• V Umadevi

      Articles written in Pramana – Journal of Physics

    • Raman study of temperature dependence of lattice modes in calcite

      A K Sood A K Arora V Umadevi G Venkataraman

      More Details Abstract Fulltext PDF

      The temperature dependence of the line width and the peak position of theEg librational mode (of nominal frequency 285 cm−1) and theEg translational mode (of nominal frequency 155 cm−1) in calcite (CaCO3) have been studied by laser-Raman spectrometry. The role of orientational relaxation as a possible process contributing to the line width has been evaluated. It is concluded that reorientations do not play a major part in relation to the present observations. It is further shown that the latter can be understood on the basis of cubic and quartic anharmonic processes. The data also suggest that certain phonon interactions earlier considered insignificant for peak shift in calcite, do contribute significantly.

    • Localized modes of ammonium ion in KCl lattice

      R Kesavamoorthy V Umadevi Debendranath Sahoo

      More Details Abstract Fulltext PDF

      Following the standard Green’s functions matrix partitioning technique, the force constant changes needed to explain the translational (6.8 THz) and torsional (10.1 THz) modes occurring in the KCl:NH4+ system are calculated. Three different defect site symmetries are considered for the ammonium ion impurity. These are (i)Oh, in which the ammonium ion is a free rotor, (ii)Td, in which it is a hindered rotor and (iii)C4v, in which it rotates freely about a N - H ... Cl axis and librates around the other two crystallographic axes.Oh defect symmetry explains only the translational mode, while in the other two symmetries both the modes are explained with reasonable changes in the force constants. It is also shown that the same set of force constant changes explains the local modes in the deuterated sample as well.

    • Infrared studies on ion irradiated quartz

      A K Sood V Umadevi R Kesavamoorthy G Venkataraman

      More Details Abstract Fulltext PDF

      Damage produced inα- and fused quartz bombarded with low energy (∼ 100 keV) D+, He+ and Ar+ ions, has been studied by observing the changes in their spectrum. Besides bulk reflectivity, the attenuated total reflection spectrum has also been studied, the latter with a view to obtaining the surface polariton frequencies. It is observed that for the same fluence, the changes following D+ irradiation are much higher compared to that for Ar+ irradiation. The variation of the surface polariton frequency inα-quartz with the damage energy deposited has the same trend as observed earlier for refractive index. Some annealing studies have also been performed in argon-irradiated samples. These studies indicate that whereas in fused quartz the damaged layer recovers completely, inα-quartz there is a residual amorphization even after annealing. A two-layer model is proposed which gives a reasonable simulation of the observedir properties.

  • Pramana – Journal of Physics | News

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

© 2017-2019 Indian Academy of Sciences, Bengaluru.