The experiments that led to the discovery of C were aimed at simulating in the laboratory the conditions under which carbon nucleates in the atmosphere of a cool carbon-rich red giant star. The specific goal of the work was to explore the possibility that long carbon chain molecules such as cyanopolyynes (HCN, n = 5 11), which had previously been detected in the interstellar medium, could form when carbon vapor nucleates in the presence of hydrogen and nitrogen (Heath, 1987).

In order to carry out these experiments, Harry Kroto, an astrophysical chemistry professor at the University of Sussex (England), travelled to Rice University (Houston, TX) to work with Professors Richard Smalley and Robert Curl. Smalley and coworkers had developed a powerful technique in which a laser vaporizes atoms of a refractory material (such as carbon) into a carrier gas (usually helium). In the carrier gas, the atoms nucleate (cluster), before being cooled by supersonic expansion, skimmed into a molecular beam, and analyzed by time-of-flight mass spectrometry. Reactive gases such as hydrogen (H) or nitrogen (N) could also be added to the carrier gas, and the reaction products of these gases with the carbon clusters could be similarly analyzed.

The Kroto/Smalley experiments showed convincingly that species such as HCN and HCN could, in fact, be produced in such laboratory simulations of the conditions in stars. However, even more significant was the unexpected discovery of C.

Note: Background material in mass spectrometry should be presented in this section.

A schematic diagram of the Smalley apparatus for generating carbon-cluster beams is shown in Figure II.B (Kroto, 1985). The vaporization laser beam strikes a rotating graphite disk, producing a plasma of vaporized carbon atoms. The pulsed nozzle passes high-density helium over the vaporization zone. The helium carrier gas provides the thermalizing conditions necessary to cool and cluster the carbon atoms in the plasma. The carrier gas also provides the "wind" that carries the cluster products through the remainder of the nozzle and the optional "integration cup". Free expansion of this cluster-laden gas at the end of the nozzle forms a supersonic molecular beam which is photoionized using an excimer laser and detected by time-of-flight mass spectrometry.

Figure II.B: Schematic diagram of apparatus used to generate and analyze carbon-cluster beams. The "integration cup" is optional.

As shown in Figure II.C, Kroto and Smalley discovered that a variety of carbon clusters were produced and that the distribution of cluster sizes depended dramatically on the experimental conditions (Kroto, 1985). When the firing of the vaporization laser was delayed until most of the helium pulse had passed, a roughly Gaussian distribution of large, even-numbered clusters with 38-120 carbon atoms resulted. The C peak was largest but not dominant. When the vaporization laser was fired at a time of maximum helium density, the C peak grew into a feature 5 times stronger than its neighbors (with the exception of C). When these conditions were duplicated but the "integration cup" was added to increase the time between vaporization and expansion, the cluster distribution was completely dominated by C.

Figure II.C: Distribution of carbon clusters produced under various experimental conditions. a) Low helium density over graphite target at time of laser vaporization. b) High helium density over graphite target at time of laser vaporization. c) Same as b), but with addition of "integration cup" to increase time between vaporization and cluster analysis.

The explanation for these results is that when hot clusters are left in contact with high-density helium, they equilibrate toward the most stable species, which appears to be a unique cluster containing 60 atoms.

Given the predominance of the C peak in their mass spectra, Smalley, Kroto, and coworkers began to think about possible 60-atom structures that would exhibit unusually high stability. They believed that in the laser vaporization, fragments of graphite were torn from the surface. Graphite has a planar structure composed of fused six-membered rings. Each carbon is bonded to three other carbons in an infinite two-dimensional array. Small graphite fragments would contain many "unsatisfied valences" at the edges; the carbon atoms around the circumference of the fragment would be bonded to only two other carbons, rather than the more-desirable three carbon atoms.