Toluene internal-rotation: Measurement and simulation of the high-resolution S-1-S-0 fluorescence excitation spectrum at 0.5 K

Rotational structure in the origin band of the S-1-S-0 fluorescence excitat ion 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 spectr a have been obtained for the parent isotopomer and for the isotopomers with mono- and di-deuterated methyl groups. These, and previously known forbidd en bands in which the internal-rotation quantum number K-i changes, are sim ulated here using ab initio internal-rotation-angle-dependent geometries, p otential functions, and electronic transition moment function. An adjustmen t 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 a llowed 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].