Probing phase relaxation by measuring fluorescence interference: polarization beating and electron-phonon coupling in conjugated polymers

We report on the measurement of coherent optical-excitation transients in the low-energy states of the strongly disordered multi-chromophore poly(para-phenylene-vinylene) at low phonon temperatures. Interferometric femtosecond (fs) excitation/probing measurements, employing freely propagating 70 fs pulses have been used to generate spatially overlapping electronic and nuclear wavepackets. High-resolution detection in combination with the unique spectral features of the sample was used to project-out two vibronic fluorescence transitions with discrete arrival states α and β. By monitoring the correlations of the α- and β-photons on the detector, the coherent superposition can be probed as a typical polarization beating. The recorded fluorescence interferograms show strong damping on a timescale of 200 fs. The relaxation of the coherent signal is indicative of, mostly, homogeneous site-dephasing and hence, gives rise to the observation of the structural relaxation of the initial Franck-Condon excitation, i.e. the formation of the dressed state