Spectroscopical and chemical manifestations of strong light-matter coupling are subjects of growing interest due to both the underlying fundamental aspects and the promise of interesting applications. In particular, the possibility that the outcome of a photochemical process can be controlled by the geometry and composition of nearby dielectric nanostructures has attracted intense interest. (In this paper we do not consider the possibility that the proximity of metallic interfaces may affect also the outcome of thermal chemical processes without any optical pumping.) Such effects have been suggested and observed in plasmonic cavities, where modification of photochemical process reflects surface-induced local modification of the electromagnetic field and/or the opening of new relaxation channels, as well as the plasmon-induced formation of hot electrons. In more recent studies performed in Fabri-Perot cavity configurations such modifications are small, and photochemistry appears to reflect mostly the excitation of polaritons -hybrid molecule-light states, where strong light-matter coupling is associated with the collective optical response of a large number of molecules. Polaritonic response of such molecules-optical cavity systems is usually manifested in the observation of Rabi splitting in absorption, emission, scattering, or reflection-transmission of the light, indicating the formation of polaritons -hybrid light-matter states. The collective nature of this interaction is expressed by the linear dependence of this splitting on the square root of the molecular density. The question as to whether this collective response is manifested in other molecular processes in cavity environemnts has been discussed both with respect to vibronic strong coupling phenomena, where cavity mode(s) are strongly coupled to electronic transitions and purely vibrational strong coupling where the cavity resonates with vibrational motions. Here, we focus on the former class of phenomena, noting that while theories associated with the behavior of a few (1-3) molecules are clear, strong coupling in Fabri-Perrot configurations arises from collective molecular response. Theoretical treatments of such situations have heretofore applied the the Holstein-Tavis-Cummings (HTC) model (1) which combines the Tavis-Cummings (TC) model of 2-level atoms interacting with an electromagnetic field mode with the Holstein polaron model to include coupling of a molecular electronic transition with a cavity mode on the one hand and a molecular (harmonic) vibration on the other. In Eq. ( ), each molecules is a two-electronic-state entity and (in the minimal version of the model) a single exciton case is considered and direct intermolecular interactions are ignored. The operator ( ) annihilates (creates) a photon of a cavity mode of frequency while and respectively affect the upward and downward transitions between the lower and upper electronic states of molecule , is the molecular electronic transition energy and is the molecule-cavity mode interaction matrix element, taken to be the same for all molecules. The above are parameters of the TC Hamiltonian.