Line broadening and line shifts in one- and two-photon single-molecule spectra

The temperature and laser-power dependent S-1 <-- S-0 spectra of single dip henyloctatetraene molecules in n-tetradecane are investigated using one- an d two-photon excitation at cryogenic temperatures. Under one photon excitat ion, the single-molecule transitions shift to lower frequencies and the lin e widths increase with increasing temperature while the line shapes are ind ependent of the laser power up to spectral saturation. The experimental dat a are consistent with the model of thermal activation of a single local pho non. Under two-photon excitation, the transitions shift linearly to lower f requencies with increasing laser power while the lines broaden only weakly. These results are discussed in terms of laser-induced ac-Stark shifts and matrix dynamics. In particular, a bottleneck effect in the thermalization o f nonequilibrium phonons is supposed to play an essential role in the obser ved broadening and shifts. The comparison of one- and two-photon excitation spectra provides information about the processes that govern the two-photo n spectra.