  Conventional approaches to probing ultrafast molecular dynamics rely on the
use of synchronized laser pulses with a well-defined time delay. Typically, a
pump pulse excites a wavepacket in the molecule. A subsequent probe pulse can
then dissociates or ionizes the molecule, and measurement of the molecular
fragments provides information about where the wavepacket was for each time
delay. In this work, we propose to exploit the ultrafast
nuclear-position-dependent emission obtained due to large light-matter coupling
in plasmonic nanocavities to image wavepacket dynamics using only a single pump
pulse. We show that the time-resolved emission from the cavity provides
information about when the wavepacket passes a given region in nuclear
configuration space. This approach can image both cavity-modified dynamics on
polaritonic (hybrid light-matter) potentials in the strong light-matter
coupling regime as well as bare-molecule dynamics in the intermediate coupling
regime of large Purcell enhancements, and provides a new route towards
ultrafast molecular spectroscopy with plasmonic nanocavities.
