Frenkel Exciton Dynamics in Anthracene under High Pressure and Quasi-Free Exciton State

The polarized absorption and luminescence spectra of anthracene crystals have been investigated under high pressures and at room temperature. A satellite absorption band which shifts to lower energy with increasing pressure is found to be related with the vibronic luminescence series. As the origin of the vibronic luminescence a model of a quasi-free exciton is proposed. The vibronic luminescence bands loose their intensity with pressure and are replaced by intrinsic self-trapped exciton luminescence at pressures higher than about 18 kbar. Self-trapping of excitons seems to be enhanced by the second order phase transition which occurs at 24 kbar. The exciton-phonon coupling constant at 22 kbar is estimated to be at least 1.6 which should be compared to 0.85 at ambient pressure.