• Volume 45, Issue 1

      July 1995,   pages  1-89

    • Diffusing wave spectroscopy of dense colloids: Liquid, crystal and glassy states

      Subrata Sanyal Ajay K Sood

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      Using intensity autocorrelation of multiply scattered light, we show that the increase in interparticle interaction in dense, binary colloidal fluid mixtures of particle diameters 0.115µm and 0.089µm results in freezing into a crystalline phase at volume fractionφ of 0.1 and into a glassy state atφ=0.2. The functional form of the field autocorrelation functiong(1)(t) for the binary fluid phase is fitted to exp[−γ(6k02Defft)1/2] wherek0 is the magnitude of the incident light wavevector andγ is a parameter inversely proportional to the photon transport mean free pathl*. TheDeff is thel* weighted average of the individual diffusion coefficients of the pure species. Thel* used in calculatingDeff was computed using the Mie theory. In the solid (crystal or glass) phase, theg(1)(t) is fitted (only with a moderate success) to exp[−γ(6k02W(t))1/2] where the mean-squared displacementW(t) is evaluated for a harmonically bound overdamped Brownian oscillator. It is found that the fitted parameterγ for both the binary and monodisperse suspensions decreases significantly with the increase of interparticle interactions. This has been justified by showing that the calculated values ofl* in a monodisperse suspension using Mie theory increase very significantly with the interactions incorporated inl* via the static structure factor.

    • A comparative dielectric relaxation study between hydrogen-and non-hydrogen-bonded liquids: Nitriles vs alcohols

      S N Helambe M P Lokhande A C Kumbharkhane S C Mehrotra

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      Dielectric properties of nitriles have been studied using time domain reflectometry in the frequency range of 10 MHz to 10 GHz over the temperature range of 0°C to 45°C. The calculated dielectric relaxation parameters are compared with values for the corresponding n-alcohols. The dielectric behaviour in n-nitrile systems is quite different from the corresponding behaviour in n-alcohols. The dipoles in n-nitrile systems show a tendency to remain antiparallel, unlike in alcohol systems. The activation energy decreases with the increase of molecular size, whereas in hydrogen bonded liquid it increases.

    • Phonon dispersion in quasiperiodic semiconductor superlattices

      Sujata Rath Prafulla K Jha Sankar P Sanyal

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      The phonon spectra of unstrained and strained quasiperiodic semiconductor superlattices (QSSL) have been calculated using one-dimensional linear chain model. We consider two types of quasiperiodic systems, namely cantor triadic bar (CTB) and Fibonacci sequences (FS), constituting of AlAs, GaAs and GaSb of which the latter two have a lattice mismatch of about 7%. The calculations have been made using transfer matrix method and also with and without the inclusion of strain. We present the results on phonon spectra of two component CTB and two as well as three component FS semiconductor superlattices (SSL), thickness and order dependence on LO mode of GaAs, effect of strain on LO frequency of GaAs. The calculated results show that the strain generated due to lattice mismatch reduces significantly the magnitudes of the confined optical phonon frequency of GaAs.

    • Effect of crystal field on the spin density at the copper nucleus in copper complexes

      V Gomathy U S Ghosh C Basu

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      The role of lower symmetry component of the crystal field in causing a mixing of excited 3dx−1 4s with the ground 3dx configuration and leading to spin density at the nucleus for iron group ions was suggested by Griffith and Orgel. This mechanism has been examined in detail for the two low-symmetry copper complexes, one square planar (D4h symmetry) and the other distorted tetrahedron (D2d symmetry) and the calculation has been performed using the powerful Racah method and tensor operator technique. It is found that for the two types of copper complexes, copper pthalocyanin (square planar, D4h symmetry) and cesium copper chloride (distorted tetrahedron, D2d symmetry) the contribution from this mechanism to the spin density at the nucleus vanishes identically.

    • Triple differential cross sections for ionization of hydrogen atoms by positrons in a Schwinger variational calculation

      J N Das A Dey K Chakrabarti

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      Schwinger variational principle has been used to calculate triple differential cross-sections for ionization of hydrogen atoms by positrons at intermediate and high energies for Ehrhardt type asymmetric geometry. The results agree in general with the calculations of Brauneret al [8] and with the second Born calculation.

    • Bound state population and dissociation dynamics of a Morse oscillator with oscillating well-depth and driven by intense radiation: Perturbative and numerical studies

      Pranab Sarkar S P Bhattacharyya

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      Bound state population dynamics in a diatom modelled by an appropriate Morse oscillator with a time-dependent well-depth is investigated perturbatively both in the absence and presence of high intensity radiation. For sinusoidally oscillating well-depth, the population of themth bound vibrational level,Pmm(t), is predicted to be a parabolic function of the amplitude of the oscillation of the well-depth (ΔD0) at a fixed laser intensity. For a fixed value of ΔD0,Pmm(t) is also predicted to be quadratic function of the field intensity (ɛ0). Accurate numerical calculations using a time-dependent Fourier grid Hamiltonian (TDFGH) method proposed earlier corroborate the predictions of perturbation theory. As to the dissociation dynamics, the numerical results indicate that the intensity threshold is slightly lowered if the well-depth oscillates. Possibility of the existence of pulse-shape effect on the dissociation dynamics has also been investigated.

    • Electronegativity and hardness profiles of a chemical process: Comparison between quantum fluid density functional theory andab initio SCF method

      S Nath P K Chattaraj

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      Temporal evolution of electronegativity and hardness associated with a collision process between a Be atom and a proton has been studied within a quantum fluid density functional framework. In the presence of a third collisional partner to take away excess energy, this collision may lead to a chemical reaction producing a BeH+ molecule. For comparisonab initio SCF level calculation (with 6–31G** basis set) on BeH+ molecule with different geometries have been performed. Electronegativity equalization and maximum hardness principles are analyzed.

    • The inversion potential for NH3 and its isotopic species

      A N Aquino A Palma J L López M A Rosales

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      Usingab initio data of the potential surface for the ground state of NH3 a function is obtained to represent, with a very high precision, the potential along the inversion coordinate. The inversion spectra of NH3, ND3, NH2D and NHD2 are found by numerically solving the Schrödinger equation for this potential. Comparison is made of the calculated inversion frequencies and the experimental values, and the molecular constants of NH3 are also compared to those found in the literature.

    • A low cost PC based card for heat-capacity measurements at low temperatures

      R S Sannabhadti S Ramakrishnan K Ghosh Arun Patade Girish Chandra

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      We describe a simple, low cost pc based card for the measurement of heat capacity using adiabatic calorimetry at low temperatures. This card provides the control pulse to the sample heater as well as trigger pulse to the nano-voltmeter which monitors the sensor voltage (Ge sensor, Lake Shore Inc., USA). We have also added a 12 bit DAC on this card and this is used for remote setting of the heater current of an old SHE (now Biomagnetic, Inc., USA, model CCS) analog constant current source. Although this card is used here for heat-capacity measurements, the same can also be used for thermo-power and thermal-conductivity measurements.

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