OBSERVATION OF TORSIONAL MOTION IN THE GROUND-STATE CATION OF JET-COOLED TOLANE BY 2-COLOR THRESHOLD PHOTOELECTRON-SPECTROSCOPY

High-resolution threshold photoelectron spectra of jet-cooled tolane (C6H5-C = C-C6H5) have been obtained by two-color (1 + 1') resonantly enhanced multiphoton ionization via the S1 state {1B1u(pi-pi-*)}. From the threshold photoelectron spectra, the adiabatic ionization energy has been determined to be I(a) = 63 917 +/- 3 cm-1 (7.9243 +/- 0.0004 eV). A vibrational progression due to torsional motion in the cation ground state was observed up to its fifth quantum level with a fundamental frequency of 54 cm-1, showing small anharmonicity. From these results, we have evaluated the potential barrier height to the torsional motion to be 1980 +/- 60 cm-1. The threshold photoelectron spectra obtained via several S1 vibronic levels exhibit dominant peaks due to the DELTA-upsilon = 0 ionization transitions. This fact indicates that no significant change occurs in molecular structure upon photoionization with removal of the antibonding pi electron from the S1 state. Threshold photoelectron bands arising from formally forbidden odd quantum transitions in the torsional model were also clearly observed with small intensities. The mechanism of the appearance of such a nontotally symmetric mode is discussed.