The Irradiation of CO₂ Ice at 30 K with Low Energy Ions

Introduction

One of the intriguing scientific questions is: how molecules in the circumstellar disk and interstellar medium (ISM) are formed? Currently we know that there are over 195 (http://www.astrochymist.org/astrochymist_ism.html) molecules in these large scale structures in space, some with long chains (eg HC₁₁N) and others containing a number of different atoms (eg isocyanic acid, HNCO). One answer to this question is that the icy mantles of interstellar dust particles (IDPs, it should be noted that the 80% of the IDPs are composed of silicates, followed by carbonaceous (20%) and remaining of minerals (eg CaAl₁₂O₁₉ in 0.002 ppm) in negligible amounts) are constantly irradiated with both electromagnetic and particle radiation, forming even more complex molecules such as amino acids – for example the amino acid glycine has been recently identified in the comet Wild 2¹. Reactions taking place within the icy mantles are said to occur in solid phase chemistry, which represents 25% of the chemistry occurring in the ISM, the remaining being assigned to gas phase chemistry.

Universe-wide, the ions H⁺, D⁺ and He⁺ (including their atomic counterparts and e^–) are dominant in terms of particle radiation (99%), with energies in the region of 1-10 MeV and are termed as low energy cosmic rays^2,3. Such ions can penetrate deep into the interior of huge dark molecular clouds are dark because primarily they are made of molecular hydrogen and IDPs with a ratio of 10¹² hydrogen atoms to 1 IDP⁶, as in Figure 1 (Horsehead nebula) and, in the process, their energy drops to below 5 keV as they hurtle deeper into the interior of the ISM; these low energy particles impinge onto the icy mantles of IDPs⁴.

 

Figure 1: shows an example of a dark molecular cloud, the Horsehead nebula. (Credit: http://www.nasa.gov/multimedia/imagegallery/image_feature_89.html)   Click here to View figure

 

The reason IDPs have icy mantles is because the interior of the clouds have temperatures in the region of 10 to 50 K^5,6 and, at such temperatures, volatiles (eg H₂O, CO, CO₂, NH₃, CH₄ etc) freeze out on the surface of the IDPs forming volatiles-rich icy mantles⁴. Low energy particles are involved in the processing of ices of comets; asteroids; objects belonging to the Kuiper Belt and Oort cloud; atmospheres and surfaces of planetary bodies – planets themselves as well as their satellites. For example, the aerosol content of Titan’s atmosphere is constantly being irradiated by ions from the magnetosphere of Saturn^7,8 and in the process Titan’s atmosphere (and its surface) contains a rich source of organic compounds⁹.

Solid carbon dioxide (CO₂) was first detected in the ISM in 1989¹⁰. Since then it has been affirmed that CO₂ is the second most common molecule within the ISM as well as in comets¹¹ (Table 1).

Species	Protostars	Comets
Water	100%	100%
CO	~15	7-20
CO2	~23	3-6
NH3	~8	1.5
O2	<7	<0.5
CH3OH	~6	~2
HCOOH	~3	~0.05
H2CO	<3	~3
CH4	~2	~1
C2H6	<0.4	~0.5
OCS	<0.04	0.5
Taken from TL Roush (2001)

Table 1: show the distribution of some common molecules in protostars and comets

It is formed on the mantle of IDPs during the photolysis and radiolysis of pure CO, CO:O₂, and CO:H₂O ices. Possible formation reactions include: pure CO reaction: via the dissociation of CO⁴.

CO:O₂ reaction: via the dissociation of O₂.

CO:H₂O reaction: via the dissociation of H₂O² (the most prevalent reactions occur in this mixture).

Such reactions generally occur in structures like the Horsehead nebula.

From a solid phase chemistry point of view, many other compounds can also be made from CO₂ via CO (Eq. 7) within the ISM. For example, the following sequence of reactions during the formation of formaldehyde (H₂CO) – biologically a very important compound in the synthesis of sugars – is possible¹².

This is one reason why CO₂ was chosen to study its astrochemistry. The aim of this experiment is to investigate the effect of  the low energy ions, H⁺, D⁺ and He⁺ on CO₂ ice at 30 K, whilst keeping their charge at +1.

Method

These experiments were carried out at Queens University, Belfast (QUB) which is the site of the UK’s low energy ion beam facilities. The Electron Cyclotron Resonance Ion Source (ECRIS) at QUB is used to produce multiple charged ions using a 9.0-10.5 GHz ECRIS¹³.  Beams of H⁺, D⁺ or He⁺ ions are extracted from the 9.0-10.5 GHz ion source. A low energy ‘floating’ beamline accelerator is then used to extract the ions by holding the beamline at 4 keV potential. The generated ions are controlled by powerful magnets with the net effect that ions are separated according to mass:charge ratio (m/Q). The ions are then focussed into the vacuum chamber by a series of deflection/focussing plates. Before irradiating the ice, the ion current was measured using a Faraday cap placed in the path of the ion beam – that is, between the origin of the beam and the vacuum chamber. During irradiation, any changes in the beam current were continuously monitored using a 90% transmitting mesh placed in the beam path in place of the Faraday cap (Figure 2).

 

Figure 2: shows a schematic diagram of the experimental setup Click here to View figure

 

A general description of the vacuum chamber is given in Dawes¹⁴ (2007). Ices of CO₂ (99.995%, Argo International) were prepared at 30 K before irradiating them with appropriate radiation ions of 1.5 keV H⁺ (300 nAmp), 2.12 keV D⁺ (4 mAmp) or 3 keV He⁺ (400 nAmp). Analysis of the irradiated CO₂ ice was carried out using in situ infrared spectroscopy derived from a N₂ purged Thermo-Nicolet Nexus 670 Fourier Transform Infrared (FTIR) spectrometer. Spectra were acquired in the range from 4 000 to 650 cm^-1 at 4 cm^-1 resolution. The sample was positioned with the infrared and ion beams perpendicular to one another and both at 45° to the substrate surface (Figure 2).

The thickness of the ices was calculated using the following equations:

Where ∫Abs(v)dv= measure of band strength in cm^-1 and A = integral absorption value cm molecules; and

Additional information required to calculate the thickness (d) of CO₂ ice is as follows: density (ρ) of CO₂ ice: 1.7 gcm^-3(15); and CO₂ integrated absorption coefficient (A value) at 3708 cm^-1: 2.5 x 10^-18 cm molecule^-1(16). The thickness of the ice obtained for 1.5 keV, 2.12 keV and 3.0 keV irradiation experiments were: 1.7mm, 2.1mm, and 2.1mm respectively.

Results

Table 2 shows the new species observed when CO₂ ice was irradiated with different types of radiation (H⁺, D⁺ or He⁺) with different energy levels. The obtained results are compared with those generated by irradiation using UV light¹⁷ (photolysis) and by radiolysis¹⁸.

Molecules and radicals observed	This work: Radiolysis of CO2 ice			Gerakines (1996) Photolysis of CO2 ice. UV Light	Brucato (1997) Radiolysis of CO2 ice. 1.5 keV H+
	Radiolysis: 1.5 keV H+	Radiolysis: 2.12 keV D+	Radiolysis:3 keV He+
	v = cm-1	v = cm-1	v = cm-1	v = cm-1	v = cm-1
O3				706
CO3	973			976
O3	1043	1043	1043	1043	1040 (H2CO3;O3)
Unidentified				1053
CO3	1078	1078	1078	1067
H2CO3					1302
H2CO3					1490
H2CO3					1736
CO3	1880	1880	1880	1883	1879 (HCO?)
CO3	2044	2044	2044	2044	2044
13CO	2093 (weak)	2093	2093	2093	2094 (weak)
CO	2141	2141	2141	2141	2143

Table 2 shows identification of bands produced during irradiation with different ions

Figure 3 depicts a section of recorded spectra when the CO₂ ice at 30 K was irradiated with H⁺ ions, to illustrate the formation of new species of molecules.

 

Figure 3: shows the displaced spectra, illustrating the formation of new species of molecules during the irradiation of CO2 ice at 30 K and using 1.5 keV H+ ions Click here to View figure

 

The three major species of interest observed during the irradiation of CO₂ ice at 30 K with H⁺, D⁺ or He⁺ ions were: carbon monoxide (CO) at v = 2141 cm^-1, ozone (O₃) at v = 1043 cm^-1 and carbonate (CO₃) at v = 1078 cm^-1. In all three cases the greatest and least amount of products generated were CO and CO₃ respectively, with O₃ being in the middle range (Figures 4a, b and c).

 

Figures 4a, b and c: shows the growth of CO, O3 and CO3 in terms of column density (N) during the irradiation of CO2 at 30 K with low energy ions: H+ (1.5 keV), D+ (2.12 keV) and He+ (3 keV) Click here to View figure

 

CO synthesis, a primary product, is essentially as in Eq. 7. The secondary products, O₃ and CO₃, are made from reaction with O produced during the radiolysis of CO₂ as follows:

and CO₃ as below:

The information from Figures 4a, b and c has been re-presented to show the growth of each new species of molecule with respect to the radiations deployed (Figures 5a, b and c) – for example, Figure 5a shows the growth of CO with respect to all three radiations.

 

Figures 5a, b and c: compare the effects of each of the radiations on the growth of CO, O3 and CO3 in terms of column density (N) during the irradiation of CO2 at 30 K Click here to View figure

 

By studying the column densities in Table 3 and Figures 5a, b and c, it can be noted that D⁺ is efficient at making CO, O₃ and CO₃; Figures 5a and b shows that He⁺ is good at producing reasonable amounts of CO and O₃ more than H⁺, but less than D⁺; lastly Figure 5c, depicts that H+ is only effective at making CO₃.

	Column density (N) molecules cm-2
	CO	O3	CO3*
1.5 keV H+ (Fluence: 1.87×1012)	3.0×1016	8.0×1015	1.0×1015
2.12 keV D+ (Fluence: 2.50×1013)	1.3×1017	2.0×1016	8.8×1015
3.0 keV He+ (Fluence: 2.50×1012)	5.0×1016	9.0×1015	1.6×1014
*estimated from Figure 6. The smooth curves in Figure 6 were obtained by using Origin software (version 6).

Table 3: shows final column densities levels (molecules cm^-2) during the irradiation of CO₂ ice at 30 K with H⁺, D⁺ and He⁺ radiations.

Discussion

The incoming ions collide with the CO₂ ice, causing the bonds to break. The type and quantity of CO₂ bond-breaking depends on the nature of the radiation (electromagnetic or particle); the fluence – the number of particles impinging onto the given surface area; the molecular composition of the ice, which will determine total stopping power of the ice; and the nature of the incoming particles, including whether these are neutral (), mono (He⁺, ) or multivalent (He²⁺, C³⁺ etc), what their energy levels are, and  their mass-to-charge ratio – all of which determines their depth of penetration and bond breaking efficiency. For the duration of ionic irradiation, as soon as the ion impinges the ice, it begins to lose energy and will come to a halt either on the surface (causing sputtering) or within the ice itself. If it is the latter, the following modifications may result: electronic transitions of the ice molecules; ionisation of the ice molecules, leading to a generation of high energy secondary electrons that go on to ionise and excite other molecules, in the process modify the ice further; or the collision may also dissociate the ice molecules, which will add to the inventory of products that can be made.

During the low energy irradiation of CO₂ ice at 30 K with H⁺, D⁺ or He⁺ ions, fluence was responsible for the destruction of CO₂. D⁺ ions are more efficient at breaking CO₂ bonds compared to the H⁺ or He⁺ ions as shown by the column densities of destruction products (CO, CO₃ and CO₃) in Table 3 and also as testified by Figures 5a, b and c. This is because D⁺ ions have the highest fluence (2.50 x 10¹³ D⁺ cm^-2), even though they have lower energy than He⁺.

It is highly probable that the He⁺ ions are sputtering on the surface of the CO₂ ice, perhaps this may be one reason for the results as depicted in Figure 5c, ie it has the lowest column density for CO₃. As the helium ion’s cross section (s) is four times the size of that of the hydrogen ion (m/Q ratio for He⁺ = 4:1; H⁺ = 1:1), suggesting that He⁺ may not be penetrating the ice upon impinging – ie causing sputtering on the surface of the ice, thereby releasing the trapped CO and O₃ as observed in Figure 5a and b.

Figure 5c also compares CO₃ generated during the irradiation of CO₂ with the three types of ionic radiation. It is probable that the formation of CO₃, in addition to being a less efficient reaction, is being removed from the system altogether.

 

Figures 6a and b: shows tentative signals for H2CO3 and D2CO3 – both at v = 1302 cm-1. However, it should be noted that, in general, the presence of D2 in D2CO3 should produce a slightly different vibrational mode from v = 1302 cm-1  Click here to View figure

 

Figure 6a (above) shows the possible, tentative signals for H₂CO₃, a product also reported by Brucato and co-workers¹⁸ (1997) as in Table 2, meaning that CO₃, during H⁺ irradiation, is being steadily removed via the Eq. 16, thus producing a curve as shown in Figure 5c.

During D⁺ irradiation (Figure 5c), the curve at 2.5×10¹⁴ molecules cm^-2 fluence levels off, again suggesting that D₂CO₃ (tentative signals indicated in Figure 6b) is being formed (Eq.17) then rapidly removed.

This may be speculative because, in general, H⁺ and D⁺ would have slightly different vibrational wavenumbers – ie both would not necessarily have v = 1302 cm^-1.

Both reactions in Eq. 16 and Eq. 17 are examples of implantation, a process whereby incident radiation partakes in the chemical reactions¹⁸.

Astrobiological implications

In this and other experiments^17,18, the same new molecules were formed regardless of the type of irradiation used (Table 2). Such species of molecules are also part and parcel of the ISM, present in the planetary atmospheres and surfaces, as well as in comets and asteroids. CO₂ and its products are important as far as astrochemistry is concerned, for example as stated in the introduction, formaldehyde and formic acid can both be produced from CO₂. Both of these compounds are important during the formation of other biologically important molecules such as those belonging to the groups: carbohydrates, amino acids, nucleotides and cofactors (ATP, acetyl-CoA, NADH, etc). As a result, the reality is that CO₂ (and C chemistry in general) is very important eg during photosynthesis and we have to explore all avenues that may help to answer the more difficult question: how did life on Earth actually get started? CO₂, and its destruction products and downstream compounds could have been delivered onto the Earth during the bombardment phase of its early history (~4.3-4.0×10⁹ years ago) and may thus have been instrumental in the inception of the origins of life on Earth.

Conclusion

The destruction of CO₂ led to the formation of CO and O, which in turn led to the formation of the secondary molecules O₃ and CO₃. These species were observed during the irradiation of CO₂ ice at 30 K with 1.5 keV H⁺, 2.12 keV D⁺ and 3 keV He⁺ ions. In addition, the following summary conclusion was also reached: fluence resulted in the modification of ice, which produced new species; the cross section of the ion as well as the stopping power of the ice determines the degree to which the impinging particle can penetrate the ice; and particles with smaller cross sections (eg H⁺) could partake in the ice modification as they penetrate deeper within the ice – eg formation of H₂CO₃ and D₂CO₃ by the impinging of H⁺ and D⁺ ions respectively – a process known as implantation.

In relation to the discovery of life’s astrobiological beginnings, the study of solid phase chemistry is important in recreating the formation of molecules under space conditions, thus demonstrating the possibility that these molecules would have been present during Earth’s early history and thus were available to play a part in the beginnings of the origin of life.

Acknowledgements

We would like to thank both the STFC and EU lassie project as well as Professor Robert W McCullough for making the facilities available at his laboratory for conducting the entitled above experiment. We also extend thanks to the Queen’s University, Belfast, BT7 1NN.

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