Coalescence reactions of fullerenes

This article reviews recent work in Los Angeles on elementary processes in fullerene vapors. The production of fullerene molecules typically involves extreme high-temperature conditions and processes which are poorly understood at date [1-3]. Once generated, these molecules may represent the most stable molecules known [4,5]. In a recent work [C. Yeretzian et al., Nature 359, 44 (1992)] we presented clear evidence for coalescence reactions between fullerene molecules. Mass spectrometric measurements on hot, dense vapors of small fullerenes (C₆₀ and C₇₀) reveal the formation of stable higher fullerenes which are multiples of the initial masses. These processes are shown to occur in the gas-phase rather than in the solid film and their dependences on laser fluence and He-gas pressure are investigated. Three distinct reactions are proposed-coalescence, emission and capture-to account for the observed distributions at higher fullerene sizes. Specifically, the heat of coalescence is released through emission of small, even-numbered fragments which, in a very dense vapor, are efficiently captured by other coalesced fullerenes. These findings have implications for the long-time stability of the fullerene vapor, and for the mechanism of fullerene formation and growth, and may open new ways to the synthesis of selected higher fullerenes and encapsulation compounds.