Theory of Molecular Polaritons. Application to the Reflection Spectra of Anthracene, 1,5-Bis(dimethylamino)pentamethinium Perchlorate, and Some Other Cationic Dye Crystals

The reflection spectra of anthracene in the 3800 Å region and 1,5-bis(dimethylamino)pentamethinium perchlorate (BDP) in the 4090 Å region are calculated using molecular polariton theory. In the crystal the anthracene transition, oscillator strength f = 0.1, involves weak to intermediate vibronic coupling of molecular exciton states, and the model calculations easily reproduce the main features of the observed reflection spectrum. In contrast, the intense transition (f = 3.31) of the dye BDP involves strong vibronic coupling of molecular excitons, and polariton theory with phenomenological damping constants does not adequately reproduce the “vibrational” structure observed in the reflection spectra. Theory and experiment are reconciled by considering the coupling among and distribution of two-exciton states in which vibronic and vibrational excitations occupy different lattice sites.