A new method based on the calculation of autocorrelation functions for
spectra measured at a high acquisition rate is developed to study spe
ctral dynamics of single molecules. The technique allows for spectrosc
opy with time resolutions down to the luminescence lifetime. The metho
d is used to study spectral diffusion in two-photon excitation spectra
of diphenyloctatetraene molecules doped in an n-tetradecane crystal m
atrix. The diffusion is light induced, and is absent in one-photon exc
itation spectra. It has a ''steplike'' time behavior, different from g
radual diffusion observed in glasses.