LABORATORY FORMATION OF FULLERENES FROM PAHS: TOP-DOWN INTERSTELLAR CHEMISTRY
Interstellar molecules are thought to build up in the shielded environment of molecular clouds or in the envelope of evolved stars. This follows many sequential reaction steps of atoms and simple molecules in the gas phase and/or on (icy) grain surfaces. However, these chemical routes are highly inefficient for larger species in the tenuous environment of space as many steps are involved and, indeed, models fail to explain the observed high abundances. This is definitely the case for the C{sub 60} fullerene, recently identified as one of the most complex molecules in the interstellar medium. Observations have shown that, in some photodissociation regions, its abundance increases close to strong UV-sources. In this Letter we report laboratory findings in which C{sub 60} formation can be explained by characterizing the photochemical evolution of large polycyclic aromatic hydrocarbons (PAHs). Sequential H losses lead to fully dehydrogenated PAHs and subsequent losses of C{sub 2} units convert graphene into cages. Our results present for the first time experimental evidence that PAHs in excess of 60 C-atoms efficiently photo-isomerize to buckminsterfullerene, C{sub 60}. These laboratory studies also attest to the importance of top-down synthesis routes for chemical complexity in space.