Comment: phonon spectrum of molecular crystals from exciton sidebands

Phonon sidebands of excitons in molecular crystals with negligibly small excitation exchange interactions, including isotopic guest systems, are not, in general, expected to be predominantly weighted by translational modes that are predictable from Brillouin zone centre (q = 0) ones. Some phonon sideband data on naphthalene, hexamethylbenzene, etc. are discussed and argued to map the extent of the phonon density-of-states as well as some of the q ≠ 0 singularities.