Photoelectron angular distributions as a probe of alignment evolution in apolyatomic molecule: Picosecond time- and angle-resolved photoelectron spectroscopy of S-1 para-difluorobenzene

We demonstrate that picosecond time-resolved photoelectron angular distribu tions (PADs) provide a sensitive probe of an evolving alignment in an excit ed polyatomic molecule. Such an evolving alignment can be caused by pure ro tational recurrences or by rotation-vibration coupling. If a molecule is ch osen for which the rotational recurrence times are well-known the method pr ovides a means of establishing the mechanism of intramolecular vibrational energy redistribution (IVR). In the case of S-1 para-difluorobenzene we obs erve striking alignment changes as a function of pump-probe time delay whic h we attribute to rotationally mediated IVR. (C) 1999 American Institute of Physics. [S0021-9606(99)00828-4].