Carbon nanowires: Phonon and pi-electron confinement.

The phonon dispersion of an isolated linear chain of carbon atoms is studied by density functional theory. It is shown that the longitudinal optical phonon at the Gamma point is affected by Kohn anomaly. Moreover the slope of this phonon branch is modulated by the degree of bond length alternation of the chain and reaches its maximum value in the case of the cumulenic structure (equalized bonds). Phonon dispersion curves of the infinite system computed for different values of the bond length alternation can be put in correspondence with the Raman active modes of finite carbon chains.