Electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL) studies were conducted onγ-irradiated CaSO₄:UO₂²⁺ to elucidate the role of the electron/hole traps in thermally stimulated reactions and to obtain the trap parameters (trap depth and frequency factor). Intense TSL glow peaks around 140, 375, 400 and 438±2K are observed and their spectral characteristics have shown that UO₂²⁺ and UO₆⁶⁻ act as luminescent centres. EPR studies have shown the peaks at 140 and 400/438K to be associated with the thermal destruction of O⁻ and SO₄⁻ radical ion in two stages respectively. The maximum rate of thermal destruction of SO₄⁻ ions (as seen by EPR) in various alkaline earth sulphate matrices investigated in our laboratory is also summarized. The activation energy which characterizes the electron transfer reaction between SO₄⁻ and the dopant ion lies in the range of (0.95±0.15 eV). This value is independent of the dopant and therefore seems to be characteristic of the binding energy of hole in the SO₄⁻ radical ion.

Electron paramagnetic resonance [EPR] and thermally stimulated luminescence [TSL] studies were conducted on self [α]-irradiated²³⁹Pu doped calcium chloro phosphate andγ-irradiated²³⁹Pu/²³⁸UO₂²⁺ doped calcium chloro phosphate to elucidate the role of the electron/hole traps in thermally stimulated reactions and to obtain trap parameters from both TSL and EPR data. TSL glow peaks around 135 K (# peak 1), 190 K (# peak 2), 435 K (# peak 5) and 490 K (# peak 7) were observed and their spectral characteristics have shown that Pu³⁺ and UO₆⁶⁻ act as luminescent centres in calcium chloro phosphate with respective dopants. EPR studies have shown the formation of the radical ions H⁰, PO₄²⁻, O⁻, O₂⁻ and [ClO]²⁻ under different conditions. Whereas the [ClO]²⁻ radical being stable up to 700 K, was not found to have any role in TSL processes, the thermal destruction of other centres was found to be primarily responsible for the TSL peaks observed. The trap depth values were determined both by using the TSL data and also the temperature variation of EPR spectra of these centres.

Thermally stimulated luminescence (TSL) studies of gamma-irradiated uraniumdoped K₂Ca₂(SO₄)₃ revealed two glow peaks around 400 K and 435 K. Electron paramagnetic resonance (EPR) studies carried out on these samples have shown the formation of the radical ions SO₄⁻, SO₃⁻, SO₂⁻ and O₃⁻. From the study of the thermal stabilities of these radical ions, it was found that the thermal destruction of SO₂⁻ and SO₄⁻ radical ions are associated with the glow peaks observed around 400 K and 435 K respectively. Uranate ion was identified as the luminescent centre for the observed TSL glow. The trap depth values for the glow peaks have been determined from TSL data.

In the fluorinated La₂CuO_4−x prepared using a solid state reaction with NH₄HF₂ as a fluorinating agent at 550 K at ambient pressure, superconductivity was detected by microwave and EPR techniques with aT_c of 35 K.

The electron/hole trapped centres created during internal irradiation in²³⁹Pu doped K₂Ca₂(SO₄)₃ were investigated using electron paramagnetic resonance (EPR), thermally stimulated luminescence (TSL) and photoacoustic spectroscopic studies (PAS). These techniques were used to identify the defects and characterize the thermally induced relaxation processes. TSL studies of self (α)/γ-irradiated²³⁹Pu doped K₂Ca₂(SO₄)₃ revealed two glow peaks around 400 and 433K. Plutonium introduced as Pu⁴⁺ was partly reduced to Pu³⁺ due to self irradiation. This was ascertained from PAS studies. EPR studies carried out on these samples showed the formation of radical ions SO₄⁻, SO₃⁻, O₃⁻, etc. The thermal destruction of SO₄⁻ ion was found to be associated with the prominent glow peak around 433K. Pu³⁺ was found to act as luminescent centre for the observed TSL glow. The trap depth for the glow peak at 433K has been determined from TSL and EPR data.

Electron paramagnetic resonance (EPR) evidence is presented for the radiation stabilization of pentavalent uranium in CaO matrix. From the theoretical predictions ofg value for U⁵⁺ in axial symmetries, it was concluded that U⁵⁺ at Ca²⁺ site is associated with a second neighbour charge compensating Ca²⁺ vacancy. EPR measurements also revealed the presence of Mn²⁺, Mn⁴⁺ and Cu²⁺ impurities in the samples. The thermal stability of U⁵⁺ was investigated using EPR and thermally stimulated luminescence (TSL) techniques. The TSL and EPR studies on gamma irradiated uranium doped calcium oxide samples had shown that the intense glow peak at 540 K is associated with the reduction in the intensity of EPR signal of U⁵⁺ ion around this temperature. This peak is associated with the process U⁵⁺+hole→U^6+*→U⁶⁺+hv. The activation energy for this process was determined to be 1.4eV.

An impurity mediated mechanism of photorefractive effect in BaTiO₃ is proposed. The photoinduced changes in the relative concentration of Fe³⁺ in BaTiO₃ results in an electro-optic coupling through a combination of the Sangster and piezoelectric effects. This is based on the examination of the extensive results on the EPR of Fe³⁺ in the BaTiO₃ lattice. This model explains the improved photorefractive behavior of BaTiO₃ on doping with Co²⁺.

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.

Photo-induced charge transfer and its kinetics were investigated in Bi₁₂SiO₂₀ in 10–300 K temperature range, using EPR of Fe³⁺ centre, underin situ illumination with copper vapour laser (CVL). The decay kinetics was found to follow double exponential behaviour. Relaxation of the photo-induced electron transfer to the preillumination condition occurred even at 10 K. Shallow traps were, therefore, associated with the electron trapping, leading to a better understanding of the fast photorefractive response of BSO.

With a view to understanding the role of photo-induced valence changes of impurities in BaTiO₃ in the phenomena of photorefraction, EPR experiments were conducted under in situ He-Ne laser illumination. These experiments gave evidence for photoinduced valence change of Fe in BaTiO₃ at room temperature. The EPR signal due to trivalent iron was found to reduce in intensity with laser illumination The kinetics of the valence change has been investigated. Under large fringe width condition, the time constant of the decay is identified as the dielectric relaxation time τ_d. The changes in line shape on laser illumination to Dysonian form, appeared most predominantly in mechanically poled crystal compared to electrically poled single domain crystals. This demonstrated the possible role of domain walls and the defects there, as source or sinks of charge carriers on photo excitation. It is observed, that there is transient growth of Fe³⁺, when the laser illumination was put on, before its decay. This was attributed to charge transfer between electrons in oxygen vacancies and Fe⁴⁺. This predicted the growth of a transient grating under depleted pump condition in a two beam coupling experiment. This was experimentally proved by following the diffracted signal of the reading beam under the depleted pump condition.

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.