Analysis and fit of the high-resolution spectrum of the Ã1Au-X˜1Ag LIF spectrum of the two-equivalent-top molecule biacetyl

The jet-cooled laser-induced visible fluorescence excitation spectrum of the Ã1Au (S1)- X˜1Ag (S0) transition in biacetyl (CH3C(O)C(O)CH3) exhibits a long progression in the torsional vibrations of the two equivalent methyl tops in this molecule, whose structure has previously been described and qualitatively understood using local mode ideas applied to the two equivalent methyl rotor torsions together with the G36 symmetry species A1, A2, A3, A4, E1, E2, E3, E4, and G. In the present rotational analysis, we have assigned a G36 symmetry species, two local-mode torsional quantum numbers, and the usual three asymmetric rotor quantum numbers JKaKc to the upper and lower torsion-rotation levels involved in the observed transitions, relying heavily on comparison of quantum-beat patterns to determine transitions with a common upper state. These torsion-rotation transitions were then globally fit using a two-equivalent-top computer program, which was written in the principal axis system of the molecule and which uses a free-rotor basis set for each top, a symmetric-top basis set for the rotational functions, and a single-step diagonalization procedure. We can fit 411 lines involving 16 torsional sublevels from states with zero to three quanta of torsional excitation in the excited electronic state, using 24 parameters to obtain a standard deviation of 0.0045 cm−1, which is quite satisfactory, but inclusion in the fit of 440 transitions from all 17 rotationally assigned torsional levels increases the standard deviation by some 25%. The present fit gives a value of V3 = 238 cm−1 for the threefold barrier height in the excited electronic state, in reasonable agreement with earlier studies.