• Volume 34, Issue 6

      June 1990,   pages  461-590

    • Wavelength dependence of phase conjugate reflectivity in absorbing media and thermal grating studies by four wave mixing

      R Krishna Mohan B Raghavendra Prasad C K Subramanian P S Narayanan

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      The development of optical phase conjugation using thermally formed gratings in absorbing media, especially the dye solutions, has besides the high efficiencies, many other characteristics which are worthy of study. Here the wavelength dependence of phase conjugate reflectivity in relation to absorption spectrum of rhodamine-6G and rhodamine-B in water, methanol and ethanol is reported. Further the Bragg condition has been verified for the dominant diffracted beam which shows that the thermal grating behaves like an optically thick grating.

    • Analysis of the absorption mechanism during laser-metal interaction

      B S Yilbas R Davies Z Yilbas A Koc

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      The present study examines the absorption of a laser beam at different wavelengths by a partially-ionized vapour during the interaction mechanism. The applicability of the theoretical models developed is discussed in detail. The interaction of the high- and low-power intensities of a laser beam with plasma is distinguished. It is shown that different metal vapours at similar temperatures and densities have absorption depths which may differ by an order of magnitude. Even more substantial is the difference between the absorption depths of light from different lasers in common use. It is also shown that the free electron temperature becomes significantly different from the heavy particle temperature for power intensities above the critical level which is typically > 1014W/m2. The free electron velocity distribution has an isotropic part which becomes non-Maxwellian for power intensities greater than the critical power intensity.

    • Anisotropic ESR parameters of Cu(II) complexes in solutions and dynamics of solvent coordination: use of “minimum linewidths”

      B S Prabhananda

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      The “minimum linewidths” seen in the ESR linewidths against temperature plots, the dependence of line widths on the63Cu nuclear magnetic quantum numbers and the Hubbard relation provide sufficient number of equations to determine the anisotropic ESR parameters in the case of axially symmetric Cu(II) complexes even when unresolved hyperfine structures make contributions to linewidths. After testing the method by reanalysing the literature data on Cu(II) bis-acetylacetonate, it has been used to obtain the anisotropic ESR parameters in the case of bis-salicylaldehydate of Cu(II). Linewidth contributions from unresolved hyperfine structures associated with the1H of coordinating CHCl3 inferred in these studies, were confirmed by comparing the widths in CHCl3 and CDCl3 under ideal conditions. The temperature dependence of this contribution and the estimate of rate constant at room temperature (∼ 1010 s−1) suggest that the coordinating solvent exchange is diffusion controlled.

    • A quantitative study of diffracted X-ray intensities from type I natural diamond crystals by high resolution X-ray diffractometry and comparison with nearly perfect silicon single crystals

      Krishan Lal S Niranjana N Goswami Ajit Ram Verma

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      Results of accurate measurements of peak and integrated intensities of$$\bar 220,\bar 440,\bar 2\bar 24,\bar 1\bar 13$$, 111 and 333 reflections of natural diamonds of type I and nearly perfect silicon single crystals are reported. Highly monochromated and collimated MoKα1 exploring beam was used. A quadrupole crystal X-ray diffractometer was employed in (+, −, +) and (+, −, +, −) settings. (111) platelets of diamond and silicon crystals with thicknesses of about 1 mm were selected. High resolution diffraction curves, stationary and traverse topographs were recorded. Diffraction curve half widths of diamond and silicon crystals were in the range: 45–200 arc sec and a few arc sec respectively. The experimental values of integrated intensitiesρ for diamond crystals were found to lie between the theoretical values for ideally perfect and ideally imperfect crystals. Experimental values ofρ for silicon were closer to the “perfect crystal” values. This is consistent with the results of diffractometric and topographic evaluation. The peak intensities of all reflections were higher for diamond crystals in comparison to the silicon crystals. The ratioIC/ISi lies in the range 1.3 (111 reflection) to 10.5$$(\bar 1\bar 13)$$ and (333) reflections. This is anomalous and cannot be accounted for by considering the degree of perfection, structure factor and difference in absorption coefficient.

    • Diffraction properties of one-dimensional finite size fibonacci quasilattice

      V S K Balagurusamy S Baranidharan E S R Gopal V Sasisekharan

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      The diffraction patterns from Fibonacci quasilattices have been calculated. Finite-size effects are evaluated for weak and strong peaks. For a smaller number of scatterers (<100) there are fluctuations in the intensities of weak and strong peaks. The fluctuations in weak peaks are greater than that in strong peaks. The fluctuations in intensities of weak and strong peaks near the origin are larger than in the corresponding cases of weak and strong peaks far away from the origin. Small shifts in peak-positions are unexpectedly found, the shifts being proportional toN−3/2 for a large number of scatterers. The diffraction pattern of a one-dimensional crystal and random structure is compared with that of the Fibonacci quasilattice. The strong peaks observed in the diffraction pattern of 1-d crystal show negligible peak-shifts, they being comparable with computational errors even when the number of scatterers is as small as 5. The implications for analysing the experiments are briefly indicated.

    • Generation of aperiodic tilings with fivefold symmetry by the method of intersecting decagons and diffraction from finite size tilings

      S Baranidharan E S R Gopal V Sasisekharan

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      A method for generating aperiodic tilings with five fold symmetry is discussed here. Basic patterns formed within decagons can be used to fill two dimensional space, by matching such suitable patterns. It appears to be possible to generate perfect tilings without retracing already established coordinates imposing conditions at the initial stages of generating them. Various possible ways to generate tilings, when perfectness is not required, are discussed. The calculated diffraction patterns for some representative finite size tilings are shown. There are subtle differences in the intensities of peaks in the diffraction patterns corresponding to different finite size tilings constructed using intersecting decagons. These effects persist for a larger number of scatterers in weak peaks than in strong peaks. They are unaffected by an introduction of systematic disorder. These effects could be termed as the finite size boundary effects. There are also small shifts in the peak positions owing to the finite size effects. The possibility of formation of large approximate square cells in large tilings is shown.

    • Vibrational spectra of Na4P2O7·10H2O

      V Ramakrishnan S Narayana Kalkura P Rajagopal

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      The IR and Raman spectra are measured and analysed for sodium pyrophosphate decahydrate. The spectra are interpreted on the basis of P2O74− ion and water vibrations. The observed results fit with the features predicted for the factor goup model. The appearance of two sets of frequencies in the stretching and bending regions of water suggests the existence of two kinds of water molecules in the crystal. This is confirmed by deuterium substitution.

    • Comments on the Meyer-Neldel rule in amorphous semiconductors

      K L Narasimhan

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      In this paper, we show that the Meyer-Neldel rule can be satisfactorily explained by the temperature dependence of the fermi level. We argue that the universality of the Meyer-Neldel rule can be explained by the fact that the dangling bond is the dominant defect in amorphous silicon.

    • A study of absorption characteristics in polyimide and polyimide fluorocarbon-2 polymer film

      M M Hossain

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      The absorption characteristics of polyimide (PM) and polyimide fluorocarbon (PMF-2) polymer film were studied at temperatures ranging from 20 to 230°C. The glass transition or critical temperature of PMF-2 was found at 160°C. Conduction of PMF-2 was observed due to the upper fluorocarbon layer below the transition temperature while the middle PM layer is important above this temperature. The time-dependent absorption/resorption current does not satisfy the traditional Curie-Von Schweidler law. The experimental results of the absorption characteristics are analysed using a simple R-C circuit.

    • A theoretical study of surface plasmon cross coupling in asymmetric corrugated metal films

      M B Pande S Dutta Gupta

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      We investigate numerically the transmission properties of a thin sinusoidally corrugated metal film bounded by two different dielectrics in the context of the experiment of Gruhlkeet al (1986). We study the dominant contributions from both the propagating and evanescent plane waves. A comparison with the experimental results reveals that the decisive role in cross coupling is played by the evanescent waves emitted by the molecular dipole. We extend our studies to different corrugation amplitudes and widths to show their effect on transmission.

    • Fluorescence quantum yield of rhodamine 6G using pulsed photoacoustic technique

      P Sathy Reji Philip V P N Nampoori C P G Vallabhan

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      Experimental method for measuring photoacoustic(PA) signals generated by a pulsed laser beam in liquids is described. The pulsed PA technique is found to be a convenient and accurate method for determination of quantum yield in fluorescent dye solutions. Concentration dependence of quantum yield of rhodamine 6G in water is studied using the above method. The results indicate that the quantum yield decreases with increase in concentration in the quenching region in agreement with the existing reports based on radiometric measurements.

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