Fullerene Solves an Interstellar Puzzle
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RESEARCH NEWS & VIEWS ASTROCHEMISTRY 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 C60 , Fullerene solves an and proposed that fullerenes may have a signif- icant role in interstellar chemistry. Arguments + to support the C60 match to the two strongest interstellar puzzle near-infrared DIBs also include the species’ extreme resistance to ultraviolet radiation, Laboratory measurements confirm that a ‘buckyball’ ion is responsible for two that the strengths and widths of the two DIBs near-infrared absorption features found in spectra of the interstellar medium, were mutually correlated, and that their band casting light on a century-old astrochemical mystery. See Letter p.322 widths were consistent with the spectrum of a rotating fullerene-like molecule2,10,11. To pro- vide unambiguous confirmation, laboratory + PASCALE EHRENFREUND & BERNARD FOING studies, theoretical modelling and astronomical gas-phase spectra of C60 were called for. observations4. As a result, a host of candidate In the current study, Campbell et al. report + ore than 400 unidentified absorption carriers has been proposed, ranging from dust the laboratory synthesis of a complex of C60 features have been recorded in grains to intriguing carbonaceous species, and helium at 5.8 kelvin, providing the best spectra of the interstellar medium of including fullerenes (a carbon allotrope)3,6. approximation so far of the gas-phase spec- + Mthe Milky Way and other galaxies. The nature High-resolution observations of fine spectro- trum of C60 . The laboratory measurements 7,8 + of these ‘diffuse interstellar bands’, which are scopic details of certain DIBs have contributed of C60 band maxima at 9,632.7 and 9,577.5 Å present in spectra of the interstellar medium to the consensus that polyatomic carbonaceous show a stunning match to the observed wave- from the near-ultraviolet to the near-infra- molecules in the gas phase are promising can- lengths of the corresponding interstellar red domains, has evaded elucidation since didate carriers. The discovery and synthesis of DIBs, with an accuracy of ±0.1 Å. Moreover, their discovery nearly 100 years ago — one fullerenes (including buckminsterfullerene) and the respective band widths of 2.2 and 2.5 Å are of the longest-standing mysteries of modern of graphene in laboratories, have revolution- consistent with the 2.8-Å widths measured for astronomy. Writing on page 322 of this issue, ized our understanding of carbon chemistry these DIBs10,11. Twenty-one years after the ini- Campbell et al.1 report laboratory spectra of and its applications ranging from astronomy and tial assignment, the authors’ superb laboratory the buckminsterfullerene, or ‘buckyball’, ion biotechnology to industrial processes. results, which were achieved with a technically + C60 . The spectra provide an excellent match In 1994, a search for the strongest signals of advanced set-up, have conclusively identified + + to two interstellar bands that we discovered C60 in interstellar-material spectra led to the C60 as the carrier of these two DIBs. 2 2 + and attributed to this ion , and securely iden- discovery of two absorption features (Fig. 1) With the two strongest C60 bands securely + tify C60 as a component of the interstellar at 9,577 and 9,632 ångströms. These coincided matched to DIBs, prospects are improving for medium. with independent measurements of electronic the identification of the carriers of the more + During the past 40 years, advances in transitions of C60 in laboratory spectra using than 400 remaining DIB features, which have astronomical instrumentation have enabled frozen-matrix techniques — in which the tar- been recorded in a variety of astrophysical set- the detection of many weak diffuse interstellar get species is diluted with an unreactive host tings. In the past decade, observations across bands (DIBs) (Fig. 1), and their various widths species (such as neon) at very low tempera- the entire optical range have established that have been accurately measured3–5. Numerous tures, and is then spectrally analysed9. This diffuse band carriers are ubiquitous in space. efforts have been made to identify the likely preliminary match of two interstellar DIBs DIBs have also recently been detected and + molecular group (the ‘carrier’) that causes with C60 showed wavelength shifts of 3–10 Å mapped in the Magellanic Clouds (the Milky the absorption bands through laboratory relative to the laboratory measurements; these Way’s satellite galaxies) and in other galaxies. The carrier molecules may therefore constitute an important reservoir of organic material 1.00 a b 1.10 throughout the Universe. The clear identification of additional DIB 0.95 1.05 carriers could have a far-reaching impact on our understanding of astrochemical reaction 0.90 + 1.00 networks. C60 was possibly the easiest and most conspicuous molecule to identify owing 0.85 0.95 to its signature of two strong bands in the * near-infrared range that is clearly evident in 0.80 0.90 interstellar-medium spectra, which have been 2,10,11 Normalized Intensity * corrected for atmospheric absorptions . 0.75 0.85 The species’ cage-like symmetry and geome- 9,580 9,620 9,660 try6 (Fig. 1), as well as its chemical properties, 0.70 increase its chances of survival in space and 4,000 5,000 6,000 7,000 8,000 9,000 A, JAN CAMI; ADAPTED FROM REF. 4/B, PASCALE EHRENFREUND 4/B, PASCALE FROM REF. CAMI; ADAPTED A, JAN its expected natural formation in astrophysi- Wavelength (Å) 4,10 & BERNARD FOING; REF. 10/ESO/JAMES HEDBERG/(CC BY-NC-SA 3.0) HEDBERG/(CC BY-NC-SA 10/ESO/JAMES REF. & BERNARD FOING; cal environments . One possible approach to identify more DIB carriers would be to detect Figure 1 | Buckminsterfullerene in space. a, High-resolution absorption spectra of the interstellar and quantify the presence in space of other medium, such as that shown here, include more than 400 unidentified diffuse interstellar9,600 bands (DIBs, white lines) spread across the optical and near-infrared domains3,5,10. Fullerenes and other fullerene compounds, which could be formed carbon-bearing molecules have been proposed as the absorbing species causing some of the interstellar by the addition of hydrogen atoms, metals or DIBs, but conclusive identification had been elusive. b, Campbell et al.1 now prove that the charged other radicals in different charge or reactive + 10 buckminsterfullerene ion C60 is indeed responsible for the two strongest interstellar DIBs in the states. near-infrared at 9,577 and 9,632 Å (indicated by stars) discovered2 in 1994. In previous analyses of mid-infrared 296 | NATURE | VOL 523 | 16 JULY 2015 | CORRECTED 24 JULY 2015 © 2015 Macmillan Publishers Limited. All rights reserved NEWS & VIEWS RESEARCH emission spectra of a young planetary nebula Astronomical surveys of DIB strengths and 3. Herbig, G. H. Annu. Rev. Astron. Astrophys. 33, (the ionized, ejected envelope of an old star) profiles in different interstellar and circumstel- 19–73 (1985). 4. Cox, N. L. J. & Cami, J. (eds) Proc. Int. Astron. Un. taken with NASA’s Spitzer Space Telescope, lar environments are needed to elucidate the Vol. 9 (2014). researchers for the first time identified12 spe- formation and fate of carriers. All these research 5. Hobbs, L. M. et al. Astrophys. J. 705, 32–45 (2009). cific molecular-vibration bands that were disciplines must be combined to provide further 6. Kroto, H. W. et al. Nature 318, 162–163 (1985). 7. Ehrenfreund, P. & Foing, B. H. Astron. Astrophys. assigned to compounds of the fullerenes C60 insights into the photochemistry, thermal con- 307, L25–L28 (1996). and C70. Vibrational spectroscopy cannot per- ditions and integration in protoplanetary-disk 8. Sarre, P. et al. Mon. Not. R. Astron. Soc. 277, L41–L43 fectly distinguish between molecules of similar material that affect the inventory and evolution (1995). 9. Fulara, J., Jakobi, M. & Maier, J. P. Chem. Phys. Lett., chemical bonds or between molecular excita- of organic molecules from molecular clouds to 211, 227–234 (1993). tion states, but it can provide a global inventory star- and planet-forming regions. ■ 10. Foing, B. H. & Ehrenfreund, P. Astron. Astrophys. of molecular groups from the pool of organic 317, L59–L62 (1997). 11. Galazutdinov, G. A. et al. Mon. Not. R. Astron. Soc. materials in space. 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