Resonant Raman-scattering spectroscopy of polydiacetylene films at high pressure

We studied the electronic energies and phonon frequencies of polydiacetylene 4-BCMU thin films at high pressures up to 50 kbar using Raman scattering under preresonant and resonant conditions. Pressure-induced changes in the resonant Raman scattering (RRS) include (1) hardening of the most strongly coupled phonon frequencies, (2) dispersion of the RRS frequencies with the laser excitation ${\mathrm{\ensuremath{\omega}}}_{\mathit{L}}$, which shift to lower energies in analogy with the red shift of the absorption band at high pressures, and (3) softening and narrowing of the phonon distribution. These observations are consistent with an increase in the average conjugation length with pressure. Amplitude-mode-model analysis of the RRS data shows that the bare phonon frequencies increase with pressure, but otherwise the one-dimensional Peierls-type relation between the phonons and the electronic gap changes very little with pressure.