Line widths of the 3800 Å singlet transitions in crystalline anthracene

The theory of resonance coupling of Frenkel excitons and phonons is extended to two molecules per unit cell. Intra- and inter-band scattering for the two Davydov components can thus be dealt with on the same footing. Restrictions to nearest neighbour interactions have been removed. Application to anthracene shows that the contribution of librational phonons to the scattering is approximately one hundred times larger than that of translational phonons. The widths of the anthracene components are shown to fit the calculated coupling and known phonon frequencies satisfactorily.