Temperature-dependent line broadening of chromophores in amorphous solids: Differences between single-molecule spectroscopy and photon echo results

Spectroscopic techniques exhibit different sensitivities for line broadening processes in amorphous solids. Photon echo and hole-burning spectroscopy yield averages over the chromophore ensemble. At low temperatures, the results can usually be fitted with a combination of a power-law term - corresponding to the relaxations of two-level systems - and of an exponentially activated contribution of pseudo-local phonon modes. Single-molecule spectroscopy (SMS), in contrast, can resolve the behavior of single dye molecules and yields a distribution of power laws as well as of activation energies. We compare photon echo results for tetra-tert-butylterrylene (TBT) in polyisobutylene (PIB) with SMS data for the same system. The latter were used to simulate numerically the data which would be obtained in an ensemble-averaging experiment. The results of the numerical calculation can be well fitted without assuming a distribution of parameters. (C) 1998 Elsevier Science B.V. All rights reserved.