Photoelectron angular distributions as a probe of alignment evolution in a polyatomic molecule: Picosecond time- and angle-resolved photoelectron spectroscopy of S₁ para-difluorobenzene

We demonstrate that picosecond time-resolved photoelectron angular distributions (PADs) provide a sensitive probe of an evolving alignment in an excited polyatomic molecule. Such an evolving alignment can be caused by pure rotational recurrences or by rotation-vibration coupling. If a molecule is chosen for which the rotational recurrence times are well-known the method provides a means of establishing the mechanism of intramolecular vibrational energy redistribution (IVR). In the case of S₁ para-difluorobenzene we observe striking alignment changes as a function of pump-probe time delay which we attribute to rotationally mediated IVR.