Photon Echo Spectroscopy in Dye-Polymer Systems

Recent experimental results on femtosecond accumulated photon echo in dye-polymer system are reviewed. This talk consists of two parts. The first half is concerned with the linear electron-phonon coupling in dye-polymer systems.1) The aim of this research is to clarify the parameter that is most important in significantly affecting the linear electron-phonon coupling in these materials,2) and to find the materials which have extremely weak linear electron-phonon coupling. In the theory of electron-phonon interaction, two parameters of Huang-Rhys factor and phonon frequency are important to characterize each material. Therefore, this research is closely related to the unsettled problem on the low frequency phonon mode in amorphous materials. It has been found that the peak frequency of the phonon sideband spectrum nearly coincides with the frequency of the boson peak that is observed in the Raman scattering spectrum of polymers,3) and that the Debye-Waller factor is larger for the polymers with higher boson peak frequency, such as hydrogen-bonded polymers. However, there seems to exist a maximum of the boson peak frequencyat around 40 cm-1 in organic polymers. This result explains the reason why it is difficult to develope the PHB memory material that works above the temperature of liquid nitrogen, so long as the dye-polymer systems are employed. As far as the dye-polymer system is concerned, the porphyrin substituted hemeproteins are, to the best of our knowledge, samples with very weak linear electron-phonon coupling.4) It is my opinion that the hydrophobic compartmentalization of the chromophores is the dominant mechanism that markedly reduces the electron-phonon coupling in the iron-free hemeproteins.