One- and two-photon spectroscopy on single molecules of diphenyloctatetraene

We report investigations on one- and two-photon excitation of single molecules of diphenyloctatetraene in n-tetradecane at cryogenic temperatures. Saturation intensities, count rates and linewidths of single molecules are compared with corresponding ensemble values obtained from statistical fine structure investigations. The single-molecule linewidths under one-photon excitation are generally two to three times narrower than in two-photon excitation spectra. This effect is discussed with respect to saturated spectral diffusion induced by the high power IR light necessary for efficient two-photon absorption