High resolution electronic spectroscopy of 4-methylanisole in the gas phase. Barrier height determinations for the methyl group torsional motion.

Rotationally resolved fluorescence excitation spectra of the S(1)<-- S(0) origin band transition of 4-methylanisole have been recorded in the gas phase. The origin band spectrum is split into two subbands owing to tunneling motions along the methyl group torsional coordinate. An analysis of this data provides information about the preferred configuration of the methyl group and the barrier opposing its motion in both the ground and excited electronic states. The results show that electronic excitation has a significant impact on the torsional dynamics of the isolated molecule.