Electronic spectra of heteroatom-containing isoelectronic carbon chains C2nS and C2nCl+ (n=1-5)

Structures and stabilities of carbon chains C2nS and C2nCl+ (n=1-5) in their ground states have been investigated by the density functional theory and the coupled cluster approach using single and double substitutions. The complete active space self-consistent-field method has been used for geometry optimization of selected excited states in both series. Calculations show that both C2nS (n=1-5) and C2nCl+ (n=3-5) have linear structures in the triplet ground state (3)Sigma(-), while C2Cl+ and C4Cl+ have nonlinear structures in the ground state (3)A('). The vertical transition energies and emission energies by the multiconfigurational second-order perturbation theory in linear clusters C2nS and C2nCl+ exhibit similar size dependences. In comparison with the available experimental observations, the predicted excitation energies for the allowed 2 (3)Sigma(-)<- X (3)Sigma(-) transitions have an accuracy of no more than 0.24 eV. Spin-orbit coupling configuration interaction calculations indicate that the spin-forbidden 2 (1)Sigma(+)<- X (3)Sigma(-) transition in these species has an oscillator strength with the magnitude of 10(-4)-10(-5), and they may be observable experimentally.