Ab-initio study of the electromagnetic response and polarizability properties of carbon chains

We present the most complete set of calculations to date of the ground state electronic properties and of the optical/UV response function of linear carbon chains CN, using ab-initio methods based on local density and on time-dependent local density approximations (LDA and TDLDA). Making use of the associated transition densities and wavefunctions we are able to provide microscopic insight into the collectivity of the corresponding plasmon spectrum in terms of correlated particle-hole excitations. From this analysis it is found that the (one-dimensional 1-D) delocalization of π (valence) electrons is responsible for the conspicuous values of the static dipole polarizability and of the high value of the exponent describing its dependence with the number of carbon atoms. Within this framework the electronic structure and linear response function of a carbon ring is also calculated. Although many properties of this function are similar to that associated with linear chains of the same number of atoms, the corresponding polarizabilities differ appreciably, providing a reliable method to distinguish between linear and close structures. The first principle results of the properties of linear carbon chains are compared with both theoretical and experimental results available in the literature, and constitute the basis for a systematic study of these 1-D s p-bonded systems, which have been found to be involved in such seemingly disparate phenomena as fullerene growth mechanism and diffuse interstellar bands.