Optical Spectroscopy of Small Carbon Clusters from Electron-Impact Fragmentation and Ionization of Fullerene-C60.

A series of cationic molecular fragments (C n+, n = 11, 12, 15, 16, 18, and 21), produced by electron-impact ionization of C60 in the gas phase, were each mass-selected and accumulated in cryogenic Ne matrices. Optical absorption measurements in the UV-vis and IR spectral ranges reveal linear carbon chain structures. In particular, we have observed the known electronic transitions of linear C11, C15, and C21. The NIR transitions of linear C15-, C16-, and C18- have also been detected, indicating that soft-landing of the corresponding cations can also involve charge-changing. Newly observed electronic absorptions at 410.3 and 429.9 nm have been assigned to linear C18 absorptions at 438.2, 443.5, 422.3, and 433.7 nm, to linear C15+, and absorption at 395.5 nm, to linear C16. Increasing deposition energy leads to fragmentation upon impact. This is indicated by absorptions of C10 (313, 316.3 nm), when depositing C n+ ( n = 11, 15, 16) as well as C12 (332 nm) or C14 (347.4, 356.6 nm), when depositing C15+ or C16+, respectively. These were previously assigned to cyclic isomers. We reassign them to linear isomers here on the basis of plausibility arguments. The observations have been supported by time-dependent density functional theory calculations for ring and chain isomers of C n+/-/0, 10 ≤ n ≤ 20 up to the vacuum-UV range. The electronic absorptions of carbon chains are at least 1 order of magnitude stronger than all NIR electronic absorptions of C60+, which have recently been attributed to several of the diffuse interstellar bands. Considering that fullerene multifragmentation yields long carbon chains that have very strong absorptions both in the UV-vis and IR spectral regions, these systems appear to be good candidates to be observed in regions of space containing fullerenes.