Electron paramagnetic resonance studies were conducted on the photoinduced charge transfer and also hyperfine interaction of U⁵⁺ stabilized in photorefractive matrix LiNbO₃. This work deals with: (i) first observation of hyperfine structure due to²³³U (I=5/2) in its pentavalent state at octahedral sites and comparison with other possible site symmetries, (ii) photoinduced charge transfer as observable by EPR and its relevance to photorefractive behaviour of LiNbO₃. The effect of chemical bonding on the hyperfine interaction of 5f¹ configuration was also studied by converting the existing literature data on²³⁵U⁵⁺ to that of²³³U⁵⁺ by standard methods. This suggests that progressive substitution of oxygen by F⁻, in the series UO₆⁷⁻, (UO₅F)⁶⁻ and (UO₄F₂)⁵⁻ drastically decreases the hyperfine coupling constantA_∥, along the local distortion axis. This trend is explained as being due to the absence of ligand ion along the distortion axis at U⁵⁺ site in trigonal LiNbO₃. The effects of illumination by copper vapor laser (CVL) on the intensity of the U⁵⁺ signal was studied in the 10–300K region. The kinetics of decay and restoration of U⁵⁺ was also studied between 10–100K range. The decay kinetics was found to obey double exponential. The reduction of concentration of U⁵⁺ with CVL-illumination and its restoration in the absence of light show that pentavalent uranium takes part in the photorefractive effects in LiNbO₃.

In order to understand the mechanism of optical image storage in photorefractive polyvinyl alcohol (PVA) films, photo-EPR and photoacoustic spectral investigations were carried out on PVA films doped with Cr³⁺ and VO²⁺. The EPR spectrum of Cr³⁺ has shown reduction in intensity onin situ illumination with copper vapor laser (CVL). The decay and recovery of Cr³⁺ signal, with and without CVL illumination respectively, was monitored at different temperatures in 10–300 K region. These were found to obey a double exponential, with one time constant independent of temperature, and the other showing significant temperature dependence. From Τ(T), activation energy for the charge carrier transport in one of the processes was estimated to be 0.016 eV. The PA spectra showed shift towards lower wavelength side on consecutive runs. On the other hand, VO²⁺ doped PVA film has not shown any significant changes in intensity on laser illumination. These observations suggest (i) interaction of PVA matrix with excited Cr³⁺ and (ii) predominant non-radiative relaxation in VO²⁺: PVA system with no change in the oxidation state.

Photo-EPR measurements carried out on Nd³⁺ -doped polyvinyl alcohol (PVA) films have shown that nearly 100% reduction occurs in the intensity of EPR of Nd³⁺ under in situ copper vapor laser (CVL) illumination (510.5 nm and 578.2 nm). The kinetics of decay and recovery were investigated. Photoacoustic (PA) spectra, observed under CVL pump condition had shown that the CVL induced changes were not due to photoinduced valence change, and that the CVL pumping creates highly favorable conditions for non-equilibrium population distribution in the excited electronic states. The complete disappearance of EPR under CVL pumping is attributed either to the possible equalization of population of |+〈 and |−〈 Zeeman components, through the decay of many excited states in the presence of magnetic field or configurational changes around Nd³⁺ shifting the resonance frequency. The former appears less probable in view of the relatively slower recovery of EPR signal.