Phonon sidebands of localized excitons in molecular crystals with methyl torsions: Hexamethylbenzene

Fluorescence and phosphorescence phonon sidebands of isotopic mixed hexamethylbenzene crystals at 2°K are presented. The external phonons can be observed separately from the semi-internal (methyl torsion) ones. The nature of the electronic or vibronic state has observable but not drastic effects on the exciton-phonon coupling function. Likewise, the exciton delocalization is of minor importance to the exciton-phonon function in hexamethylbenzene. The coupling between external and internal vibrations is also weak in this system. The exciton-phonon coupling appears comparable for the optical and acoustic phonons in hexamethylbenzene. The phonon sidebands give some of the phonon singularities of the low-temperature crystal.