Electroabsorption (EA) or Stark effect spectroscopy is a widely used method for characterizing the dopant
molecules in polymer films and the films themselves. In this method the spectrum of a small (~0.1%) absorbance
change induced by a strong (~1 MV/cm) electric field is measured. Analysis of the spectra provides information
about the change in dipole moment and the change in polarizability accompanying an electronic transition. We
measured near-UV absorption and EA spectra of indole-doped poly (methy1 methacrylate) (PMMA) film prepared
by a spin coating method. In the normal-incidence EA spectrum a negative contribution, being proportional
to the absorption, was observed. In the case of magic angle between the electric vectors of the absorbed light
and the applied field this contribution disappeared. These observations have been explained by field-induced
orientation/alignment of the ensemble of polar indole molecules. Since the ground state dipole of indole is nearly
parallel with the transition dipole, the absorption rate of light, which is polarized perpendicularly to the applied
field, should be smaller. Our study demonstrated that the angular movement of indole-sized molecules in PMMA
at room temperature is hindered, but not stopped.
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