The vinyl radical (C2H3)Ã 2A″←X̃ 2A′ spectrum has been measured between 530 and 385 nm using cavity-ringdown spectroscopy. The active vibrational progressions involve C–C stretching and alpha H–C–C bending vibrations. Optimal rotational constants and linewidths were determined for the first four vibrational bands by modeling the spectrum as an asymmetric top. The best-fit rotational constants obtained for the excited electronic state are consistent with the molecular geometry predicted by ab initio calculations. The lifetime of the vibrationless level in the excited electronic state is estimated to be a few picoseconds, and increasing vibrational excitation leads to a decrease in the lifetime, based upon an increasing linewidth. Various possibilities for the predissociation mechanism are discussed. The most likely is judged to be a conical intersection or seam of intersections. A preliminary CASSCF calculation has found the point on the relevant potential energy surfaces at which the ground and electronically excited states are closest. While the geometry and other properties of this crossing point are in accord with the experimental results, the calculated position of the point of closest approach of the two electronic states lies considerably (>1 eV, including zero-point energy) above the already predissociative Ã 2A″ state origin. Other mechanisms are also discussed to account for the observed rapid predissociation. Clearly there is a need for a higher level theoretical work on this problem.

ABSTRACT:A theoretical study has been carried out on the allyl radical in its ground and first excited electronic states. Complete active space self-consistent field (CASSCF) calculations show the presence of a conical intersection between the ground and first excited electronic states (ϳ400 cm Ϫ1 above the adiabatic excited state energy), reached by decreasing the COCOC angle and twisting the COC bonds. The presence of this conical intersection provides a likely explanation for the very rapid predissociation in the excited electronic state.