  Time-resolved coincidence imaging of photoelectrons and photoions represents
the most complete experimental measurement of ultrafast excited state dynamics,
a multi-dimensional measurement for a multi-dimensional problem. Here we
present the experimental data from recent coincidence imaging experiments,
undertaken with the aim of gaining insight into the complex ultrafast
excited-state dynamics of 1,3-butadiene initiated by absorption of 200 nm
light. We discuss photoion and photoelectron mappings of increasing
dimensionality, and focus particularly on the time-resolved photoelectron
angular distributions (TRPADs), expected to be a sensitive probe of the
electronic evolution of the excited state and to provide significant
information beyond the time-resolved photoelectron spectrum (TRPES). Complex
temporal behaviour is observed in the TRPADs, revealing their sensitivity to
the dynamics while also emphasising the difficulty of interpretation of these
complex observables. From the experimental data some details of the wavepacket
dynamics are discerned relatively directly, and we make some tentative
comparisons with existing ab initio calculations in order to gain deeper
insight into the experimental measurements; finally, we sketch out some
considerations for taking this comparison further in order to bridge the gap
between experiment and theory.
