Fullerenes in space

In 1987, C60+ was predicted to be a diffuse interstellar band (DIB) carrier. In 2015, Maier's group assigned four DIBs unequivocally to C60+. This was the first time any had been identified in nearly 100 years! It was a brilliant achievement.

On

1987 prediction

The present observations indicate that C₆₀ might survive in the general interstellar medium (probably as the ion C₆₀⁺) protected by its unique ability to survive processes so drastic that, most if not all, other known molecules are described.

Sir Harry Kroto

1987 fullerenes in space prediction

They seek it here, they seek it there,
they seek that magenta Buckyball everywhere,
but space is full of high energy photons too,
and so it is ionised and has changed its hue.

People forget that the diffuse clouds of space, where the carriers of the diffuse interstellar bands reside, are bathed in up to 13.6 eV photons (the ionisation potential of the H atom). Therefore, C60 will be ionised and have changed colour.

C60+ is in space

It's official, C60+ is in space, all over the galaxy and almost certainly all over the universe!

'Laboratory confirmation of C60+ as the carrier of two diffuse interstellar bands', E K Campbell, M Holz, D Gerlich and J P Maier, 2015.

Abstract

The diffuse interstellar bands are absorption lines seen towards reddened stars. None of the molecules responsible for these bands have been conclusively identified.

Two bands at 9,632 A˚ and 9,577 A˚ were reported in 1994, and were suggested to arise from C60+ molecules, on the basis of the proximity of these wavelengths to the absorption bands of C60+ measured in a neon matrix. Confirmation of this assignment requires the gas-phase spectrum of C60+.

Here we report laboratory spectroscopy of C60+ in the gas phase, cooled to 5.8 kelvin. The absorption spectrum has maxima at 9,632.7 +/- 0.1 A˚ and 9,577.5 +/- 0.1 A˚ and the full widths at half-maximum of these bands are 2.2 +/- 0.2 A˚ and 2.5 +/- 0.2 A˚, respectively.

We conclude that we have positively identified the diffuse interstellar bands at 9,632 A˚ and 9,577 A˚ as arising from C60+ in the interstellar medium.

Read the C60+ DIB original manuscript (PDF, 118KB).

DIB graph — Gas-phase laboratory spectra of C60+ at 5.8 K. Gaussian fits to the experimental data (circles) are represented by the solid black lines. The vertical red lines are the rest wavelengths, 9,577.4 +/- 0.2A˚ and 9,632.6 +/- 0.2A˚ of two reported DIBs.

The Basel group of John Maier has made an outstanding breakthrough in unequivocally assigning two diffuse interstellar bands to the ion C60+. They have succeeded in identifying a species in space which has eluded astronomers and other scientists for nearly 100 years.

This breakthrough opens up highly exciting, new fields of focused laboratory and astronomical studies to see whether any other fullerene analogues are carriers of other DIBs.

A significant number of C60 analogues must also be present in the interstellar medium.

It also yields important new data which impacts significantly on the understanding of the chemistry of the ISM, as well as the clouds in which stars and planets form.

Sir Harry Kroto's reflections

"In 1919, Heger published observations of some curious absorption lines in the spectra of stars. These lines were later shown to be due to material in the interstellar medium in the line of sight and not associated with the background star. The lines were broader than atomic lines (and so possibly molecular) and have since been called the diffuse interstellar bands (DIBs).

"At the present time, some 400 are known and they have been well documented observationally. Although scores of suggestions have been made as to the possible carriers, until now no single DIB has been identified. This is strange as the carrier or carriers must be stable in the interstellar medium and yet no terrestrial species has been found to correlate with any observed band. A consensus has grown that the carriers are probably molecular species although some favoured absorption by atoms in grains.

"At last, after nearly 100 years, the group of John Maier in Basel has made the first breakthrough by assigning two DIBs unequivocally to C60+, the positive ion of C60. This remarkable achievement was no accident. It was the result of many years of painstaking work developing state of the art spectroscopic techniques to create conditions in the laboratory which simulated the extremely low temperatures and low pressures in the interstellar medium.

"The C60 molecule Buckminsterfullerene was serendipitously discovered in our experiments in 1985 which had as their second aim the identification of the DIBs. The fact that this molecule was spontaneously created under conditions in which possible contenders for DIB carriers might also be produced seemed like an intriguing coincidence worth following up.

"In 1987, I suggested that if C60 was a likely DIB contender in interstellar space, than the ambient radiation field would almost certainly ensure that it must be ionised as C60+. Maier, in 1993, measured the spectrum of the ion in a matrix and found two strong lines the frequencies of which were used by Foing and Ehrenfreund to search for new DIBs close of these frequencies. They found two lines in reasonable correspondence, however significant matrix shifts meant that for an unequivocal assignment the spectrum had to be measured in the gas phase at low-temperature; a daunting task.

"Maier has spent much of the intervening time developing superb state-of-the-art experimental techniques to achieve just this, and this brilliant breakthrough is a result of his determined approach to solving one of the most important and long-lasting puzzles in science.

"It is a bit sad of course that this puzzle has now been solved, at least in part, however there are several hundred lines still to identify. It now seems highly likely that other C60 analogues are also present in the ISM. One might conjecture many of the other lines are due to these analogues, which could be endohedral species in which an atom of sodium or calcium is trapped inside the fullerene cage, or the atom is attached to the outside of the cage.

"Such species should display quite strong spectra and be detectable in the future although laboratory measurement will still be a daunting task."

It is incredible to think that one of the most abundant set of species in interstellar space may be these carbon cages, and yet they are almost non-existent in the terrestrial environment and took until nearly the end of the 20th century to discover. The molecule was under our noses all the time in flames (indeed it is now made in bulk by combustion of methane) and at the same time its signature was being recorded whenever astronomers observed stellar spectra.

Sir Harry Kroto

22-pole ion trap

Producing cold molecules becomes more difficult as the size of the species increases. This is due to the fact that larger molecules have more internal vibrations where the energy can be stored. Therefore very big molecules, like polycyclicaromatic hydrocarbons (PAHs) and fullerenes, cannot be cooled efficiently during the short-time scales involved in a supersonic expansion.

An alternative approach is to collect the ions in an ion trap and cool them down through collisions with a cold buffer gas, like helium or argon.

Analysis

An analysis of why the C60+ diffuse interstellar band assignment is so scientifically convincing.

It is important to realise that this is not just a simple case of assigning two DIB lines (observed in 1994 in the near infrared, based on the Basel groups initial 1993 laboratory study). It is an elegant sequence of several step-by-step research observations over many years.

These observations have uncovered a series of several circumstantial scientific results, ending with a perfect fit achieved by the Basel group. This makes the whole story so much more scientifically convincing, than if two lines had just happened to fit.

Steps that led to the discovery

1919-22 - Heger discovers some odd lines in the spectra of various stars

Around 100 years ago, Mary Lea Heger discovered diffuse bands in the spectra of stars due to some sort of material maybe molecules in the space between stars and earth. A map of the data from the Sloan Digital Sky Survey, by a team from Johns Hopkins produced this map. Red indicates areas with the most abundant DIB molecules, blue the least.

A map of the data from the Sloan Digital Sky Survey.

The detection of the unusual lines happened originally in 1919 (Lick Observatory) but possibly earlier.

Takeshi Oka notes: "The strongest DIB at 4430 is mentioned on page 288 of Volume 9-10-11 of the Henry Draper catalog published from 1911 to 1919. A. D. Code in Publ. Astron. Soc. Pac. 70, 404 (1958) examines the plate taken by Annie Jump Cannon and confirmed it.

"The plate is not dated and George Herbig (ApJ 196, 129, 1975) said “sometime between 1911 and 1919”. Others argue that the plate is much older and may even be of the late 19th century! (Cannon lived 1863-1941). In any case I think it is safe to say that it is more than 100 years since the first diffuse interstellar band was observed."

1934-36 - Merrill shows they are interstellar lines

In 1934, Merrill showed one band did not share the varying Doppler shifts exhibited by atomic stellar lines associated with the orbital motion of the binary background star. He demonstrated that the band was interstellar.

1969 - Becklin et al discover the Red Giant Star IRC+10216

E E Becklin et al. (December 1969). “The Unusual Infrared Object IRC+10216”. Astrophysical Journal 158: L133

1974-78 - The microwave spectra of long carbon chains measured at Sussex University

Read more about the microwave spectra of the cyanopolyynes.

1975-78 - Radioastronomy detection of the long chains in space by the Sussex/NRC team

1977 - Conjecture that carbon chains might be carriers of the DIBs

A E Douglas, Nature 269, 130−132 (1977).

1985 - The discovery of C60, a Rice/Sussex collaboration

Sir Harry Kroto, "When we discovered the fullerenes in 1985, in laboratory experiments which simulated the chemical conditions in carbon rich red giant stellar atmospheres, they stimulated me to think about their possible existence in stellar outflows and in the general interstellar medium.

"In fact there were two reasons for carrying out the 1985 experiments. One was to simulate the chemical conditions which were responsible for production of the long carbon chains which we had discovered in the late 1970s. I thought these must have been created in the atmospheres of cool carbon red giant stars.

"The second reason was to follow up on the idea that carbon chains are possible carriers of the diffuse interstellar bands. Our long chain discoveries were carried out at NRC in Canada and this stimulated Alec Douglas to suggest that the carbon chains might be responsible for them.

"This second experiment was to be a resonant two photon ionisation experiment in which the first photon would be at a DIB frequency. We would hope to see a specific chain detected by mass spectrometry.

"Needless to say the unexpected appearance of C60 diverted us from this extremely tricky second experiment."

1987 - 2015 - Arguments in favour of C60+ and its analogues as carriers of the diffuse interstellar bands

Particularly intriguing was the possibility that C60 or some analogues (ionised species such as C60+, endohedral fullerenes M@C60+, and analogues M+…C60) might be carriers of the diffuse interstellar bands. It seemed that it might not be a coincidence.

The radiation field in the diffuse clouds of interstellar space is such that molecules must be ionised. The possibility that some sort of carbon molecules might be involved in the DIB mystery was one of the reasons for carrying out the original 1985 C60 discovery experiments.

H Kroto, 'Polycyclic Aromatic Hydrocarbons in Astrophysics' (PDF, 1.02MB) (eds A Léger, L d’Hendecourt and N Boccara) 197−206 (Reidel, Dordrecht, 1987).

In this 1987 paper, there are predictions made on the possible existence of C60 and its analogues in stars and interstellar space in particular as the positive ion C60+ as well as complexes M…C60+ and M@C60+ with other species M such as atoms (H, He, Na, K, Ti, Ca, N, O etc) and also as possible carriers of the diffuse interstellar bands.

H Kroto, 'The role of linear and spheroïdal carbon molecules in interstellar grain formation', Ann. Phys. Fr., Vol. 14, N°2 1989, pp. 169-180.

H Kroto, 'Space, stars, c60, and soot', Science. 1988 Nov 25;242(4882):1139-45.

H Kroto and J Hare, 'A postbuckminsterfullerene view of carbon in the galaxy' (PDF, 961KB), 1992.

M Jura and H Kroto paper on dust and interstellar chain formation (PDF, 148KB).

M Jura and H Kroto, 'Circumstellar and interstellar fullerenes and their analogues' (PDF, 125KB), Astronomy and astrophysics, 1992.

1990 - Extraction of C60 making laboratory measurements possible

1993 - The key laboratory measurement by the Basel group of the matrix spectrum of C60+

In 1993, John Maier’s Basel group published some fascinating results on C+60 trapped in a neon matrix. They detected two strong bands at long wavelength.

'Electronic and infrared spectra of C+60 and C−60 in neon and argon matrices', J Fulara, M Jakobi, J P Maier, 1993.

This study is arguably the most important one of the final phase of the C60+ detection because it stimulated an astrophysical search based on reliable laboratory measurements. Two lines are much better than one.

1994 - A search based on the Basel matrix spectra detected two new DIBs close to the laboratory frequencies

Foing and Ehrenfreund followed up the measurements of the two lines by the Basel group and searched in the Infra Red, a region which has only recently been developed. They found two lines in tantalisingly close agreement with the laboratory measurements, taking into account matrix solid-state shifts. Thus indicating gas phase study is vital for unequivocal assignment.

'Detection of two interstellar absorption bands coincident with spectral features of C60+', B H Foing and P Ehrenfreund, Nature 369, 296 – 298, 26 May 1994.

Unequivocal verification of this possible assignment of C+60 as a DIB carrier will require the accurate laboratory measurement of these bands in the gas phase at very low temperature, a somewhat daunting task.

Read Harry Kroto's thoughts in this article (PDF, 146KB).

2010 - C60 detected in space

The amazing Canadian discovery, using Spitzer satellite data, that C60 is flowing out of stars indicated the hunt might soon be up.

'Detection of C60 and C70 in a young planetary nebula', Science. 2010 Sep 3;329(5996):1180-2.

2015 - After nearly 100 years, the hunt is up for the carriers of the diffuse interstellar bands

The icing on the cake is a fantastic 20 year effort to develop state-of-the-art technology to obtain the gas phase spectrum of C60+ under conditions of very low temperature and pressure. They find that they agree perfectly with the astronomers’ measurements (Basel University).

Sir Harry Kroto, "The age-old awe that man has had for the heavens has driven almost all aspects of human culture and knowledge and resulted in many useful technologies.

"As spectroscopic techniques developed and were allied with telescopes many fascinating discoveries were made. One early surprise was that space – between stars – was not empty; indeed interstellar space was found to contain numerous atoms, molecules and dust particles.

"The development of radio telescopes has revolutionised our understanding of the molecular constitution of the interstellar medium ISM. A recent surprise that the element carbon had up its sleeve was the existence of C60, Buckminsterfullerene, the third well-defined form of carbon.

"The possibility that C60 and derivatives exist in space was suggested by the fact that the original discovery was made serendipitously during our laboratory experiments in 1985, designed to simulate the atmospheric conditions in cool red giant carbon stars. This conjecture was beautifully confirmed by NASA’s Spitzer satellite telescope and this suggests that C60, or some analogues, may be responsible for some of the diffuse interstellar bands.

"The first set of these DIBs was observed in 1919 and many more have since been observed. They have puzzled scientists ever since. In 1993, John Maier’s group in Basel detected two lines of C60+ in matrix studies and these observations were followed up by Foing and Ehrenfreund who observed two new diffuse interstellar bands very close to Maier’s observations, which are of course subject to solid-state matrix shifts.

"Although the astrophysical lines were very close, indeed tantalisingly so, unequivocal confirmation requires laboratory measurement of C60+ lines in the gas phase at very low temperature… an extremely demanding task indeed. The very close proximity of the laboratory and astrophysical measurements however make the measurement well worth attempting.

"The fact that this species seems to have been hiding in the dark recesses of the galaxy since time immemorial brings to mind the mysterious character, Harry Lime, lurking underground and in the dark streets of Vienna, made famous by Orson Welles in the classic movie The Third Man. In fact, we now know that the molecule forms fleetingly within sooting flames, but seems to be immediately destroyed by fast aggregation reactions and/or as it passes through the flame barrier into an oxygen containing atmosphere.

"This is yet another example of the remarkably synergistic relationship between terrestrial and space science. In these difficult times it lends useful support for the fundamental value of “Blue Skies” or perhaps more accurately black skies cross-disciplinary research."

Relevant articles, documents and videos

'A Conversation with John Maier - the spectroscopist discusses the search for buckyballs in deep space' (PDF, 1.2MB), ACS Central Science, 2015.
'Chains and grains in interstellar space' (PDF, 1.02MB), Harry Kroto discussing the possibility of C60 and its analogues in space and as DIB carriers.
Klemperer's letter to John Maier (PDF, 47.1KB)
'From space, knowledge', Chemistry World, 2015.
'Buckyballs solve century old mystery about interstellar space', NPR, 2015.
'Buckyballs in space solve 100 year old riddle', Scientific American, 2015.
'Bountiful Buckyballs resolve interstellar mystery', Physics World, 2015.
'After 20 years, Buckyballs have been confirmed in space', IFL Science.
'Buckyballs in space: Massive molecules stealing starlight', Los Angeles Times, 2015.
'In distant galaxies, new clues to century-old molecule mystery', NASA.
'A mystery between the stars', FSU news.
'Fullerene's faint fingerprint?' (PDF,146KB), Nature, 1994.
'On the discovery of the diffuse interstellar bands', The Royal Society, 2013.
'The search for interstellar C60', G Herbig, 2000.
Jim Watson identified CH as the carrier of an unusually narrow DIB which was much narrower than the standard ones. Read more in this paper (PDF, 3.21MB).
'Weird 'Buckyballs' may be at root of milky way mystery', Space.com, 2015.
C A Rice, F-X Hardy, O Gause and J P Maier, '(1)1A’- X1A’ electronic transition of protonated coronene at 15K', JPCL 5, 942-945, 2014.
F-X Hardy, O Gause, C A Rice and J P Maier, 'Absorptions in the visible of protonated pyrene collisionally cooled to 15K', ApJL 778 (2), L30, 2013.
S Chakrabarty, M Holz, E K Campbell, A Banerjee, D Gerlich and J P Maier, 'A novel method to measure electronic spectra of cold molecular ions', J. Phys. Chem. Lett. 4, 4051-5054, 2013.
S Chakrabarty, C A Rice, F J Mazzotti, R Dietsche and J P Maier, 'Electronic absorption spectrum of triacetylene cation for astronomical considerations', J. Phys. Chem. A 117 (39), 9574-9577, 2013.
'Astrophysical problems involving carbon re-appraised' in Astrochemistry of Cosmic Phenomena, J P Hare and H Kroto.

Full US version here in four parts entitled “The Race to Catch the Buckyball”. A nonsense title as there was no race really. Unfortunately the original UK version isn't available, beautifully narrated by Juliet Stevenson, entitled “Molecules with Sunglasses”. A much better scientifically and historically accurate title, especially in retrospect as a molecule seems to be the most resilient one with respect to the radiation fields in interstellar space!

Sir Harry Kroto

Photograph taken by Jon Hare of John Maier, Harry Kroto and Don Huffman. Taken on 15 July 2015 at a symposium celebrating the 30th anniversary of the discovery of C60, where John announced the identification of one carrier of the diffuse bands.

Sir Harry Kroto