Linear electron-phonon interaction in dye-doped polymers studied by femtosecond accumulated photon echo

Abstract

The purpose of this research is to investigate the linear electron-phonon interaction in dye-doped polymers by using Fourier-transform spectroscopy based on femtosecond accumulated photon echo [1]. This technique enables us to extract the homogeneous spectrum from the inhomogeneously broadened absorption band. Thus obtained homogeneous spectrum, in particular the phonon sideband spectrum is analyzed according to the theory which takes into account the linear electron-phonon interaction. This analysis has revealed that once the phonon sideband spectrum and the Debye-Waller factor are measured at a sufficiently low temperature, the temporal behaviour of the echo at any temperature can be precisely predicted. A typical example is shown in Fig. 1. The sample employed in this experiment is octaethylporphine doped in polystyrene. The Qx(0,0) band was excited by a synchronously pumped kiton red dye laser. The laser bandwidth has exceeded the spectral width of the Qx(0,0) band. Observed temperature dependence of the echo signal and its Fourier-transformed spectrum are shown in Figs. 1(a) and (b),respectively. The clearly resolved vibronic quantum beat is observed around the time origin. The Fourier spectrum indicates that the echo signal is generated from three kinds of optical transitions including zero-phonon line, the phonon sideband located at 15 cm−1 and two vibronic lines at 150 and 280 cm−1. According to the theory which takes into account the linear electron-phonon interaction, we tried to reproduce numerically the temperature dependence of the phonon sideband spectrum. The result is shown in Fig. 1(c), from which the temperature dependences of the zero-phonon line width and the Debye-Waller factor have been obtained. These results indicate that the ultrafast decay of the echo signal observed at higher temperatures is due to the excitation from the thermally occupied phonon state in the ground satae and is not due to the spectral broadening of the zero-phonon line [2]. Further we have found that the peak frequency of the phonon sideband spectrum is an important parameter, which affects seriously the tempearture dependence of the homogeneous spectrum [3]. The polymer dependence of the peak frequency and spectral shape of the phonon sideband spectrum are discussed from the viewpoint of fractal structure in polymers [4]. The low frequency Raman spectrum in polymers from which fracton density of states has been obtained is also presented.