Elucidating quantum number-dependent coupling matrix elements using picosecond time-resolved photoelectron spectroscopy.

We measure quantum beating patterns of photoelectron intensity caused by intramolecular vibrational energy redistribution following the excitation of a low-lying ring breathing state in S(1) parafluorotoluene. Analysis of the beating patterns reveals an exceptional sensitivity to details of the evolving wave packet which is found to contain two incoherent components, one of which rapidly dephases. This analysis enables the determination of coupling matrix elements, which are shown to depend strongly on torsional and rotational quantum numbers.