The First Discovery of Fullerenes

This was the experiment performed by three Nobel Price Winners in 1985, Robert. F. Curl, Sir Harold W.Kroto and Richard E. Smalley.Their experiments aimed at understanding the mechanisms by which long chained carbon molecules are formed in interstellar space and circumstellar shells¹, graphite was vaporized by laser irradiation, producing a remarkably stable cluster consisting of 60 carbon atoms. Concerning the question of what kind of 60-carbon atom structure might give rise to a superstable species, they suggested a truncated icosahedron, a polygon with 60 vertices and 32 faces, 12 of which are pentagonal and 20 hexagonal. This object was commonly encountered as the football shown in Fig.1. The C₆₀ molecule which resulted when a carbon atom was placed at each vertex of this structure had all valences satisfied by two single bonds and one double bond, had many resonance structures, and appeared to be aromatic.

Fig.1 A football (in the United States, a soccerball) on Texas grass. The C60 molecule featured in this letter was suggested to have the truncated icosahedral structure formed by replacing each vertex on the seams of such a ball by a carbon atom.

The technique they used to produce and detect this unusual molecule involved the vaporization of carbon species from the surface of a solid disk of graphite into a high-density helium flow, using a focused pulsed laser. The vaporization laser was the second harmonic of Q-switched Nd:YAG producing pulse energies of ~30mJ. The resulting carbon clusters were expanded in a supersonic molecular beam, photoionized using an excimer laser, and detected by time-of flight mass spectrometry. In the experiment the pulsed valve was opened first and then the vaporization laser was fired after a precisely controlled delay. Carbon species were vaporized into the helium stream, cooled and partially equilibrated in the expansion, and travelled in the resulting molecular beam to the ionization region. The clusters were ionized by direct one-photon excitation with a carefully synchronized excimer laser pulse. The apparatus was fully described previously.^2-5

The vaporization of carbon was studied previously in a very similar apparatus⁶. In that work clusters of up to 190 carbon atoms were observed and it was noted that for clusters of more than 40 atoms, only those containing an even number of atoms were observed. In the mass spectra displayed in ref.6, the C₆₀ peak was the largest for cluster sizes 40 atoms, but it was not completely dominant. They re-examined the system and found that under certain clustering conditions the C₆₀ peak could be made about 40 times larger than neighbouring clusters.

Figure 2 shows a series of cluster distributions resulting from variations in the vaporization conditions evolving from a cluster distribution similar to that observed in ref.3, to one in which C₆₀ is totally dominant. In Fig.2c, where the firing of the vapourization laser was delayed until most of the He pulse had passed, a roughly gaussian distribution of large, even-numbered clusters with 38-120 atoms resulted. The C₆₀ peak was largest but not dominat. In Fig.2b, the vaporization laser was fired at the time of maximum helium density; the C₆₀ peak grew into a feature perhaps five times stronger than its neighbours, with the exception of C₇₀. In Fig.2a, the conditions were similar to those in Fig.2b but in addition the intergrating cup depicted in was added to increase the time between vaporization and expansion. The resulting cluster distribution was completely dominated by C₆₀, in fact more than 50% of the total large cluster abundance was accounted for by C₆₀; the C₇₀ peak has diminished in relative intensity compared with C₆₀, but remained rather prominent, accounting for ~5% of the large cluster population.