The selenoketyl (HCCSe) radical has been positively identified for the first time as a product of an electric discharge through selenophene vapor. Laser-induced fluorescence, wavelength resolved emission, and fluorescence decay studies of jet-cooled HCCSe and DCCSe have given a detailed picture of the ground and excited state. The 418-400 nm band system of the HCCSe radical is assigned as (A) over tilde (2)Pi(i)-(X) over tilde (2)Pi(i) and the available evidence suggests that the radical is linear in the ground state and quasilinear in the excited state. The fluorescence decays of some upper state rotational levels show field-free molecular quantum beats, ascribed to an internal conversion interaction with high vibrational levels of the ground state. A comparison of the molecular structures and bonding in the HCCX (X=O,S,Se) free radicals shows that nonlinear ground state HCCO is best described as the ketenyl radical (H-C=C=O) with the unpaired electron on the terminal carbon atom, whereas HCCS and HCCSe have linear ground state acetylenic (H-Cequivalent toC-X) structures with the unpaired electron on the heteroatom. On electronic excitation, (B) over tilde (2)Pi HCCO reverts to the linear acetylenic structure, and (A) over tilde (2)Pi HCCS and HCCSe become quasilinear with the allenic structure. (C) 2004 American Institute of Physics.