  The photochemistry of a molecular ensemble coupled to a resonance cavity and
triggered by a femtosecond laser pulse is investigated from a real-time,
quantum dynamics perspective with the multi-configuration time-dependent
Hartree method. Coherent excitation of a superposition of electronic states in
the ensemble leads to superradiant energy transfer to the cavity characterized
by quadratic scaling with the number of molecules. Electronic decoherence
associated with loss of nuclear wave packet overlap among those states destroys
superradiant energy transfer, returning to a linear regime. For equal pump
laser conditions, the photoexcitation probability per molecule decreases with
increase of the number of molecules inside the cavity. This is caused by a loss
of resonance condition of the laser with the bright electronic-photonic states
of the coupled cavity-ensemble system. Increase of the laser bandwidth restores
the energy transferred per molecule and the trigger probability remains
independent of the number of molecules in the cavity.
