High-resolution electronic spectroscopy of a nonlinear carbon chain radical $C6H4+$

A high-resolution gas-phase spectrum of a molecular absorption band around 604 nm is assigned as due to an electronic transition of a nonlinear $C6H4+$ planar species starting from its $A″2$ electronic ground state. The spectrum is observed in direct absorption by cavity ringdown spectroscopy through a supersonic planar discharge through a mixture of acetylene in helium. The spectrum has a clear rotational and K-type structure. This allows an accurate determination of the B and C rotational constants and an estimate for the A rotational constant in ground and electronically excited states. The resolved spectrum of the fully deuterated species $C6D4+$ has been obtained as well. The results are compared both to the outcome of ab initio geometry optimizations and low-resolution absorption spectra in 6 K neon matrices obtained after mass-selective deposition.