Electronic spectra of linear HC5H and cumulene carbene H2C5

The 13Σ−u←X3Σ−g transition of linear HC5H (A) has been observed in a neon matrix and gas phase. The assignment is based on mass-selective experiments, extrapolation of previous results of the longer HC2n+1H homologues, and density functional and multi-state CASPT2 theoretical methods. Another band system starting at 303 nm in neon is assigned as the 11A1←X˜1A1 transition of the cumulene carbene pentatetraenylidene H2C5 (B)