For a qualitative understanding of this observation consider the ideal case where the Condon approximation is valid so that the electronic excitation does not change the vibrational state. Let an Nmolecules system start in its ground electronic/vibrational state, where and denote the ground electronic state and the corresponding vibrational state of molecule j. A sudden single photon absorption creates an excitation (an exciton outside and a polariton inside the cavity) that involves a molecular bright vibronic state of the form (17) where, using for the electronically excited molecular state,