Stability of C60 and C70 fullerenes toward corpuscular and γ radiation

F. Cataldo G. Strazzulla S. Iglesias-Groth 0 Instituto de Astrofisica de Canarias , Via Lactea s/n, E-38200 La Laguna, Tenerife, Canary Islands, Spain 1 Istituto Nazionale di Astrofisica - Osservatorio Astrofisico di Catania , Via S. Sofia 78, 95123 Catania, Italy A B S T R A C T The stability of C60 and C70 fullerenes in the interstellar medium deposited on dust surface or embedded in meteorites and comets has been simulated with irradiation and with He+ ion bombardment. It is shown by vibrational spectroscopy that a radiation dose of 2.6 MGy (1 Gy = 1 joule absorbed energy per kilogram) causes partial oligomerization of both C60 and C70 fullerenes. Oligomers are made by fullerene cages chemically connected each other which can yield back free fullerenes by a thermal treatment. The amount of irreversibly polymerized fullerenes caused by 2.6 MGy as deduced as the toluene insoluble fraction has been determined as 1.7 and 15 per cent by weight, respectively, for C60 and C70 fullerene. The radiation dose generated by radionuclides decay and expected to be delivered to fullerenes buried at a depth of more than 20 m in comets and meteorites is about 3 MGy per 109 yr. Since fullerenes are by far resistant to such radiation dose they can survive for at least some billion years inside comets and meteorites and in fact have been detected inside certain carbonaceous chondrites. On the other hand, the direct exposure of fullerenes to cosmic rays for instance when they are adsorbed or deposited on the surface of carbon dust corresponds to the delivery of a radiation dose comprised between 30 and 65 MGy per 109 yr. Experimental bombardment of both C60 and C70 fullerenes for instance with He+ ions has shown that the complete amorphization occurs at about 250 MGy. Thus in 4 Gyr exposure to cosmic rays it is expected a complete amorphization. 1 I N T R O D U C T I O N The dust particles account for 13 per cent of the interstellar matter mass but they have a very strong effect in dimming the light of distant stars. This light dimming or extinction is not uniform throughout the entire electromagnetic spectrum being maximum in the ultraviolet (UV) region, gradually falling off with increasing wavelength. In fact in the UV an absorption bump located at 217 nm can be observed in the interstellar light extinction curve. Such feature is commonly attributed to carbon dust particles either graphitic (Unsold & Baschek 2002) or amorphous carbon (Cataldo 2002, 2004) although none of these model particles matches perfectly the absorption peak at 217 nm. Hydrogenated and proton irradiated carbon appears to be one of the best model which is able to match the absorption at 217 nm (Mennella et al. 1997; Mennella, Brucato & Colangeli 2001). The driving force which led to the discovery of fullerenes derived from the need to explain the origin of the absorption feature observed in the interstellar light extinction curve (Kroto et al. 1985; Bagott 1996). In an attempt to reproduce in laboratory such feature Kratschmer and colleagues succeeded to produce a carbon soot containing fullerenes (Kratschmer 1991). It is thought that fullerenes are formed in the circumstellar envelope of carbon-rich stars and ejected in the interstellar medium together with carbon dust and carbon-rich molecules such as polycyclic aromatic hydrocarbons and polyynes (Kroto 1992). More precisely the carbon vapour is thought to be the real precursors of fullerenes and, depending on the cooling conditions and the quenching gas present, may yield polyynes which may or may not cross-link to fullerenes, may produce carbon soot or polycyclic aromatic hydrocarbons (Heath 1992; Hirsch 1994). The latter case is favoured by an excess of hydrogen in the quenching gas. Laboratory studies and simulations in carbon dust formation have shown that fullerenes are always produced embedded in amorphous carbon black matrix and can be separated from the dust only by sublimation or by solvent extraction (Kratschmer, Fostiropoulos & Huffman 1990a; Kratschmer et al. 1990b). If we apply these results to the circumstellar conditions it is straightforward to think that a large part of the fullerenes eventually formed should remain embedded into the carbon soot and only a fraction should be released in free form. In fact, fullerenes have a low vapour pressure and consequently should have a tendency to condense on the surface of the dust particles when the temperature is below 1100 K. Although C60 and higher fullerenes were never firmly detected in space there are numerous experimental hints and indications about their existence (Foing & Ehrenfreund 1994; Kroto 1994; Sonnentrucker et al. 1997; Garcia-Lario et al. 1999; Galazutdinov et al. 2000; Sassara et al. 2001; Rietmeijer 2006) and also numerous theoretical and observational papers dealing with their existence in space have been appeared in recent years (Webster 1997; Fulara & Krelowski 2000; Iglesias-Groth & Breton 2000; Sassara et al. 2001; Sellgren 2001; Stoldt, Maboudian & Carraro 2001; Iglesias-Groth 2004, 2005, 2006, 2007). Furthermore, the formation of fullerenes together with polycyclic aromatic hydrocarbons in dense interstellar clouds has been predicted (Bettens & Herbst 1997) and their possible fate and derivatization was studied experimentally (Petrie & Bohme 2000). Particularly original was the proposal of fullerene formation in interstellar space from the decomposition of hydrogenated amorphous carbon proposed by Scott, Duley & Pinho (1997). Despite fullerenes can form in space and may play a role in astrochemistry either directly or through their derivatives, the socalled fullerene-like structures, for instance in molecular hydrogen formation (Cataldo 2003) or even in the synthesis of relatively complex pre-biotic molecules, their stability in the interstellar space has not yet fully clarified. Certain authors have considered exclusively the stability of C60 fullerene toward radiation (Cataldo 2000b; Sakaguchi et al. 2007) even at extremely high doses (Albarran et al. 2004; Basiuk et al. 2005) neglecting completely to consider their resistance of fullerenes to cosmic rays which are composed by corpuscular radiation. Similarly, other authors have studied the decomposition of C60 under the action of swift heavy ions (Kalish et al. 1993; Takayama 1993; Yogo, Majima & Itoh 2003; Todorovic-Markovic et al. 2007; Singhal et al. 2008) which represent a minor, practically negligible components of cosmic rays (Stanev 2004). In the present work we wish to summarize earlier experimental data of fullerene stability toward both high energy and corpuscular radiation adding new and original experimental data in the frame of realistic interstellar conditions of both high energy and corpuscular radiation. 2 E X P E R I M E N TA L 2.1 Materials and equipment C60 and C70 fullerene were 99.5+ and 98 per cent pure grades, respectively, and were obtained from Southern Chemicals Group LLC, USA. Toluene (...truncated)