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Fig. 1.3. Schematic view of a laser ablation source of fullerenes. The sequence of events is that a pulsed valve opens, flowing He gas over the surface. Near the middle of the He gas pulse, the laser pulse strikes the graphite surface, vaporizing the carbon material to atoms and ions. The carbon material is entrained in the He carrier gas and carbon clusters condense in the gas. After clustering the carbon material in the integration cup, the gas mixture undergoes supersonic expansion into a vacuum with resultant cooling [1.16].

clusters. The clustering was permitted to occur for 30-200/i,s, after which the gas stream was allowed to expand and cool, thereby terminating cluster growth. This laser ablation source was coupled to the front end of a time-of-flight mass spectrometer which allowed the mass spectrum of the clusters and molecules formed by the source to be measured [1.16, 18]. The oft-cited mass spectrum of Fig. 1.4 shows two main groupings of carbon clusters: (1) those with lower masses (<30 carbon atoms) containing predominantly odd numbers of carbon atoms and (2) the higher-mass clusters (above 36 carbon atoms) containing only even numbers of carbon atoms. Many years ago Pitzer and Clementi predicted that large carbon clusters could be formed from the vapor phase [1.19]. The spectrum of Fig. 1.4 was one of the first to show these high mass clusters experimentally. The mass spectrum above 40 carbon atoms shows a prominence for clusters of mass C60. This preponderance of the C60 line in the mass spectra was recognized by Kroto and Smalley at an early stage. By allowing the plasma to react for a longer time, they found that the C60 peak could be enhanced relative to the other peaks [1.16].

Previous experimental and theoretical work had shown that the most stable form for carbon clusters up to about 10 atoms is a linear chain [1.18]. Furthermore, for clusters in the 10-30 carbon atom range, rings of carbon atoms are the most stable [1.20]. Certain ring sizes are more favorable energetically, as can be seen in Fig. 1.4, where local maxima in the peak

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