Line “narrowing” in strong optical fields

Fluorescence of single molecules coupled to a pseudo localized vibration (PLV) and a thermal bath is studied both experimentally and theoretically. The coupled system molecule-PLV is described by master equations which are tested in an experiment where a shape of an excitation spectral line of a single molecule is accurately measured over six orders of magnitude of the exciting laser power. Significant deviations from the predictions of the standard Bloch equations are found but the master equations work very well. The role of critical parameters—the correlation time of the bath, the Rabi oscillation period, and the coupling constant between the bath and the molecule is discussed.