  Molecular aggregates on plasmonic nanoparticles have emerged as attractive
systems for the studies of cavity quantum electrodynamics. They are highly
tunable, scalable, easy to synthesize and offer sub-wavelength confinement, all
while giving access to the ultrastrong light-matter coupling regime at room
temperature and promising a plethora of applications. However, the complexity
of both the molecular aggregate and plasmonic nanoparticle introduces many more
processes affecting the excitation and its relaxation, than are present in
atom-cavity systems. Here, we follow the complex relaxation pathways of the
photoexcitation of such hybrid systems and conclude that while the metal is
responsible for destroying the coherence of the excitation, the molecular
aggregate significantly participates in dissipating the energy. We rely on
two-dimensional electronic spectroscopy in a combined theory-experiment
approach, which allows us to ascribe the different timescales of relaxation to
processes inside the molecules or the metal nanoparticle. We show that the
dynamics beyond a few femtoseconds has to be cast in the language of hot
electron distributions and excitons instead of the accepted lower and upper
polariton branches, and furthermore set the framework for delving deeper into
the photophysics of excitations that could be used in hot electron transfer,
for example to drive photocatalytic reactions.
