Toluene internal-rotation:: Measurement and simulation of the high-resolution S1-S0 fluorescence excitation spectrum at 0.5 K

Rotational structure in the origin band of the S-1-S-0 fluorescence excitation spectrum of toluene has been measured at 0.012 cm(-1) resolution with a rotational temperature of 0.5 K using a pulsed beam apparatus. Such spectra have been obtained for the parent isotopomer and for the isotopomers with mono- and di-deuterated methyl groups. These, and previously known forbidden bands in which the internal-rotation quantum number K-i changes, are simulated here using ab initio internal-rotation-angle-dependent geometries, potential functions, and electronic transition moment function. An adjustment of some of the ab initio parameters allows a close fitting of the spectra to be made, and this can only be achieved if bond lengths and angles are allowed to vary with internal rotation. The resulting geometries for ground and excited-state toluene are the most accurate to date. (C) 2000 American Institute of Physics. [S0021-9606(00)01601-9].