Femtosecond coherence in poly(p-Phenylene- vinylene) ? polarization beatings and phase-relaxation probed by wavepacket fluorescence interferometry

Coherent, optical excitation transients have been measured in the bottom-states of the dynamically disordered multi-chromophore poly(p-phenylenevinylene) at low phonon temperatures. Interferometric femtosecond (fs) excitation/probing experiments using freely propagating 70 fs pulses have been employed to generate spatially overlapping electronic and nuclear wavepackets. Narrow-bandwidth detection tuned to two vibronic 0→1 S1→S0 fluorescence transitions with discrete arrival-states α, β, respectively, projects-out two distinct site-subensembles from the pulsed target-area of site excitations. By monitoring the interferences of the fluorescence correlations for α- and β-photons on the detector, the coherent superposition can be probed as a typical polarization beating. The fluorescence interferograms show strong damping on an overall time-scale of 200 fs with typically, half-to-single cycle beating waves. The relaxation of coherence is indicative of a predominantly homogeneous site-dephasing and hence, gives rise to the observation of the structural equilibration of the initial Franck–Condon (FC)-excitation, i.e. the formation of the dressed state.