Infrared spectroscopy and integrated molar absorptivity of C60 and C70 fullerenes at extreme temperatures

Mon. Not. R. Astron. Soc. 413, 213–222 (2011) doi:10.1111/j.1365-2966.2011.18124.x Infrared spectroscopy and integrated molar absorptivity of C60 and C70 fullerenes at extreme temperatures Susana Iglesias-Groth,1 Franco Cataldo2,3 and Arturo Manchado1,4,5 1 Instituto de Astrofı́sica de Canarias, Via Lactea s/n, E-38200, La Laguna, Tenerife, Spain 2 Istituto Nazionale di Astrofisica – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy 3 Actinium Chemical Research, Via Casilina 1626/A, 00133 Rome, Italy 4 Consejo Superior de Investigaciones Cientı́ficas, Spain 5 Departamento de Astrofı́sica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain Accepted 2010 November 29. Received 2010 November 28; in original form 2010 October 26 ABSTRACT Key words: astrochemistry – methods: laboratory. 1 I N T RO D U C T I O N Infrared measurements from the Spitzer space telescope have led to the detection of the C60 fullerene and have hinted at the possible presence of the larger homologous C70 in a young planetary nebula Tc 1 (Cami et al. 2010). The detection of fullerenes in such an object comes as a surprise as it is known that the formation of fullerenes requires environments completely free from hydrogen (Ehrenfreund & Foing 2010). Thus, the fullerenes were thought to be formed in the inner core region of Tc 1, which is supposed to be carbon-rich, hydrogen-poor and dusty (Cami et al. 2010). However, Garcia-Hernandez et al. (2010) have also found, with the Spitzer space telescope, the presence of C60 in the circumstellar medium of a series of different planetary nebulae, including SMP SMC 16, an extragalactic source. Astonishingly, C60 and C70 were found in a hydrogen-rich environment mixed with polycyclic aromatic hydrocarbons (PAHs). As a likely explanation for the simultaneous presence of fullerenes and PAHs, Garcia-Hernandez et al. (2010) have proposed that fullerenes may be formed by the photochemical processing of hydrogenated amorphous carbon (HAC). Experimental studies on the hydrogenation of fullerenes have shown that the infrared spectrum of hydrogenated fullerenes (called fulleranes) is  E-mail:  C 2011 The Authors C 2011 RAS Monthly Notices of the Royal Astronomical Society  similar to the infrared spectra recorded in certain protoplanetary nebulae (Cataldo 2003). Fullerenes must necessarily be formed in a hydrogen-depleted environment, but once formed and exposed to hydrogen undergo easily the hydrogenation to fulleranes (Cataldo & Iglesias-Groth 2010). The ultraviolet radiation of fulleranes causes the release of hydrogen and the formation of aromatic derivatives from fullerenes, while the thermal processing of fulleranes produces back the fullerenes (Cataldo & Iglesias-Groth 2009, 2010). The fullerene C60 has also been detected in the interstellar medium and, more precisely, in the reflection nebulae NGC 7023 and 2023 (Sellgren, Werner & Ingalls 2009; Sellgren et al. 2010). Once formed, the fullerenes are stable toward high-energy radiation and cosmic rays (Cataldo, Strazzulla & Iglesias-Groth 2009). The C60 and C70 fullerenes are the largest molecules detected in space, exceeding in the number of atoms the cyanopolyyne H–C11 N, confirmed more than a decade ago (Bell et al. 1997), and the anthracene C14 H10 , very recently tentatively detected (Iglesias-Groth et al. 2010). The detection of C60 fullerenes in space represents the culmination of a long-standing prediction about their existence formulated by Kroto (1988, 1992) and Hare & Kroto (1992) about 20 yr ago. The search for fullerenes was concentrated in the circumstellar environment of late-type carbon-rich stars where probably C60 is embedded or mixed with other forms of elemental carbon. Particularly remarkable places to search for fullerenes were the R Coronae The detection of C60 and C70 fullerenes in young planetary nebulae and in reflection nebulae suggests that these molecules are more common in certain astrophysical environments than previously thought. The dependence on temperature of the positions and widths of the infrared bands of the C60 and C70 fullerenes is needed for a firm qualitative detection of these molecules in space. Furthermore, the integrated molar absorptivity  (in km mol−1 ) of each infrared absorption band is required for a quantitative determination of the abundance of C60 and C70 in space. In this paper, we report on the temperature dependence of the wavelength shift and integrated molar absorptivity of the infrared bands of the C60 and C70 fullerenes. The measurements have been made in a KBr matrix in the temperature range between −180◦ C and +250◦ C. The experimental data have been extrapolated to derive both the infrared band shift and the integrated molar absorptivity of the C60 and C70 fullerenes at absolute zero temperature. 214 S. Iglesias-Groth, F. Cataldo and A. Manchado 2 E X P E R I M E N TA L D E TA I L S 2.1 Materials and equipment The fullerenes C60 and C70 were high-purity grades (99.95 per cent and 99.0+ per cent, respectively) from MTR Ltd, (USA). Potassium bromide (infrared spectroscopy grade) was obtained from SigmaAldrich. The infrared spectra were recorded on a Thermo-Fischer Fourier transform infrared (FTIR) spectrometer model Nicolet IR-300 at a resolution of 1 cm−1 . The KBr pellet used in the infrared spectra measurement were produced in a Silfradent press equipped with a pressure meter; the KBr pellets were produced at a pressure comprised between 3.0 and 3.5 tonnes cm−2 . The thickness of the KBr pellets was measured with a Somet digital micrometer having a sensitivity of 0.01 mm. The low-temperature apparatus consisted of a variable temperature cell from Specac model P/N 21525 equipped with KBr windows and a sample holder, which is able to work in the range between +250◦ C to −196◦ C. The variable temperature cell was evacuated with a Buchi vacuum pump model V-710 equipped with four diaphragm heads and a three-stage vacuum creation process, which delivers 3.1 m3 h−1 and an absolute vacuum of 2 mbar. 2.2 Measurement of the infrared spectrum and the molar absorptivity and integrated molar absorptivity of C60 from low to high temperatures C60 (2.2 mg) was quickly mixed with 245.4 mg of KBr in an agate mortar and the two components were finely ground together. The powder was transferred into a macro–micro KBr pellet die and compressed at 3.3 tonnes cm−2 with the Silfradent press. The resulting pellet, with a measured thickness of 0.71 mm, was mounted into the sample holder of the Specac variable temperature cell and inserted into the cell. The cell was then evacuated with the aid of the Buchi pump to a vacuum of 0.1 torr and then heated gradually at +60◦ C in order to permit the humidity, which is eventually absorbed on the internal surfaces of the cell and in the KBr pellet, to evaporate. In order to go below room temperature, use was made of liquid nitrogen, added cautiously and in small amounts in the ca (...truncated)