  We have developed a femtosecond ultra-voilet (UV) stimulated Raman
spectroscopy (UV-FSRS) for $N$ molecules in optical cavities. The scheme
enables a real-time monitoring of collective dynamics of molecular polaritons
and their coupling to vibrations, along with a crosstalk between polariton and
dark states. Through multidimensional projections of the UV-FSRS signal, we
identify clear signature of the dark states, e.g., pathways and timescales that
used to be invisible in resonant technique. A microscopic theory is developed
for the UV-FSRS, so as to reveal the polaritonic population and coherence
dynamics that interplay with each other. The resulting signal makes the dark
states visible, thereby providing a new technique for probing dark state
dynamics and their correlation with polariton modes.
