• R Kesavamoorthy

      Articles written in Pramana – Journal of Physics

    • Localized modes of ammonium ion in KCl lattice

      R Kesavamoorthy V Umadevi Debendranath Sahoo

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      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

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      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.

    • Structure factor of charged colloidal suspensions using Brownian-dynamics simulation: Comparison of Yukawa and Sogami pair potentials

      B V R Tata A K Sood R Kesavamoorthy

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      Static structure factorsS(Q) are obtained for dilute charged colloidal systems using Brownian dynamics simulation method for the widely used screened Coulomb repulsive Yukawa potential and the recently proposed Sogami pair potential. The latter potential has, in addition to the usual repulsive part, an attractive term which is necessary to understand the reentrant phase transition reported in these colloids. It is shown for the first time thatS(Q) obtained using the Sogami potential for parameters favourable for liquid-like order agrees well with that measured experimentally. Thus it appears that the Sogami potential explains features of a homogeneous liquid as well as phase separated states, whereas Yukawa potential does not.

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