The laser-induced fluorescence spectrum, Renner-Teller effect, and molecular quantum beats in the A (2)Pi(i)-X (2)Pi(i) transition of the jet-cooled HCCSe free radical.

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 (2)Pi(i)-X (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[Single Bond]C[Double Bond]C[Double Bond]O) with the unpaired electron on the terminal carbon atom, whereas HCCS and HCCSe have linear ground state acetylenic (H[Single Bond]C[Triple Bond]C[Single Bond]X) structures with the unpaired electron on the heteroatom. On electronic excitation, B (2)Pi HCCO reverts to the linear acetylenic structure, and A (2)Pi HCCS and HCCSe become quasilinear with the allenic structure.