- Email: [email protected]
Implantation of reactive and unreactive ions in silicates and ices
cm .__
__ BB
Nuclear Instruments and Methods in Physics ResearchB 116(19%) 289-293
NOMB
Beam Interactions with Materials 8 Atoms
ELSEVIER
Implantation of reactive and unreactive ions in silicates and ices G. Strazzulla aT* , J.R. Brucato a, M.E. Palumbo a, M.A. Satorre b a Osservatorio
Astrojisico and Istituto di Astronamia, Vniversit& di Catania, Citth Vniversitaria, I-95125 Catania, Italy b Departament Fisica Aplicada EPSA (Escola Politecnica Superior Alcoi), Alicante, Spain
Abstract
We present selected results of a series of experiments, performed in our laboratory, on the implantation of = 1 keV/amu reactive (H, C, N, 0) and unreactive (He, Ar) ions into silicon, silicates and frozen gases. The purpose to irradiate the last two samples, is to simulate solar wind ion implantation on planem objects. In particular we investigated on the possibility of producing chemical bonds between the projectile and the target atoms searching for the appearance of characteristic IR bands. When the impinging ion is reactive (H, C, N, 0) we found several cases of newly produced molecular species that include the projectile.
1. Introduction
In space there are many places, including interstellar dust and planetary objects, where energetic (keV-MeV) particles impinge on solid surfaces made of refractory (carbonaceous and/or silicates) materials and/or frozen ices. Ion irradiation produces a number of effects whose study has been based on laboratory simulations of relevant targets bombarded with fast charged particles under physical conditions more or less similar to the astrophysical ones (see e.g. Refs. [1,2]). In this paper we present some results of a series of experiments, undertaken in our laboratory, on the implantation of reactive (H, C, N, 0) and unreactive (He, Ar) ions into silicon, silicates and ices of astrophysical relevance.
2. Experimental
apparatus
To study the effects induced by ions we used “in situ” IR spectroscopy in the 4400-400 cm-’ (2.27-25 p,m) range. The experimental apparatus has been already described [3]: it is essentially constituted by a scattering chamber faced, through IR-transparent windows, to an IR spectrophotometer. Vacuum is better than 10v7 mbar. Frosts are accreted onto a substratum, put in contact with a cold finger (lo-300 K), by admitting gas (mixtures) into the chamber, through an opportune valve. The substratum is a silicon single crystal, transparent in the LR.The spectra are ratioed to a background including the substratum. Ion
* Corresponding author. Tel.: + 39 95 7332213; fax: + 39 95 330592; e-mail: [email protected] (internet).
currents are maintained lower than few PA/cm’ to avoid macroscopic heating of the target. Ions have been obtained from a 30 keV ion implanter. The doses are often given in eV per small molecule (we use 16 amu) because this is a convenient way to characterize chemical changes [4] and compare them with experiments on other targets and, with some cautions, to other projectiles.
3. Experimental 3.1. H implantation
into silicon
and silicates
It has been suggested that a clear test of proton implantation could be the observability of the -SiH stretching band on the surfaces of atmosphereless planetary objects [5]. The wavenumber of that stretching varies from 2278 cm- ’ (4.39 pm) in oxidized silicates. to 2110 cm-’ (4.74 Frn) in a reduced solid [5,6]. In a recent paper we presented the results of experimental studies on the development of -SiH stretching band by H+ (1.5 keV) implantation on silicon, quartz, palagonite and feldspar [7]. Fig. 1 shows the 1800-2300 cm-’ (5.56-4.35 pm) IR transmittance spectra, of single silicon crystal (111; 600 p,rn thick) and of quartz, feldspar and palagonite after implantation with 7.5 X lOI Hf (1.5 keV)/cm’. The implanted quartz sample was a window about 4 mm thick. Palagonite and feldspar were prepared by pressing dust on previously prepared KE+r pellets in order to obtain samples whose thickness was of a few pm. Palagonite is a broad geologic term, used to mean an amorphous hydrated ferric-iron silica gel, formed by the alteration of
0168-583X/%/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SOl68-583X(96)00063-8
results
290
G. Strauulla et al. / Nucl. Instr. and Meth. in Phys. Res. B I16 (1996) 289-293
mafic volcanic glass. The sample used here has been collected from Mt. Etna: it is weathered and reworked pyroclast from a historic eruptive activity. Feldspar is a commercially available standard. It is a mixture of orthoclase (K-feldspar) and albite (Na-feldspar). We have also irradiated olivine (prepared as a dust layer on KRS-5 by laser ablation of a bulk sample) and muscovite, a phillosilicate from which we obtained selfsupporting films whose thickness is of the order of a few p.m. Their spectra are shown in Fig. 2. From Figs. 1 and 2 it is clear that the selected samples, but pure silicon, do not exhibit any clear evidence of the presence of the -SiH stretching band. Note that the spectrum of the quartz sample is shown only in the 2100-2300 cm-’ (4.76-4.35 pm) region because quartz is opaque at lower wavenumbers. These results seem to argue against the possibility of observing asteroidal (or cometary) spectral features due to -SiH groups formed by implantation of solar wind protons in silicate surfaces. However we planned to perform additional experiments on different silicon samples (amorphous and polycrystalline) and silicates with different experimental conditions (temperature, surface roughness, pre-irradiation with different ions).
wavelength 5.3
5.1
4.9
(pm) 4.7
4.5
zi % 2
1.5 keV El’ 6.2
lO”/Crn’
on muscovite
1900 2000 2100 2200 wavenumber
(cm-‘)
Fig. 2. IR transmittance spectra(1800-2300 cm-‘) of olivine and muscovite after implantation of H+ (1.5 keV) at the given doses.
3.2. C, N, 0 implantation
into silicon and silicates
Some experimental results on the implantation of 40 keV C ions into SiO, grains or thin films leads to the synthesis of CO and CO, by bonds formed between the projectiles and the lattice 0 atoms [2,8]. These molecules have been identified by the appearance in the IR spectrum of the bands located at 2143 cm- ’ (CO) and = 2350 cm-’ (CO,). We started a program with the purpose to implant C, N, 0 ions in silicon and silicates. In Fig. 3 the results obtained for pure silicon are presented. We can see that as reactive ions are implanted these are able to form bonds with the Si atoms in the target. Implantation with argon (see the two bottom curves in Fig. 3) rules out the possibility that the new bands are due to a damage into the crystalline structure of silicon. Concerning silicates we present, in Fig. 4, the very first results obtained implanting 30 keV C+ in quartz: the formation of CO and CO, is made clear from the appearance of the bands located at -2140 cm-’ (CO) and = 2350 cm-’ (CO,). 3.3. Ion implantation into ices
I I,,,,I,,,,I,.~,I,~,,I,,,,I la00 2000 2100 2200 wavenumber
(cm-‘)
Fig. 1. IR transmittance spectra (1800-2300 cm-‘) of several samples after irradiation of 7.5 X lOI H+ (1.5 keV)/cm*.
Strazzulla et al. [9] presented some results of nitrogen ions implantation in frozen hydrocarbons mixed with water. They observed the formation of a new broad band around 2130 cm- ’ both for an oil (aliphatic hydrocarbon)-water mixture and for a benzene (aromatic hydrocarbon)-water mixture. However the attribution of that band is not stmightforward: in this region in fact, both the X-C=N and the X-CzC-H stretches absorb as well as frozen CO [lo]. In fact, although CO has a much lower sublimation temperature it could be, trapped in the matrix and evaporate at higher T. The observed bands show two
G. Strauulla et aL/Nucl.
fnstr. and Meth. in Phys. Res. 3 I16 (19961289-293
291
12 eV/l6amu
a
-
2
_ c)
s”_ a
(3 keV He+)
14 ke’f N’ V’
2
-
% 8
- d) 30 keV
2 E a
_ e)
5
_
100
0+
keV Ar* 0 4000
3000 Wavenumber
2000 (cni’)
1000
5. IR spectra (10 K; 4000-450 cm-‘) of a H,O:CO, (1: 1) mixture before and after irradiation with 3 keV He ions at two different doses. Fig.
Fig. 3. IR spectra of implanted silicon: (a) 7.5~ lOI Hf (30 keV)/cm*; (b) 1.1 X 10” C+ (30 keV)/cm’; (c) 2.2~ lOI N+ (14 keV)/cm*; (d) 1.7X 1O’7 O+ (30 keV)/cm’; (e) 1.5~ lOI Arf (100 keV)/cm*; (f) 1.5 x 1OL6A?+ (60 keV)/cm’.
30 kev
C* on
Sio,
2100 2200 2300 wavenumber (cm-‘) Fig. 4. IR transmittance spectra (2100-2400 cm-‘) of quartz before and after implantation of C+ (30 keV) at tbe given doses.
peaks: at 2113 cm-’
and at 2140 cm-‘.
Other experi-
ments are planned. in order to discriminate among the different possible contributions. In particular we want to
irradiate pure hydrocarbons, in order to avoid the possible formation of CO. It is impox&mt to note that the band at 2113 cm-’ is probably due to X-C&-H bonds: this band in fact is observed also in benzene irradiated with He ions and attributed to mono-substituted acetylenes [3]. In another set of experiments we irradiated frozen CO, and H$O: CO, (1: 1) mixtures. The spectrum of an unirradiated H,O: CO, (1: 1) mixture is shown in Fig. 5, together with those obtained after irradiation with helium iotis (3 keV) at two different doses: it is evident the appearance, upon irradiation, of several new bands that testify to the formation of new species. CO is easily detected from the band at 2140 cm-‘. The other new features are at about the same position as detected by Moore and Khanna [ 111. As a matter of fact Moore and Khanna [ 1l] irradiated a mixture H,O: CO, (1: 1) with 700 keV protons. The thickness of the bombarded layers was of the order of 4 p,m, smaller than the penetration depth of the used ions. The stopping power of 700 keV protons in that mixture is = 30 eV cm2 lo-l5 molecules-’ and, as the experiment was made with a total fluence of 1.5 X 10” protons cm-‘, the total dose was = 12 eV/16 amu. After irradiation they observed an IR spectrum where several new bands appeared: CO was clearly detected along with some radicals and carbonic acid (H,CO,). This is an elusive molecule whose IR spectrum was unknown before that work. It has
G. Strauda
292
co, 0.3
t
after
c
et al./Nucl.
Imtr. and Meth. in Phys. Res. B 116 (1996) 289-293
10K 1.5 keV H’
(Is evj16am~)
II
Fig. 6. IR spectra (at 10 K) of a CO, frozen layer after irradiation with 1.5 keV protons.
The similarity of the spectra exhibited in Fig. 5 with those obtained in Ref. [I l] is evidenced in Table 1 where the peak positions (at low T) of their newly produced bands (first column) are reported together with those observed in our experiments with helium (column 2). Columns 3 and 4 report data obtained by proton irradiation as we discuss below. The species suggested as responsible for the bands are reported in the last column. To verify the effects induced by the implantation of a reactive ion we irradiated a layer of pure CO, ice with 1.5 keV protons. We irradiated pure CO, in order to have no H atoms in the target except for those implanted. The spectrum of the proton-irradiated CO, sample is shown in Fig. 6. We can see, also in this case, the appearance of several new bands which are the same observed for a H,O: CO, mixture irradiated with unreactive ions (see columns 2 and 4 in Table 1). This result implies that the incoming protons are bonded with target atoms to form carbonic acid. Ion implantation of reactive species is able to form chemical species that include the projectile, that is atoms not originally present on the target (for details see Ref. [ 141).
4. Conctusion been demonstrated that once ions provide the necessary activation energy ( = 50 kcal mol- ‘) carbonic acid can be stabilized at low temperature. Warming up the irradiated layers to 215-250 K a residual film was left over and its IR broad features were tentatively attributed to carbonic acid. Recently this attribution has been confirmed [12]: carbonic acid has been synthesized by protonation of bicarbonate and characterized by FTIR spectroscopy.
The ion population in the interplanetary medium is essentially due to solar wind and flare ions. Solar wind ions are produced by a plasma expansion whose velocity, at a distance of few solar radii, becomes supersonic, of the order of 400 km/s (i.e. ions at energies of = 1 keV/amu are expelled). At 1 astronomical unit (1 AU = 1.49 X 10’ km) the wind has a density of the order of 5 protons cmv3,
Table 1 Peak position (cm-‘) of newly formed bands observed in H,O: CO, = 1: 1 mixtures irradiated by energetic ions (T= lo-20 K) 700keVH+a 12 eV/16 amu
3 keV He+ 12 eV/16 amu
1.5 keV Hf 13 eV/16 amu
1.5 keV H+b 19eV/16amu
wing 2599 2237 2143 2094 2044 1943 1879 1714 1482 1357 1294 1038 1015
2842 2576
2850 2580
2900-2600
2141 very weak 2043
2142 very weak 2044
2142 very weak 2044 weak
1710 1484 1384 1297 1040
weak 1701 1500 1380 1303 1040
811
811
811
a
Ref. [ll].
b on pure co,.
Assignment H&O,
&CO,
1736 1490 1302 1040
C,O,? co %o CO, ? HCO? H&O, H&O, ? H,CO,
H,CO,; O,? ? H2CO3
G. Strauufla et al. /Nuc~. fnstr. and Meth. in Phys. lies. 3 116 (I996) 289-293
corresponding to a flux of = 2 X lo8 protons cm-* s-l. The flux varies as the inverse of the square of the distance from the Sun. Thus, for example, at a distance of the order of 3 AU the number of protons impinging on a hypothetical asteroidal surface is of the order of 10 l7 cm-* in only 100 yr. Such a high flux is expected to produce a number of effects such as sputtering [15] and ion implantation. In this paper we have presented some selected results of a study whose purpose is to investigate the possibility of the formation, by solar wind implantation, of chemical bonds evidenced by the appearance of infrared spectral bands. Acknowledgements
This research has been supported by the Italian Space Agency (ASI). References [l] G. Strazzulla, in: Solar System Ices 1996, eds. B. Schmitt, C. de Bergh, M. Festou (Kluwer Academic Publishers, Dordrecht) in press. [2] J.P. Bibring and F. Rocard, Adv. Space Res. 4 (1984) 103.
293
[31 G. Strazzulla and G.A. Baratta, Astron. Astrophys. 241 (1991) 310. 141 G. Strazzulla and R.E. Johnson, in: Comets in the Post-Halley Era, eds. R. Newbum Jr., M. Neugebauer and J. Rahe (Kluwer Academic Publishers, Dordnxht, 1991) p. 243. 151 J.A. Nuth, M.H. Moore and T. Tanabk, Icarus 98 (1992) 207. b1 M.H. Moore, T. Tana% and J.A. Nuth, Astrophys. J. Lett. 373 (1991) L31. M A. Buemi, G. Cimiuo, G. Leto and G. Strazzulla, Icarus 108 (1994) 169. k31J.P. Bibring and F. Rocard, Radiat. Eff. 65 (1982) 159. 191 G. Strazzulla, J.R. Brucato, G. Cimino, G. L&o and F. Spinella, Adv. Space Sci. 15(10) (1995) 13. DOI M.E. Palumbo and G. Strazzulla, A&on. Astrophys. 269 (1993) 568. illI M.H. Moore and R.H. Khanna, Spcctrochim. Acta 47 (1991) 255. WI W. Hage, A. Hallbrucker and E. Mayer, J. Am. Chem. Sot. 115 (1993) 8427. 1131 R.E. Johnson, Energetic charged particle interactions with atmospheres and surfaces, ed. L.J. Lanzerotti (Springer, 1990). 1141 J.R. Bmcato, M.E. Palumbo and G. Strazzulla, Icarus (1996) submitted. Ml K. Thiel, U. Sassmannshausen, H. Kulzer and W. Herr, Radiat. Eff. 64 (1982) 83.
__ BB
Nuclear Instruments and Methods in Physics ResearchB 116(19%) 289-293
NOMB
Beam Interactions with Materials 8 Atoms
ELSEVIER
Implantation of reactive and unreactive ions in silicates and ices G. Strazzulla aT* , J.R. Brucato a, M.E. Palumbo a, M.A. Satorre b a Osservatorio
Astrojisico and Istituto di Astronamia, Vniversit& di Catania, Citth Vniversitaria, I-95125 Catania, Italy b Departament Fisica Aplicada EPSA (Escola Politecnica Superior Alcoi), Alicante, Spain
Abstract
We present selected results of a series of experiments, performed in our laboratory, on the implantation of = 1 keV/amu reactive (H, C, N, 0) and unreactive (He, Ar) ions into silicon, silicates and frozen gases. The purpose to irradiate the last two samples, is to simulate solar wind ion implantation on planem objects. In particular we investigated on the possibility of producing chemical bonds between the projectile and the target atoms searching for the appearance of characteristic IR bands. When the impinging ion is reactive (H, C, N, 0) we found several cases of newly produced molecular species that include the projectile.
1. Introduction
In space there are many places, including interstellar dust and planetary objects, where energetic (keV-MeV) particles impinge on solid surfaces made of refractory (carbonaceous and/or silicates) materials and/or frozen ices. Ion irradiation produces a number of effects whose study has been based on laboratory simulations of relevant targets bombarded with fast charged particles under physical conditions more or less similar to the astrophysical ones (see e.g. Refs. [1,2]). In this paper we present some results of a series of experiments, undertaken in our laboratory, on the implantation of reactive (H, C, N, 0) and unreactive (He, Ar) ions into silicon, silicates and ices of astrophysical relevance.
2. Experimental
apparatus
To study the effects induced by ions we used “in situ” IR spectroscopy in the 4400-400 cm-’ (2.27-25 p,m) range. The experimental apparatus has been already described [3]: it is essentially constituted by a scattering chamber faced, through IR-transparent windows, to an IR spectrophotometer. Vacuum is better than 10v7 mbar. Frosts are accreted onto a substratum, put in contact with a cold finger (lo-300 K), by admitting gas (mixtures) into the chamber, through an opportune valve. The substratum is a silicon single crystal, transparent in the LR.The spectra are ratioed to a background including the substratum. Ion
* Corresponding author. Tel.: + 39 95 7332213; fax: + 39 95 330592; e-mail: [email protected] (internet).
currents are maintained lower than few PA/cm’ to avoid macroscopic heating of the target. Ions have been obtained from a 30 keV ion implanter. The doses are often given in eV per small molecule (we use 16 amu) because this is a convenient way to characterize chemical changes [4] and compare them with experiments on other targets and, with some cautions, to other projectiles.
3. Experimental 3.1. H implantation
into silicon
and silicates
It has been suggested that a clear test of proton implantation could be the observability of the -SiH stretching band on the surfaces of atmosphereless planetary objects [5]. The wavenumber of that stretching varies from 2278 cm- ’ (4.39 pm) in oxidized silicates. to 2110 cm-’ (4.74 Frn) in a reduced solid [5,6]. In a recent paper we presented the results of experimental studies on the development of -SiH stretching band by H+ (1.5 keV) implantation on silicon, quartz, palagonite and feldspar [7]. Fig. 1 shows the 1800-2300 cm-’ (5.56-4.35 pm) IR transmittance spectra, of single silicon crystal (111; 600 p,rn thick) and of quartz, feldspar and palagonite after implantation with 7.5 X lOI Hf (1.5 keV)/cm’. The implanted quartz sample was a window about 4 mm thick. Palagonite and feldspar were prepared by pressing dust on previously prepared KE+r pellets in order to obtain samples whose thickness was of a few pm. Palagonite is a broad geologic term, used to mean an amorphous hydrated ferric-iron silica gel, formed by the alteration of
0168-583X/%/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved PII SOl68-583X(96)00063-8
results
290
G. Strauulla et al. / Nucl. Instr. and Meth. in Phys. Res. B I16 (1996) 289-293
mafic volcanic glass. The sample used here has been collected from Mt. Etna: it is weathered and reworked pyroclast from a historic eruptive activity. Feldspar is a commercially available standard. It is a mixture of orthoclase (K-feldspar) and albite (Na-feldspar). We have also irradiated olivine (prepared as a dust layer on KRS-5 by laser ablation of a bulk sample) and muscovite, a phillosilicate from which we obtained selfsupporting films whose thickness is of the order of a few p.m. Their spectra are shown in Fig. 2. From Figs. 1 and 2 it is clear that the selected samples, but pure silicon, do not exhibit any clear evidence of the presence of the -SiH stretching band. Note that the spectrum of the quartz sample is shown only in the 2100-2300 cm-’ (4.76-4.35 pm) region because quartz is opaque at lower wavenumbers. These results seem to argue against the possibility of observing asteroidal (or cometary) spectral features due to -SiH groups formed by implantation of solar wind protons in silicate surfaces. However we planned to perform additional experiments on different silicon samples (amorphous and polycrystalline) and silicates with different experimental conditions (temperature, surface roughness, pre-irradiation with different ions).
wavelength 5.3
5.1
4.9
(pm) 4.7
4.5
zi % 2
1.5 keV El’ 6.2
lO”/Crn’
on muscovite
1900 2000 2100 2200 wavenumber
(cm-‘)
Fig. 2. IR transmittance spectra(1800-2300 cm-‘) of olivine and muscovite after implantation of H+ (1.5 keV) at the given doses.
3.2. C, N, 0 implantation
into silicon and silicates
Some experimental results on the implantation of 40 keV C ions into SiO, grains or thin films leads to the synthesis of CO and CO, by bonds formed between the projectiles and the lattice 0 atoms [2,8]. These molecules have been identified by the appearance in the IR spectrum of the bands located at 2143 cm- ’ (CO) and = 2350 cm-’ (CO,). We started a program with the purpose to implant C, N, 0 ions in silicon and silicates. In Fig. 3 the results obtained for pure silicon are presented. We can see that as reactive ions are implanted these are able to form bonds with the Si atoms in the target. Implantation with argon (see the two bottom curves in Fig. 3) rules out the possibility that the new bands are due to a damage into the crystalline structure of silicon. Concerning silicates we present, in Fig. 4, the very first results obtained implanting 30 keV C+ in quartz: the formation of CO and CO, is made clear from the appearance of the bands located at -2140 cm-’ (CO) and = 2350 cm-’ (CO,). 3.3. Ion implantation into ices
I I,,,,I,,,,I,.~,I,~,,I,,,,I la00 2000 2100 2200 wavenumber
(cm-‘)
Fig. 1. IR transmittance spectra (1800-2300 cm-‘) of several samples after irradiation of 7.5 X lOI H+ (1.5 keV)/cm*.
Strazzulla et al. [9] presented some results of nitrogen ions implantation in frozen hydrocarbons mixed with water. They observed the formation of a new broad band around 2130 cm- ’ both for an oil (aliphatic hydrocarbon)-water mixture and for a benzene (aromatic hydrocarbon)-water mixture. However the attribution of that band is not stmightforward: in this region in fact, both the X-C=N and the X-CzC-H stretches absorb as well as frozen CO [lo]. In fact, although CO has a much lower sublimation temperature it could be, trapped in the matrix and evaporate at higher T. The observed bands show two
G. Strauulla et aL/Nucl.
fnstr. and Meth. in Phys. Res. 3 I16 (19961289-293
291
12 eV/l6amu
a
-
2
_ c)
s”_ a
(3 keV He+)
14 ke’f N’ V’
2
-
% 8
- d) 30 keV
2 E a
_ e)
5
_
100
0+
keV Ar* 0 4000
3000 Wavenumber
2000 (cni’)
1000
5. IR spectra (10 K; 4000-450 cm-‘) of a H,O:CO, (1: 1) mixture before and after irradiation with 3 keV He ions at two different doses. Fig.
Fig. 3. IR spectra of implanted silicon: (a) 7.5~ lOI Hf (30 keV)/cm*; (b) 1.1 X 10” C+ (30 keV)/cm’; (c) 2.2~ lOI N+ (14 keV)/cm*; (d) 1.7X 1O’7 O+ (30 keV)/cm’; (e) 1.5~ lOI Arf (100 keV)/cm*; (f) 1.5 x 1OL6A?+ (60 keV)/cm’.
30 kev
C* on
Sio,
2100 2200 2300 wavenumber (cm-‘) Fig. 4. IR transmittance spectra (2100-2400 cm-‘) of quartz before and after implantation of C+ (30 keV) at tbe given doses.
peaks: at 2113 cm-’
and at 2140 cm-‘.
Other experi-
ments are planned. in order to discriminate among the different possible contributions. In particular we want to
irradiate pure hydrocarbons, in order to avoid the possible formation of CO. It is impox&mt to note that the band at 2113 cm-’ is probably due to X-C&-H bonds: this band in fact is observed also in benzene irradiated with He ions and attributed to mono-substituted acetylenes [3]. In another set of experiments we irradiated frozen CO, and H$O: CO, (1: 1) mixtures. The spectrum of an unirradiated H,O: CO, (1: 1) mixture is shown in Fig. 5, together with those obtained after irradiation with helium iotis (3 keV) at two different doses: it is evident the appearance, upon irradiation, of several new bands that testify to the formation of new species. CO is easily detected from the band at 2140 cm-‘. The other new features are at about the same position as detected by Moore and Khanna [ 111. As a matter of fact Moore and Khanna [ 1l] irradiated a mixture H,O: CO, (1: 1) with 700 keV protons. The thickness of the bombarded layers was of the order of 4 p,m, smaller than the penetration depth of the used ions. The stopping power of 700 keV protons in that mixture is = 30 eV cm2 lo-l5 molecules-’ and, as the experiment was made with a total fluence of 1.5 X 10” protons cm-‘, the total dose was = 12 eV/16 amu. After irradiation they observed an IR spectrum where several new bands appeared: CO was clearly detected along with some radicals and carbonic acid (H,CO,). This is an elusive molecule whose IR spectrum was unknown before that work. It has
G. Strauda
292
co, 0.3
t
after
c
et al./Nucl.
Imtr. and Meth. in Phys. Res. B 116 (1996) 289-293
10K 1.5 keV H’
(Is evj16am~)
II
Fig. 6. IR spectra (at 10 K) of a CO, frozen layer after irradiation with 1.5 keV protons.
The similarity of the spectra exhibited in Fig. 5 with those obtained in Ref. [I l] is evidenced in Table 1 where the peak positions (at low T) of their newly produced bands (first column) are reported together with those observed in our experiments with helium (column 2). Columns 3 and 4 report data obtained by proton irradiation as we discuss below. The species suggested as responsible for the bands are reported in the last column. To verify the effects induced by the implantation of a reactive ion we irradiated a layer of pure CO, ice with 1.5 keV protons. We irradiated pure CO, in order to have no H atoms in the target except for those implanted. The spectrum of the proton-irradiated CO, sample is shown in Fig. 6. We can see, also in this case, the appearance of several new bands which are the same observed for a H,O: CO, mixture irradiated with unreactive ions (see columns 2 and 4 in Table 1). This result implies that the incoming protons are bonded with target atoms to form carbonic acid. Ion implantation of reactive species is able to form chemical species that include the projectile, that is atoms not originally present on the target (for details see Ref. [ 141).
4. Conctusion been demonstrated that once ions provide the necessary activation energy ( = 50 kcal mol- ‘) carbonic acid can be stabilized at low temperature. Warming up the irradiated layers to 215-250 K a residual film was left over and its IR broad features were tentatively attributed to carbonic acid. Recently this attribution has been confirmed [12]: carbonic acid has been synthesized by protonation of bicarbonate and characterized by FTIR spectroscopy.
The ion population in the interplanetary medium is essentially due to solar wind and flare ions. Solar wind ions are produced by a plasma expansion whose velocity, at a distance of few solar radii, becomes supersonic, of the order of 400 km/s (i.e. ions at energies of = 1 keV/amu are expelled). At 1 astronomical unit (1 AU = 1.49 X 10’ km) the wind has a density of the order of 5 protons cmv3,
Table 1 Peak position (cm-‘) of newly formed bands observed in H,O: CO, = 1: 1 mixtures irradiated by energetic ions (T= lo-20 K) 700keVH+a 12 eV/16 amu
3 keV He+ 12 eV/16 amu
1.5 keV Hf 13 eV/16 amu
1.5 keV H+b 19eV/16amu
wing 2599 2237 2143 2094 2044 1943 1879 1714 1482 1357 1294 1038 1015
2842 2576
2850 2580
2900-2600
2141 very weak 2043
2142 very weak 2044
2142 very weak 2044 weak
1710 1484 1384 1297 1040
weak 1701 1500 1380 1303 1040
811
811
811
a
Ref. [ll].
b on pure co,.
Assignment H&O,
&CO,
1736 1490 1302 1040
C,O,? co %o CO, ? HCO? H&O, H&O, ? H,CO,
H,CO,; O,? ? H2CO3
G. Strauufla et al. /Nuc~. fnstr. and Meth. in Phys. lies. 3 116 (I996) 289-293
corresponding to a flux of = 2 X lo8 protons cm-* s-l. The flux varies as the inverse of the square of the distance from the Sun. Thus, for example, at a distance of the order of 3 AU the number of protons impinging on a hypothetical asteroidal surface is of the order of 10 l7 cm-* in only 100 yr. Such a high flux is expected to produce a number of effects such as sputtering [15] and ion implantation. In this paper we have presented some selected results of a study whose purpose is to investigate the possibility of the formation, by solar wind implantation, of chemical bonds evidenced by the appearance of infrared spectral bands. Acknowledgements
This research has been supported by the Italian Space Agency (ASI). References [l] G. Strazzulla, in: Solar System Ices 1996, eds. B. Schmitt, C. de Bergh, M. Festou (Kluwer Academic Publishers, Dordrecht) in press. [2] J.P. Bibring and F. Rocard, Adv. Space Res. 4 (1984) 103.
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