Optical Relaxation Processes in Polymers at Low Temperatures
The investigation of microscopic parameters of amorphous materials with the means of optical spectroscopy is rather difficult, the main difficulty being the poor optical resolution due to inhomogeneous broadening effects. The nature of the inhomogeneous broadening is rather well understood on a qualitative level. In a typical experiment, one dopes an amorphous polymeric material with molecules absorbing in the electronic bandgap of the polymer host material. The individual molecules will then occupy different ‘sites’; in each site their microscopic environment will be somewhat different. This is symbolically depicted in Fig. 1 for three identical molecules occupying three different sites. Due to ‘crystal field effects’ the absorption energy of the three sites will differ; the molecules will absorb at the frequencies ωl, ω2 and ω3 respectively, as is symbolically depicted in Fig. 1. The frequency spread and, hence, the inhomogeneous bandwidth is a measure of both, the molecule-matrix interaction and the degree of disorder. High quality single crystals are characterized by extremely small inhomogeneous broadening (0.1 – 1 cm−1), amorphous materials show large inhomogeneous broadening effects (100 – 1000 cm−1).
KeywordsHigh Energy Side High Quality Single Crystal Crystal Field Effect Homogeneous Linewidth Ground State Depletion
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