Molecular Spectroscopy and Dynamics below the Doppler Width

The density of states in polyatomic molecules is so high that many spectroscopic transitions are only observed as large groupings even in high-resolution spectroscopy. This is due to the fact that many of these states are buried beneath the Doppler width of the optical transition. We here discuss new sub-Doppler experiments for optical transitions in molecular spectra. For benzene we have obtained a resolution of less than 80 MHz, more than one order of magnitude below Doppler width. From these measurements precise rotational constants of the excited state are reported for the first time. Doppler-free experiments were also performed with biacetyl when measuring the molecular quantum beat in the decay after fluorescence excitation in a supersonic jet. This technique permits the spectroscopy of nearby coupled states well below the Doppler limit and also below the bandwidth of the excitation laser.