Vibronic interactions in the photodetachment spectroscopy of phenide anion.

Photodetachment spectroscopy of phenide anion C6H5- is theoretically studied with the aid of electronic structure calculations and quantum dynamical simulations of nuclear motion. The theoretical results are compared with the available experimental data. The vibronic structure of the first, second, and third photoelectron bands associated with the ground X 2A1 and low-lying excited A 2B1 and B 2A2 electronic states of the phenyl radical C6H5 is examined at length. While the X state of the radical is energetically well separated and its interaction is found to be rather weak with the rest, the A and B electronic states are found to be only approximately 0.57 eV apart in energy at the vertical configuration. Low-energy conical intersections between the latter two states are discovered and their impact on the nuclear dynamics underlying the second and third photoelectron bands is delineated. The nuclear dynamics in the X state solely proceeds through the adiabatic path and the theoretically calculated vibrational level structure of this state compares well with the experimental result. Two Condon active totally symmetric (a1) vibrational modes of ring deformation type form the most dominant progression in the first photoelectron band. The existing ambiguity in the assignment of these two vibrational modes is resolved here. The A-B conical intersections drive the nuclear dynamics via nonadiabatic paths, and as a result the second and third photoelectron bands overlap and particularly the third band due to the B state of C6H5 becomes highly diffused and structureless. Experimental photodetachment spectroscopy results are not available for these bands. However, the second band has been detected in electronic absorption spectroscopy measurements. The present theoretical results are compared with these absorption spectroscopy data to establish the nonadiabatic interactions between the A and B electronic states of C6H5.