Oriented Se6 ring clusters in zeolite single crystals: Polarized Raman microscopy, optical absorption spectra and photo-induced effects

Abstract

Polarized Raman and optical absorption spectra of oriented Se 6 ring clusters incorporated into free spaces of CHA, MOR and AFI zeolite single crystals are studied. The ring orientation was determined via polarization dependence of the bond-bending mode Raman activities which are high in the configuration with both incident and scattered lights polarized parallel to the ring plane. Se 6 rings seem to be compressed in CHA cavities, their original D 3d symmetry remaining preserved with the ring 3-fold axis orientation along the 3-fold axis of CHA. No symmetry distortion and less compression were found for the Se 6 rings in the AFI channels, with their 3-fold axes being oriented along the 3-fold axis of AFI. A noticeable structural distortion is found for Se 6 rings in MOR channels which are oriented by their 3-fold axes along the b -axis of MOR. Two broad peaks at 3.1–3.2 eV and 3.5–3.65 eV with rather clear polarization dependence were found in the optical absorption spectra of Se 6 rings. Photo-induced Se 6 ring fragmentation is demonstrated at illumination with the photon energies of ~1.96 eV and ~2.41 eV at the temperature of ~77 K, formation of Se 2 − (Se 3 ) and Se 8 molecules being observed in CHA and AFI, respectively. • Polarized Raman spectra of inner modes and librations of oriented Se 6 ring clusters in zeolites CHA, MOR and AFI are obtained. • The ring orientation was univocally determined via polarization dependencies of the bond-bending mode Raman activities. • The original D 3d symmetry of the Se 6 ring is preserved in CHA and AFI. While its strong structural distortion is observed in MOR.. • Two polarization-dependent peaks at 3.1–3.2 eV and 3.5–3.65 eV were found in the optical absorption spectra of the Se 6 rings. • Photo-induced Se 6 ring fragmentation at the temperature of ~77 K with the formation of Se 2 − or Se 3 in CHA-Se and formation of Se 8 in AFI-Se is demonstrated..