OPTICAL SPECTROSCOPY OF EU3+-DOPED SILICA-GELS

Laser techniques have been applied to study the spectroscopy of Eu3+ in silica gels. Site selection spectroscopy, resonant fluorescence line narrowing and lifetime measurement show a continuous evolution of the system as a function of the thermal treatments which produce the gel-to-glass transition. Two different environments for the Eu3+ ion, a liquid-like and a dry environment, have been observed during the initial steps of the dehydration process. At heat treatments below 250 degrees C the non-radiative relaxation channels due to coupling with the O-H vibrational modes are very efficient. As the densification continues, clustering of Eu3+ induces strong energy transfer also at a relatively low concentration of the rare-earth ions. Site selection by laser excitation is indeed efficient only for non-densified samples. Dynamic interactions are site dependent as shown by the temperature dependence of the homogeneous linewidth taken at different energies within the D-5(0) <-> F-7(0) line.