• N K Porwal

      Articles written in Pramana – Journal of Physics

    • Electron paramagnetic resonance studies in photorefractive crystals I: Hyperfine interaction and photoinduced charge transfer in233U5+ and238U5+ doped LiNbO3

      N K Porwal Mithlesh Kumar M D Sastry

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      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.

    • Electron paramagnetic resonance studies of photorefractive crystals II: Fe3+ doped Bi12SiO20 with copper vapour laser illumination in 10–100 K range

      N K Porwal R M Kadam Y Babu M D Sastry M D Aggarwal Putcha Venkateswarlu

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      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.

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