Picosecond time-resolved and frequency domain coherent Raman scattering study of conjugated polymeric films: A soluble polydiacetylene, poly-4-BCMU

We report the first study of both time-resolved and frequency domain coherent Raman scattering in a conjugated polymer, specifically a soluble polydiacetylene called poly-4-BCMU. Both CARS and CSRS spectra were recorded at room temperature and at 4 K in the region of -C=C- stretch on a ∼12 μm thick film of poly-4-BCMU in the red amorphous form. The relevant CSRS spectra of the blue crystalline form are also reported. Frequency domain study reveals the vibrational resonance frequency to be independent of the polymer molecular weight in the red form, but different for the blue and red form. The line shapes are asymmetric, but do not seem to fit the predictions of a simple model involving dominant two-photon resonance contributions. The observed vibrational dephasing in the time-resolved study is very fast at both room temperature and 4 K, being within the time-resolution available. From the combined analysis of the frequency domain and time-resolved studies at both room temperature and 4 K, an inhomogeneous mechanism of dephasing is inferred. The inhomogeneous dephasing arises from simultaneous coherent excitation of a distribution of vibrational frequencies originating from a distribution of polymer conjugation length.