TORSION ROTATION INTERACTIONS IN A 2-TOP MOLECULE - HIGH-RESOLUTION S1[-S0 ELECTRONIC-SPECTRUM OF 2,3-DIMETHYLNAPHTHALENE

A rotationally resolved fluorescence excitation spectrum of the O0(0) band in the S1 <-- S0 transition of 2,3-dimethylnaphthalene has been obtained using a cw laser/molecular beam spectrometer. More than 3000 lines were observed, each exhibiting a width of about 3 MHz. Despite the proximity of the two methyl groups, the observed rotational structure can be interpreted satisfactorily using an uncoupled rotor model. However, extensive torsion-rotation interactions are observed. Accurate measurements of these perturbations are used to determine the effective threefold hindering potentials in both electronic states, V3 (S0) = 652 cm-1 and V3 (S1) = 391 cm-1, respectively. Rotor-rotor couplings do influence these barriers as well as the patterns of lines observed in the rotationally resolved spectra of higher vibronic bands.