  The $1 ^3\Sigma_u^- \leftarrow X^3\Sigma_g^-$ transition of linear HC$_5$H
(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
HC$_{2n+1}$H homologues, and density functional and multi-state CASPT2
theoretical methods. Another band system starting at 303 nm in neon is assigned
as the $1 ^1 A_1 \leftarrow X ^1 A_1$ transition of the cumulene carbene
pentatetraenylidene H$_2$C$_5$ (B).
