strong m3 fundamental line at 2346 cm1 [14]; this species is the most abundant of all detectable molecules in the experiment. A peak at 1989 cm1 is commonly attributed to the C2O molecule [8]. Despite its simple formation reaction, this molecule has one of the lowest abundances of all detectable molecules in the present experiment. C3O is observed through a small shoulder at 2247 cm1 overlapped with an intense absorption line of C3O2 at 2242 cm1 [9]. The O3 molecule is identified by its m3 vibration at 1041 cm1 [8], the occurrence of this species indicates that the abundance of oxygen atoms in the ice should be high. In the current experiment, the m3 vibration of the C5O2 molecule was observed in the top of intense band at 2060 cm1, similarly to the results of Palumbo et al. [8], Jamieson et al. [9], and Trottier and Brooks [10]. Fig. 2 shows the changes of the column density of all the species described above as a function of ion fluence. The C3 molecule is identified by its 2019 cm1 line [9]. C3 and C3O molecules appear only at the fluence of 1.5  1014 cm2 (see Fig. 2).

Fig. 3. Oxygen electronic (Se) and nuclear (Sn) stopping power calculated by SRIM [5] for condensed CO film. Arrows show the conditions of the current experiment at 1.75 keV/u.

Fig. 4. Evolution of the measured net yields (absolute value of destruction less formation yields) for the CO and CO2 molecules. For low fluences, the CO formation yield and the CO2 destruction yields are negligible.

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A.L.F. de Barros et al. / Nuclear Instruments and Methods in Physics Research B 269 (2011) 852–855

Table 1 Values of destruction yield (Yd) for the CO molecules and formation yields (Yf) for the daughter species, also the carbon budget (number of C molecules) obtained by measuring the net yield (YNET).

a b c d e

Molecules

CO

CO2

C2O

O3

C3

C3O

C3O2

C5O2

Position (cm1) A-value (1017 molecule1 cm) Yd (molecules/impact) Yf (molecules/impact) Carbon-budget

2139 1.1a 35.4 – –

2346 7.6b – 6.80 6.80

1989 2.4c – 3.10 6.20

1041 1.53d – 0.70 –

2019 13c – 0.08 0.24

2247 2.5d – 0.38 1.14

2242 13e – 2.60 7.80

2060 7.4c – 0.30 1.50

Ref. [13]. Ref. [14,15]. Ref. [9]. Refs. [7,16]. Ref. [11].

3.2. Yield calculations In our previous measurements [3,4], the projectile energies were in the tens and hundreds MeV range, a situation in which the projectile traverses completely the ice layer losing a relative small fraction of its kinetic energy. Under these conditions, molecular dissociation and formation cross sections could be determined for the corresponding projectile-ice system at the impinging energy. In the keV range (here 28 keV O ions), the situation is quite different: (i) at the beginning the stopping power is 50% nuclear and 50% electronic, but as the ion slows down the nuclear regime begins to dominate (see Fig. 3); (ii) the projectile gets implanted and, therefore, a series of collisions occur for all projectile energies from the impact kinetic energy down to zero; (iii) the projectile trajectory inside the ice is a zig-zag and not a nearly straight line; (iv) the potential energy of highly charged projectiles may play a major role in the interaction. In this low-energy scenario, cross sections vary strongly during the slowing down of the projectile and its analysis is difficult. The determination of integrated quantities such as dissociation yield, Yd – the number of ice molecules destroyed or dissociated per projectile impact, and formation yield, Yf – the number of daughter molecules formed of a given species per projectile, and Ys – the number of sputtered molecules of a given species per projectile, are more appropriate and useful. Since the infrared spectra allow determination of the ice column density of a given molecular species as a function of fluence, it is possible to measure the net yield, YNET, defined as the variation of the number of molecules of that species per projectile impact in the target. Besides layering or condensation, all the three processes (formation, dissociation/destruction and sputtering/desorption) may be responsible for such variation; the respective integrated yields are related as:

Y NET ¼ Y f  Y d  Y s

ð1Þ

Microscopically, the relation between Yf and rf (the formation cross section) for a molecular species k is:

Yf ¼

dNk ¼ dF

Z 0

NL

rðEÞ dN1 ¼ q1

Z

L

rf ðEðxÞÞ dx

ð2Þ

0

where F is the fluence, Nk is the number of produced k molecules per projectile, N1 is the precursor column density along the track, q1 is the number of precursor molecules per volume, E(x) is the projectile energy after a path x in the ice and L is the projected path range of the projectile. The value of L should correspond to the threshold Eth, below which no chemical reaction occurs for the k species. The average projected range (around 1600 Å), as well as defect distributions are obtained from the code SRIM [5]. Similar expression can be defined for Yd, either for the precursor dissociation or for daughter molecule dissociation, when qk (F,x) needs to be introduced.

Since Yk = Yf,k  Yd,k is the quantity measured by FTIR, the Yf,k value can be obtained in the beginning of irradiation, when the daughter density qk is negligible compared to q1. Fig. 4 illustrates the experimental net yields for the CO and CO2 species. Table 1 gives the values obtained for Yd for the CO molecules and Yf for the identified daughter species and the carbon budget. Schou and Pedrys [12] reported the sputtering yield produced by 9 keV protons to be 34 CO/H+. At the same time, a nonvolatile residue was produced with a rate of 25 CO/H+, which actually is surprisingly close to the value of Yd 35/ion. This is remarkable, since the stopping of the protons is purely electronic – and also much less than for 28 keV O ions. This may be an indication of the relative effect of the two different stopping mechanisms, i.e., the electronic stopping seems much more efficient for molecular destruction. 4. Conclusion In this work, a (new) FTIR methodology for analyzing sputtering and chemical reaction yields is discussed and employed for keV ion bombardment on ices. For the case presented as illustration, 28 keV oxygen ions on 14 K CO ice, no CO sputtering occurred due to water layering on the top of the target ice preventing its emission. The column density of CO was reduced during bombardment while new species were formed: CO2, O3, C3O2, C2O, C5O2, C5O, C3 and C3O. The CO destruction yield is around 35 molecules per projectile and the CO2 production yield is 7 molecules per projectile. After a fluence of 1016 projectiles/cm2, the net (destruction + formation) yield of CO destruction decreases by a factor of 5, that for CO2 formation decreases by a factor of 6, approaching to a dynamical equilibrium, as can be seen at Fig. 4. Acknowledgements The Brazilian agencies CNPq, Capes/Cofecub and Faperj are acknowledged for partial support. The experiment was performed at GANIL and we thank T. Been, J.-M. Ramillon, F. Noury, and L. Maunoury for technical support. References [1] P. Ehrenfreund, W. Irvine, L. Becker, J. Blank, J.R. Brucato, L. Colangeli, S. Derenne, D. Despois, A. Dutrey, H. Fraaije, et al., Rep. Prog. Phys. 65 (2002) 1427. [2] E. Seperuelo Duarte, P. Boduch, H. Rothard, T. Been, E. Dartois, L.S. Farenzena, E.F. da Silveira, Astron. Astrophys. 502 (2009) 599. [3] S. Pilling, E. Seperuelo Duarte, E.F. da Silveira, E. Balanzat, H. Rothard, A. Domaracka, P. Boduch, Astron. Astrophys. 509 (2010) A87. [4] E. Seperuelo Duarte, A. Domaracka, P. Boduch, H. Rothard, E. Dartois, E.F. da Silveira, Astron. Astrophys. 512 (2010) A71. [5] J.F. Ziegler, J.P. Biersack, Version 02, 2006, . [6] F. Aumayr, H.P. Winter, Philos. Trans. R. Soc. Lond. A 362 (2004) 77.

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