Interference of optical coherent transients in pentacene/p-terphenyl: Femtosecond beats probed by correlated fluorescence fluctuations

By using two-pulse excitation with phase-randomized pulses in an interferom eter and analyzing the variance of the fluctuating fluorescence intensity, femtosecond beatings have been observed in the binary crystal pentacene/p-t erphenyl, P/pTP at low phonon temperatures. The THz oscillations evolve fro m fluctuating quantum interferences prepared by the coherent superposition of different, electronic transition dipoles in spatially different constitu ents (O1-O4) of the pentacene impurity. The polarization beats can be expla ined by a model of uncoupled two-level systems including inhomogeneous deph asing as the predominant coherence relaxational pathway.