• J Ramakrishna

      Articles written in Pramana – Journal of Physics

    • Temperature dependence of chlorine NQR in 2-chloro 5-nitrobenzoic acid and 4-chloro 3-nitrobenzoic acid

      V S S Sastry J Ramakrishna

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      Temperature dependence of chlorine nuclear quadrupole resonance in 2-chloro 5-nitrobenzoic acid and 4-chloro 3-nitrobenzoic acid has been investigated in the region 77° K to room temperature. No phase transition has been observed. The results are analysed to obtain the torsional frequencies and their temperature dependence. A nonlinear temperature dependence is obtained for the torsional frequencies.

    • A simple and inexpensive two channel boxcar integrator

      R K Shenoy J Ramakrishna K R Jeffrey

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      A two-channel boxcar integrator with an analog to digital converter was constructed using integrated circuits wherever convenient. The digital output can be instantaneously displayed or displayed after accumulating many samplings in the totaliser. The totaliser mode provides averaging at the digitiser level and hence the integrator has an infinite holding time. When used in the double boxcar mode the instrument overcomes the problem of any base line instability.

    • Zeeman effect of NQR in 4,6-dichloropyrimidine and 6-chloro 2,4-dimethoxypyrimidine

      V S S Sastry J Ramakrishna

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      Zeeman (35Cl) NQR studies in polycrystalline samples of 4,6-dichloropyrimidine and 6 chloro 2,4 dimethoxypyrimidine show that the asymmetry at the four chemically inequivalent chlorine sites in the former is about 10%, while in the latter (one line) the asymmetry is almost zero. Using a valence-bond picture, C-Cl bonds in 4,6-dichloropyrimidine have been characterised, and the results are also compared with those in a corresponding benzene compound using a simple molecular orbital calculation. The axial symmetry of C-Cl bond in the second compound has been attributed to mesomeric effects.

    • A solid state pulsed NMR spectrometer

      R K Shenoy J Ramakrishna K R Jeffrey

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      A 10 MHz pulsed NMR spectrometer, built using mostly solid state devices, is described. The pulse programmer provides 2-pulse, 3-pulse, saturation burst and Carr-Purcell sequences both in repetitive and manual modes of operation. The transmitter has a maximum power output of ∼ 2 kW with a 75 Ω output impedance termination. The total gain of the receiver system is around 120 dB with a minimum band width of 2 MHz. The recovery time of the receiver is ∼ 7 µsec. A two-channel boxcar integrator capable of working in the single channel, differential and double boxcar modes provides signal to noise ratio improvement. The sensitivity and the linearity of the boxcar integrator are ∼ 2 mV and ∼ 0.1% respectively.

    • Proton magnetic relaxation in (TMA)2HgBr4 and (TMA)2HgI4

      B S Sarma J Ramakrishna

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      Internal motions of the protonic groups have been studied in polycrystalline [(CH3)4N]2HgBr4 and [(CH3)4N]2HgI4 from the temperature dependence of proton spin relaxation time (T1) and the data analysed according to the spin lattice relaxation model due to Albert and coworkers. The temperature dependence ofT1 in the above compounds is compared with that in (TMA)2HgCl4 and (TMA)2ZnCl4.

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