Electronic spectroscopy of the nonlinear carbon chains C4H4+ and C8H4+

The electronic spectrum of a nonlinear carbon chain radical C4H4+ was observed after mass-selective deposition in a 6 K neon matrix. The corresponding gas-phase spectra of C4H4+ and C4D4+ have been observed in the 512 to 513 nm region and at 710 nm for C8H4+. These were detected in direct absorption by cavity ringdown spectroscopy through a supersonic planar discharge. The electronic transition energies of these nonlinear carbon chain radicals correlate well with those of the polyacetylene cations HCnH+ (n = 4, 6, 8). The observed profiles are reproduced with rotational constants obtained by ab initio geometry optimizations and extrapolation between the ground and excited electronic states. Key words: nonlinear carbon chain, carbon cation, electronic transition, diffuse interstellar bands, molecular structure.