Electronic absorption spectrum of a nonlinear carbon chain: trans-C6H4+.

The 2Bg-X2Au transition of a nonlinear carbon chain trans-C6H4+, and trans-C6D4+, has been observed in the gas phase. The spectra were detected in the 580 nm region by direct absorption with a cavity ringdown technique through a supersonic planar discharge. Though the rotational structure is not resolved, the band profiles were analyzed using a "total spectral fitting" procedure using ground-state constants calculated by the CASSCF and CASPT2 methods.