Optical response of small carbon molecules

We apply the time-dependent local density approximation (TDLDA) to calculate dipole excitations in small carbon molecules. Linear chains are found to have a strong low-frequency mode which varies with chain size as the classical resonance of a conducting needle; here the theory agrees well with observation on Cn molecules in the range n = 3 − 15. The lowest collective transition in rings occurs at about twice the frequency, which may also be understood in simple terms. TDLDA also explains a weak transition found in even-n chains at the frequency of the HOMO-LUMO gap. PACS numbers:33.20.Ea,36.40.Wa,78.30.Hv,36.40.Vz Typeset using REVTEX ∗E-mail [email protected] 1