Rotational isomers of hydroxy deuterated o- and m-cresols studied by ultraviolet high resolution experiments.

The laser induced fluorescence spectra of several torsional transitions of the S1 <-- S0 electronic transition were recorded for the hydroxy deuterated o- and m-cresols. Both cis and trans rotamers were observed in a high resolution molecular beam experiment. The spectra were analyzed using a genetic algorithm assisted automatic assignment. The Hamiltonian used included rotational, torsional and rotation-torsion components. Both, high resolution rotationally resolved spectra and low resolution torsional frequencies, were combined to obtain the rotational constants, the direction of the methyl group axis, and the V3 and V6 barriers to internal rotation of the methyl top in the ground (S0) and excited (S1) states. The lifetime of the S1 state is also reported. Quantum interference effects due to the interaction of the internal and overall rotation allowed for determination of the absolute sign of the angle between transition moment and the a principal axis.