Design of a Supercritical Fluid Expansion Source for Gas-Phase Spectroscopy of Fullerenes

Introduction The fullerenes, including C 60 and C 70 , were discovered by Kroto et al. [1] in 1985 during experiments meant to simulate the chemistry of carbon stars. It was initially believed that these molecules would be formed primarily in carbon-rich, hydrogen-poor regions where formation of polyaromatic hydrocarbons (PAHs) would be limited. This hypothesis seemed to be supported by the first detection of extraterrestrial C 60 and C 70 by Cami et al. [2] in the planetary nebula Tc1, the remnant nebula of a carbon star, where no PAH emission features were detected. Later work by García-Hernández et al. [3] pointed out that Tc1 was not necessarily hydrogen-poor and also presented data showing fullerene and PAH emission features together in other planetary nebulae. They suggested that fullerenes may be formed through photochemical reactions of hydrogenated amorphous carbon grains. The precise mechanism by which fullerenes are formed in space is an active area of research [4,5,6]; obtaining high-resolution gas-phase spectra of C 60 and C 70 would allow astronomers to search for fullerenes in regions of space not accessible by emission spectroscopy and aid in accurate quantitation, potentially clarifying the formation process. In addition to planetary nebulae, fullerenes have also been detected in the interstellar medium (ISM). Interstellar C 60 was first detected by Sellgren et al. [7] in reflection nebulae via emission. They calculated that C 60 could account for 0.1% to 0.6% of all interstellar carbon, but were hampered by uncertainties in its ultraviolet absorption cross-section. Absorption measurements could provide more precise estimates. In addition to the presence of neutral fullerenes in the ISM, relatively low ionization potentials mean that considerable quantities of ionized fullerenes could be present. Two features of the diffuse interstellar bands have been attributed to C 60 + by Foing and Ehrenfreund [8], although these have not been confirmed. An absorption spectrum of C 60 + could allow the claim to be confirmed or refuted, and solving the challenges of neutral fullerene spectroscopy would be the first step towards 1 obtaining the spectra of their molecular ions. The primary challenges of gas-phase fullerene spectroscopy are low volatility and the substantial partition functions associated with such large molecules (see Fig. 1). Previous efforts in our group have focused on the production of C 60 vapor via high temperature oven and vibrational cooling through supersonic expansion [9]. This approach has failed largely due to the difficulty …