Chemical irradiation effects in ices

Nuclear Instruments and Methods in Physics Research B32 (1988) 349-353 North-Holland, Amsterdam

CHEMICAL

IRRADIATION

J. BENIT, J-P. BIBRING

EFFECTS

349

IN ICES

and F. ROCARD

L.aboratowe Rene Berms du CSNSM,

91406 Orsay, France

We present some chemical effects of the irradiation of ices by keV light ions. We have detected the synthesis of new compounds by means of in situ infrared spectroscopy. We discuss the results obtained by the irradiation of pure H,O or CH, ices, as well as mixtures: (H,O+CO,); (H,O+NH,); (CO* + NH,). In all cases, we observe a rich variety of small species, not always unambiguously identified. Broad features are also generally present, which are likely the signature of more complex molecules. In some cases residues are formed, which are stable after annealing at room temperature.

1. Introduction We have shown, by in situ infrared spectroscopy, that the irradiation of ices, at 80 K, by keV or MeV ions leads to their “erosion” through the dissociation of the molecules all along the track of the ions [l-4]. In the model we proposed, this dissociation is followed by the recombination of the products into new molecules, as H, and 0, in the case of H,O ice, likely to diffuse out of the irradiated samples. The erosion mechanism does not merely result from the ejection of the molecules constituting the targets. It proceeds from the molecular synthesis induced by the irradiation. We have studied this synthesis in the case of irradiation either by inert gases (He+) or reactive ions (H+, C’). In particular, we have shown that the irradiation of H,O ice by C+ ions leads to the synthesis of CO, with a yield close to 50% [S]. In this paper, we present our results concerning the irradiation of organic material (section 3) and mixtures of ices (sections 4-6) by ai’ and He+ ions, with energy 10 to 40 keV. In the last section, we compare our results to those obtained previously [6].

2. Experimental procedure The entire experimental procedure has been described previously [2,4,5]. We analyse the films by FTIR spectroscopy during the irradiation. Icy films are condensed by vacuum deposition onto a KBr substrate, in thermal contact with a cryostat. To condense composite ices, we start with gaseous mixtures whose abundance ratios are controlled by pressure measurements. The minimum temperature we can reach is 30 K, when we use a closed-cycle He cryostat instead of the liquid nitrogen device we used before. The lower temperature is chosen in order to allow the condensation of organic gases, in particular CH,. We have to take into account 0168-583X/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

that in these conditions, species like O2 and N,, present within the residual vacuum, do also condense onto the KBr cold substrate. In order to minimize their concentration in the ices we irradiate, we have improved the thermal screens surrounding the target.

3. Irradiation of CH, CH, films, at 35 K, of various thicknesses have been irradiated with 40 keV Hi. When we start with a thickness of the order of the range of the ions (8000 i% to 1 pm), we observe after a fluence of lOI H.cmm2 the presence of a variety of new absorption features in the 4300-400 cm-’ IR spectrum (table 1). They correspond to the synthesis of species containing C, H and 0 atoms. Two main groups of C-containing molecules are present, one in the 3000-2800 cm-’ region (fig. l), the other between 1700 and 1300 cm-’ (fig. 2). We have tentatively attributed the different peaks to compounds by comparison with library spectra in similar spectral regions: in the 2960-2850 cm-’ region, the bands correspond to the vibrational stretching of C-H bounds in the form of CH,- and CH,- functional groups, whereas in the spectral region in excess of 3000 cm-‘, the bands correspond more likely to unsaturated hydrocarbons, containing preferentially multiple boundings. In the 1700-1300 cm-’ region, the bending of C-H and the vibration of C-C dominate. The irradiation is also responsible for the synthesis of H,O, which appears as a band centered at 3287 cm-‘. H,O probably results from the dissociation of CH, and 0, (see section 2). Upon thermal annealing, by warming up the target to room temperature, the IR spectrum is modified: the volatile species (H,O ice at 3287 cm-‘, CH, ice at 1300 and 3011 cm-‘, synthesized CO at 2135 cm-‘) are removed. Some new features appear, either correspondVI. SPUTI’ERING/DESORPTION

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J. Benrt et al. / Chemical irradiation effects in ices

ing to species synthesized during the annealing or rendered detectable after the removal of the volatile species. In the high frequency region, the spectrum of the residue is dominated by features between 3200 and 2800 cm-’ (fig. lc). Two small peaks, at 2975 and 2930 cm-‘, are slightly visible superimposed on a broad band. Optical observation of the residue indicates a yellow-brown stuff. We note that these peaks have positions similar to those observed in astrophysical spectra, either of the Halley inner coma [7] or IR sources [ 81. We have observed that under subsequent irradiation, the residue is progressively transformed. Its IR spectrum exhibits fewer features. After irradiation with an additional fluence of 1017 H cm-*, we observe a dark residue with very tiny spectral structures. \ \”

300K

i 4. Irradiation of (H,O + CO,)

.

. 4000

3000

3500

2500

cm-1

Fig. 1.4000-2500 cm-’ spectra of a CH, ice film; (a) at 35 K, before irradiation; (b) at 35 K, after irradiation with 10” H cmm2 at 30 keV; (c) after warming up at room temperature. The ordinate expansion in the spectrum (a) is 10 times smaller than in the spectra (b) and (c). The positions of the bands are given in table 1.

2000

1500

1

cm-1 Fig. 2. 2000-1000 cm-’ spectra of a CH, ice film. The upper and lower curves correspond respectively to the curves (b) and (c) of figure 1. The positions of the bands are given in table 1.

We have irradiated two mixtures of (HZ0 + CO,) ice, at 80 K, with abundance ratios H,O/CO, = 2 and 0.2, respectively. The primary beam consisted of 1.5 X 1Or6 cm-* He+ ions at 30 keV energy. Table 2 summarizes the dominant species detected by their absorption features. The identification of the species is based on the comparison with known spectra obtained either in gaseous form or blocked in rare gases matrixes. The second column gives a rough estimate of the validity of our identification. Case (a) corresponds to the less ambiguous one: the various major bands of each compound appear, with their relative intensities, at their expected spectral position ( + 10 cm-‘). Case (b) corresponds to species for which each major band is present, but with relative intensity differing from the gaseous ones. Case (c) corresponds to molecules for which some expected bands have not been detected. Fig. 3 illustrates the modification of the ice induced by the irradiation. Newly synthesized species appear by their IR bands. For example, the CO stretching mode, at 2139 cm-‘, is clearly present. It appears that most of the synthesized molecules are oxides and not hydrides. A rough estimate of the overall synthesis yields have been obtained, using an oscillator strength of lo-l7 cm molecule-’ for the dominant mode of each species. The irradiation of the Ha0 rich mixture leads to the synthesis of - 30 molecules per incident ion, whereas in the case of the CO, rich one, the yield is - 60 molecules per ion.

5. Irradiation of (Hz0 + NH,) The ice consists of a mixture of (H,O + NH,) at 80 K, with a ratio H,O/NH, = 5. The irradiation was performed with 30 keV He+ ions, at fluences of 3 X lOi cm-*. The spectrum after irradiation does not exhibit

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J. Bemt et al. / Chemtcal irradiation effects in ices

Table 1 Position and width of the bands detected in the irradiated CH, ice film at 35 K (right) and 300 K (left). The central column gives tentative identifications with some functional groups. The letters beside the position values refer to the intensity: (vs) very strong; (s) strong; (m) medium; (w) weak; (VW) very weak; (sh) shoulder. 300K

Functional

35 K

Wavenumber (cm-‘)

Bandwidth (cm-‘)

group

Wavenumber (cm-‘)

4147 4044

250

s

OH

50

3370 w

3287 vs 3011

H2O

3020 2930

’

550

2285 vw

40

CH, unsaturated [CH] saturated [CH]

2972-2961-2930 2907-2876-2853

1295 1262 1130 1063 1030 962 872 620

55

w sh s m w

150 70 25

C=C bend CH, or (CHs),-C bend CH, or (CH,)-C CH,

70 50 40 50 100 130

(CH,),-C

’

240

2260 vw 2135 s 2111 s 1650 vs 1590 vw 1440s 1374 m 1300 m

co 2104 1720 1590 1441 1374

Bandwidth (cm-‘)

10 20 70 300 20 70 40 30

mw m vw wv m s s

co2/NH3=5

h i

77

K

Ri

5

I 2000

1500

1000

cm-t Fig. 3. 2200-1000 cm-’ spectra of (H,O+CO,) ice mixture (H,O/CO, = 0.2) at 77 K: (a) before irradiation; (b) after irradiation with 1.5 x 1Or6 He+ cm-2 at 30 keV; (c) subtraction of the spectra (b) and (a). The positions of the bands are given in table 2.

lBO0

1600

1400

1200

cm-t Fig. 4. 1850-1200 cm-’ spectra of a (CO, +NH,) ice mixture (CO,/NHs = 5) at 77 K: (a) before irradiation; (b) after irradiation with 1.5 X lOI He+ cme2 at 30 keV. The positions of the bands are given in table 4.

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Table 2 Mixture (H,O+CO,), at 77 K, irradiated with He+ for 2 abundance ratios: HrO/CO, = 2 and 0.2 labelled A and B respectively. The proposed identifications are divided into 3 categories (a, b, c) which indicate the degree of confidence (see text). The given intensities correspond to the integral of the absorption bands; (9) means that the band is identified after a subtraction with a spectrum corresponding to an unirradiated ice; (sh) shoulder; (-) not detected. Identification

Cate8OIy C

(9 (3 CH,CO CH, H,CO H&O co 03 C3 (?) P)

HCO H,CO (7) H&O CH,CO

C

a

CH4

H&O H&O CH,CO CO3 03

Wavenumber (cm-‘)

Bandwidth (cm-‘)

3470 * 3345 * sh 3212 * 3120 * sh 3070 * 3ooO * sh 2840 2780 sh 2138 2090 2040 1946 * 1919 * 1878 1714 1617 1494 1391-1378 1300 1230 1173 1107 1071 1010-1030

400

B

14.2 8.6

400

co3

b, b

CH,CO CH,CO

b b

976 812-820 501 480 sh

2.1

Cat.

(2 N3

C

45-100

0.6

< 0.01

C ;jH2

0.14 < 0.01 0.01 _ 1.81 0.1 0.04 0.45 < 0.01 _ 0.02 0.01 0.09

1.85 0.02 0.27 0.04 0.01 0.18 3.16 0.03 1.24 0.17 3.65 0.04 0.01 0.01 0.37

20 40 55

< 0.01

0.04 0.17

0.16

0) m (?) (9

Bandwidth (cm-‘)

Intensity

2165

50 110 ? 10 10 15 150 loo?

0.04 0.12 ? 0.01 i 0.01 0.01 0.69 0.04 ?

1480 860 824 754 730 510 497 sh

b a, b c,c

1635 sh 1700 1548 1490 1399 1345 1300 1264 902 * 827 * 515 *

CO,CH,CO

NO2

E

(9 WNH,),

C

NH:

C

(9

I:; (9

50 15 15 20 15 30 140 20 110 40 100 30 25 15-25 15 35

Wavenumber (cm-‘) 2110-2000

N2O

(6

&Hz

Table 3 Mixture (H,O+NH,), at 77 K, irradiated with 3X 1016 He+ cmm2 at 30 keV (H,O/NH, = 5). See also table 2. Identification

Cat.

KHz,

180

CH ,CO, m

Identifications

HCO, CONH, CO(NH,),

Intensity A

Table 4 Mixture (CO, + NH3), at 77 K, irradiated with 1.5 x lOi He+ cm-* at 30 keV (C02/NH3 = 5). See also table 2.

N,O

c, b C

Wavenumber (cm-‘)

Bandwidth (cm-‘)

Intensity

2227

20

2164 214Osh 1853

70

< 0.01 0.38

30

0.03

90 75 60 80 30 50 30 65 20 35

0.60 0.52 0.42 0.63 0.03 0.09 0.05 0.19 0.03 0.10

intense IR features. Some very small bands have been identified, and reported in table 3. The irradiation does not synthesize detectable amounts of NO, NO,, N,O, N,H,, N,H, and NH : ; only NH, and N, seem present in low concentration. Our interpretation is that N,, which is not detectable in the IR, is the major N-containing synthesized species.

6. Irradiation of (CO, + NH,) The CO,/NH, abundance ratio of the mixture we condensed was 5, and its temperature 80 K. We used 1.5 X lOi He+ cmP2 at 30 keV, as primary beam. The irradiation led to a ‘rich variety of new compounds (table 4). However, their identification is difficult, because large regions of the spectrum contain the intense signatures of CO,, NH, and H,O. The latter constituent, present in small amounts before the irradiation due to the condensation of some residual vapour, is efficiently synthesized by the irradiation. Hz0 and CO are unambiguously identified. NO, and N,O seem also present. The problem for their identification comes from the presence of a very broad feature, from 1300 to 1800 cm-‘, superimposed on their major peaks (fig. 4). This broad band might correspond to a more complex molecule, similar to that observed in the case of irradiated CH,.

7. Comparison with previous work The only experiments dealing with the in situ detection of species synthesized by ion irradiation in icy

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J. Benit et al. / Chemical wradlation effects m ices

mixtures have been reported by Moore et al. [6]. Their main IR results may be summarized as follows: the irradiation of (H,O + N, + CO) leads to the synthesis of CO, and of some CH,; (H,O + N, + CO,) leads to CO, some NO and CH,; (Hz0 + NH, + CH,) leads to CO,, C,H,, CO and N,. No compounds containing C and N have been detected. Their experiments differ from ours by the energy of the incident ions (1 MeV protons). However, in both cases, the electronic stopping power dominates, and has similar values. Concerning the C-rich molecules, a main difference concerns the synthesis of H&O, which constitutes a major product of the irradiation of (H,O + CO,) in our experiment. In the case of N-containing mixtures, the comparison of the data presented by Moore et al. with ours seems to indicate that the synthesis of compounds with both C and N is favoured if one starts with a mixture containing CO, instead of CH,. Finally, it appears that the ion irradiation in both the keV and MeV regions is responsible for the synthesis of a rich variety of species, from small molecules to much more complex structures. The relative abundances of synthesized products are very sensitive to the composition of the initial ice as well as to the fluence of the irradiating beam. In particular, we have observed a

progressive depletion of H in the products as a function of the irradiation fluence, leading to stable refractory residue highly enriched in C, exhibiting very weak IR features.

References [l] J. Benit, J-P. Bibring, S. Della Negra, Y. Le Beyec and F. Rocard, Radiat. Eff. 99 (1986) 105. [2] F. Rocard, These de Doctorat d’Etat, Universite Paris XI (1986). [3] J. Benit, J-P. Bibring, S. Della Negra, Y. Le Beyec, M. Mendenhall, F. Rocard and K. Standing, Nucl. Instr. and Meth. B19/20 (1987) 838. [4] J. Benit, These de doctorat, Universite Paris XI (1987). [5] J-P. Bibring and F. Rocard, Adv. Space Res. 4 (12) (1984) 103. [6] M.H. Moore, B. DOM, R. Khanna and M.F. A’Heam, Icarus 54 (1983) 388. [7] M. Combes, V.I. Moroz, J-F. Criffo, J-M. Lamarre, J. Charra, N.F. Sanka, A. Soufflot, J-P. Bibring, S. Cazes, N. Coron, J. Crovisier, C. Emerich, T. Encrenaz, R. Gispert, A.V. Grigoryev, G. Guyot, V.A. Krasnopolsky, Y.V. Nikolsky and F. Rocard, Nature 321 (1986) 266. [8] S.P. Willner, R.C. Puetter, R.W. Russell and B.T. Soifer, Astrophys. Space Sci. 65 (1979) 95.

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