The first observation to the authors' knowledge of electromagnetic surface waves in a two-dimensional dielectric crystal is reported. By using the coherent microwave transient spectroscopy technique, surface waves are shown to exist at frequencies within the photonic band gap for certain lattice terminations. Energy at gigahertz frequencies is coupled into the surface mode using a prism coupling technique. The experimental results are in excellent agreement with theoretical predictions.
The ultraviolet absorption spectra for polyimide and several polyimide model compounds have been measured and the spectroscopically parametrized CNDO/S3 model is used to provide a detailed quantitative description of the spectra. Analysis of the optical absorption spectrum yields a value of 30° for the phenylimide torsional angle in these systems. In addition, the ultraviolet absorptions are found to be intramolecular charge transfer transitions.
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