Laboratory measurements confirm that a 'buckyball' ion is responsible for two near-infrared absorption features found in spectra of the interstellar medium, casting light on a century-old astrochemical mystery. See Letter p.322

More than 400 unidentified absorption features have been recorded in spectra of the interstellar medium of the Milky Way and other galaxies. The nature of these 'diffuse interstellar bands', which are present in spectra of the interstellar medium from the near-ultraviolet to the near-infrared domains, has evaded elucidation since their discovery nearly 100 years ago — one of the longest-standing mysteries of modern astronomy. Writing on page 322 of this issue, Campbell et al.1 report laboratory spectra of the buckminsterfullerene, or 'buckyball', ion C 60 +. The spectra provide an excellent match to two interstellar bands that we discovered and attributed to this ion2, and securely identify C 60 + as a component of the interstellar medium.

During the past 40 years, advances in astronomical instrumentation have enabled the detection of many weak diffuse interstellar bands (DIBs) (Fig. 1), and their various widths have been accurately measured3,4,5. Numerous efforts have been made to identify the likely molecular group (the 'carrier') that causes the absorption bands through laboratory studies, theoretical modelling and astronomical observations4. As a result, a host of candidate carriers has been proposed, ranging from dust grains to intriguing carbonaceous species, including fullerenes (a carbon allotrope)3,6. High-resolution observations of fine spectroscopic details of certain DIBs7,8 have contributed to the consensus that polyatomic carbonaceous molecules in the gas phase are promising candidate carriers. The discovery and synthesis of fullerenes (including buckminsterfullerene) and of graphene in laboratories, have revolutionized our understanding of carbon chemistry and its applications ranging from astronomy and biotechnology to industrial processes. Figure 1: Buckminsterfullerene in space. a, Jan Cami; adapted from ref. 4/b, Pascale Ehrenfreund & Bernard Foing; ref. 10/ESO/James Hedberg/(CC BY-NC-SA 3.0) a, High-resolution absorption spectra of the interstellar medium, such as that shown here, include more than 400 unidentified diffuse interstellar bands (DIBs, white lines) spread across the optical and near-infrared domains3,5,10. Fullerenes and other carbon-bearing molecules have been proposed as the absorbing species causing some of the interstellar DIBs, but conclusive identification had been elusive. b, Campbell et al.1 now prove that the charged buckminsterfullerene ion C 60 + is indeed responsible for the two strongest interstellar DIBs10 in the near-infrared at 9,577 and 9,632 Å (indicated by stars) discovered2 in 1994. Full size image

In 1994, a search for the strongest signals of C 60 + in interstellar-material spectra led to the discovery2 of two absorption features (Fig. 1) at 9,577 and 9,632 ångströms. These coincided with independent measurements of electronic transitions of C 60 + in laboratory spectra using frozen-matrix techniques — in which the target species is diluted with an unreactive host species (such as neon) at very low temperatures, and is then spectrally analysed9. This preliminary match of two interstellar DIBs with C 60 + showed wavelength shifts of 3–10 Å relative to the laboratory measurements; these were attributed to known effects caused by the neon matrix. Nonetheless, the interstellar- medium study2 estimated that about 0.6% of interstellar carbon may be in the form of C 60 +, and proposed that fullerenes may have a significant role in interstellar chemistry. Arguments to support the C 60 + match to the two strongest near-infrared DIBs also include the species' extreme resistance to ultraviolet radiation, that the strengths and widths of the two DIBs were mutually correlated, and that their band widths were consistent with the spectrum of a rotating fullerene-like molecule2,10,11. To provide unambiguous confirmation, laboratory gas-phase spectra of C 60 + were called for.

In the current study, Campbell et al. report the laboratory synthesis of a complex of C 60 + and helium at 5.8 kelvin, providing the best approximation so far of the gas-phase spectrum of C 60 +. The laboratory measurements of C 60 + band maxima at 9,632.7 and 9,577.5 Å show a stunning match to the observed wavelengths of the corresponding interstellar DIBs, with an accuracy of ±0.1 Å. Moreover, the respective band widths of 2.2 and 2.5 Å are consistent with the 2.8-Å widths measured for these DIBs10,11. Twenty-one years after the initial assignment, the authors' superb laboratory results, which were achieved with a technically advanced set-up, have conclusively identified C 60 + as the carrier of these two DIBs.

With the two strongest C 60 + bands securely matched to DIBs, prospects are improving for the identification of the carriers of the more than 400 remaining DIB features, which have been recorded in a variety of astrophysical settings. In the past decade, observations across the entire optical range have established that diffuse band carriers are ubiquitous in space. DIBs have also recently been detected and mapped in the Magellanic Clouds (the Milky Way's satellite galaxies) and in other galaxies. The carrier molecules may therefore constitute an important reservoir of organic material throughout the Universe.

The clear identification of additional DIB carriers could have a far-reaching impact on our understanding of astrochemical reaction networks. C 60 + was possibly the easiest and most conspicuous molecule to identify owing to its signature of two strong bands in the near-infrared range that is clearly evident in interstellar-medium spectra, which have been corrected for atmospheric absorptions2,10,11. The species' cage-like symmetry and geometry6 (Fig. 1), as well as its chemical properties, increase its chances of survival in space and its expected natural formation in astrophysical environments4,10. One possible approach to identify more DIB carriers would be to detect and quantify the presence in space of other fullerene compounds, which could be formed by the addition of hydrogen atoms, metals or other radicals in different charge or reactive states.

In previous analyses of mid-infrared emission spectra of a young planetary nebula (the ionized, ejected envelope of an old star) taken with NASA's Spitzer Space Telescope, researchers for the first time identified12 specific molecular-vibration bands that were assigned to compounds of the fullerenes C 60 and C 70 . Vibrational spectroscopy cannot perfectly distinguish between molecules of similar chemical bonds or between molecular excitation states, but it can provide a global inventory of molecular groups from the pool of organic materials in space. Electronic spectroscopy, such as that used by Campbell and colleagues, can pinpoint the fingerprints of individual molecules. Additional observations of circumstellar nebulae have shown that, under the right conditions, fullerenes can and do form efficiently in space4.

Other carbon-bearing molecules known as polycyclic aromatic hydrocarbons (PAHs) — fused benzene rings — have also been proposed13,14 as carriers of vibrational bands that are observed in the mid-infrared domain. These are estimated to be the most abundant complex organic molecules in space, comprising more than about 15% of cosmic carbon, and have also been proposed as potential DIB carriers. However, in contrast to C 60 +, PAHs can undergo partial loss of hydrogen atoms (dehydrogenation), and would therefore exist in a large number of molecular isomers, diluting their spectral signatures15. This might explain why, despite several dedicated efforts16, no specific PAH electronic feature has been clearly associated with DIBs. Some metal–PAH complexes may eventually be shown to have special molecular configurations that have few isomers and strong oscillator strengths — these would have undiluted spectral features, which would permit their electronic spectroscopic identification and matching to astrophysical DIBs4.

The relationships between PAHs and fullerenes in space represent an intriguing puzzle. Laboratory studies have shown17 that C 60 + can form following sequential light-induced fragmentation of large PAH cations. Infrared observations obtained by the Spitzer and the European Space Agency's Herschel Space Observatory have also shown18 that C 60 can form in the tenuous and cold environment of an interstellar cloud illuminated by strong ultraviolet starlight — possibly as the product of the conversion of PAHs to graphene, and then to fullerenes.

As Campbell and colleagues' findings show, the identification of space molecules can be achieved only by measuring laboratory spectra under astrophysically relevant conditions, and the spectra must unambiguously match the astronomical data. This task requires theoretical modelling to determine other plausible fullerene- and PAH-candidate DIB carriers and a systematic programme of laboratory spectroscopy of these in frozen neon matrices (for screening) and in the gas phase (for identification). Astronomical surveys of DIB strengths and profiles in different interstellar and circumstellar environments are needed to elucidate the formation and fate of carriers. All these research disciplines must be combined to provide further insights into the photochemistry, thermal conditions and integration in protoplanetary-disk material that affect the inventory and evolution of organic molecules from molecular clouds to star- and planet-forming regions.Footnote 1

Change history 24 July 2015 An earlier version of this article omitted the full credit information and relevant reference citations for Figure 1. This has now been corrected.

Notes

References 1 Campbell, E. K., Holz, M., Gerlich, D. & Maier, J. P. Nature 523, 322–328 (2015). 2 Foing, B. H. & Ehrenfreund, P. Nature 369, 296 (1994). 3 Herbig, G. H. Annu. Rev. Astron. Astrophys. 33, 19–73 (1985). 4 Cox, N. L. J. & Cami, J. (eds) Proc. Int. Astron. Un. Vol. 9 (2014). 5 Hobbs, L. M. et al. Astrophys. J. 705, 32–45 (2009). 6 Kroto, H. W. et al. Nature 318, 162–163 (1985). 7 Ehrenfreund, P. & Foing, B. H. Astron. Astrophys. 307, L25–L28 (1996). 8 Sarre, P. et al. Mon. Not. R. Astron. Soc. 277, L41–L43 (1995). 9 Fulara, J., Jakobi, M. & Maier, J. P. Chem. Phys. Lett., 211, 227–234 (1993). 10 Foing, B. H. & Ehrenfreund, P. Astron. Astrophys. 317, L59–L62 (1997). 11 Galazutdinov, G. A. et al. Mon. Not. R. Astron. Soc. 317, 750–758 (2000). 12 Cami, J., Bernard-Salas, J., Peeters, E. & Malek, S. E. Science 329, 1180–1182 (2010). 13 Leger, A. & Puget, J. L. Astron. Astrophys. 137, L5–L8 (1984). 14 Allamandola, L. J., Tielens, A. G. G. M. & Barker, J. R. Astrophys. J. 290, L25–L28 (1985). 15 Vuong, M. H. & Foing, B. H. Astron. Astrophys. 363, L5–L8 (2000). 16 Salama, F. et al. Astrophys. J. 728, 154–162 (2011). 17 Zhen, J., Castellanos, P., Paardekooper, D. M., Linnartz, H. & Tielens, A. G. G. M. Astrophys. J. 797, L30–L34 (2014). 18 Berné, O. & Tielens, A. G. G. M. Proc. Natl Acad. Sci. USA 109, 401–406 (2012). Download references

Author information Affiliations Pascale Ehrenfreund is at the Space Policy Institute, George Washington University, Washington DC 20052, USA. Pascale Ehrenfreund Bernard Foing is at the European Space Research and Technology Centre of the European Space Agency, ESA-ESTEC, 2200 Noordwijk, the Netherlands. Bernard Foing Authors Pascale Ehrenfreund View author publications You can also search for this author in PubMed Google Scholar Bernard Foing View author publications You can also search for this author in PubMed Google Scholar Corresponding authors Correspondence to Pascale Ehrenfreund or Bernard Foing.

Related audio Your browser does not support the audio element. Lizzie Gibney asks John Maier about bucky-balls in space.

Rights and permissions Reprints and Permissions