Phonon sidebands of electronic transitions in molecular crystals and mixed crystals

An approximate theoretical description of phonon participation in the electronic transitions of molecular crystals and of mixed crystals is presented, and it is suggested that the ΔD dispersion shift term is a determining factor of the phonon-sideband spectra in triplet and weak singlet exciton spectra. We show that the phonon sidebands map the density of phonon states and do not correspond to discrete transitions to Raman-active states of the crystal lattice. We also show that the spectra of guest molecules should be accompanied by mainly resonance mode phonon sidebands and that the coupling and the selection rules are different for neat and for mixed crystals. Spectra are presented to illustrate some of the main points of the discussion: in particular, the differences between neat crystal and mixed crystal phonon sidebands; the effect of exciton bandwidth on phonon sidebands; the appearance of local modes; and the differences between phonon sidebands in absorption and luminescence spectra.