Luminescence and resonant Raman scattering of color centers in irradiated crystalline L-alanine

The color centers in electron irradiated L-alanine crystals are studied by means of resonant Raman scattering and photoluminescence. We find a selective resonant enhancement of some of the Raman-active vibrations. The Raman resonant modes are either localized at the methyl group or belong to lattice modes comprising displacements of the ammonium group in the damaged amino acid molecules. The luminescence intensity wanes with increasing temperature above an onset of ~150 K, indicating a strong interaction of the photoexcited electrons in the defects with the lattice modes. These results are compared with the electron-paramagnetic-resonance absorption brought forth by these defects. A simple model calculation that explains the experimental results qualitatively is presented. It is shown that optical spectroscopy provides a valuable technique to study color centers in organic crystals on account of the spatial information of the wave function that can be obtained by resonant Raman scattering.