A systematic study of the size dependence of the vibrational modes of carbon clusters with 5-70 atoms is presented. The vibrational frequencies are measured using photoelectron spectroscopy of mass-separated negatively charged clusters. The clusters are carefully annealed. Only the most stable isomers are present in the beam after the annealing. Observed vibrational modes can be assigned to four dominant isomeric structures: linear chains (${\mathrm{C}}_5^{-}$, ${\mathrm{C}}_7^{-}$, and ${\mathrm{C}}_9^{-}$), monocyclic rings (${\mathrm{C}}_{10}^{-}$, ${\mathrm{C}}_{12}^{-}$, ${\mathrm{C}}_{14}^{-}$, ${\mathrm{C}}_{16}^{-}$, and ${\mathrm{C}}_{18}^{-}$), bicyclic rings (${\mathrm{C}}_{20}^{-}$, ${\mathrm{C}}_{24}^{-}$, and ${\mathrm{C}}_{28}^{-}$), and fullerenes (even $n>\mathrm{}{{\mathrm{C}}_{30}}^{-}$).

• Received 22 April 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.1095