A comparison between microscopic and empirical estimates of the density dependence of effective interactions

The effective interaction W which was recently deduced empirically from saturation considerations as an addition to the Sussex matrix elements is compared with earlier estimates of corrections arising from an assumed short-range repulsion and from the tensor force in second order.     

Self distortion of a Gaussian electromagnetic pulse in a plasma

On the basis of perturbation theory, the phenomenon of nonlinear self distortion of an electromagnetic pulse (Gaussian in time) in a collisional plasma has been studied. The nonlinearity appears through the modulation of electron collision frequency by the power absorption from the wave. At moderate energies of the pulse, considerable distortion in the amplitude envelope has been predicted.     

Phonon spectrum and Kohn anomaly in platinum

The phonon frequencies of platinum are calculated using a model potential approach. The results are in fair agreement with the experimental data but do not show any Kohn anomaly or any kink, as revealed experimentally. The magnitude of the anomaly has been theoretically estimated and comes out to be very small.     

Indirect electronic transitions in alkali halate crystals-II (sodium bromate and iodate) crystals

In the case of NaClO₃ and KClO₃ crystals, analysis of the long wavelength tail of their fundamental absorption revealed the active participation of the internal vibrations of the chlorate ion (Part I). In order to test the validity of the above interpretation the absorption spectra of two more halates with different anions namely sodium bromate and sodium iodate are analysed in a manner similar to that given in Part I. It is found that the principle internal vibrations of bromate and iodate ions are involved in the indirect transitions. The variation of indirect band gap with temperature is found to be ?2·5 × 10^?4 eV/K and ?2·9 × 10^?4 eV/K for sodium bromate and sodium iodate respectively.     

Angular forces in the lattice dynamics of diamond crystal lattices

The lattice dynamics of the diamond lattice has been investigated using a model in which the interatomic forces include, in addition to central forces, the angular forces of the type employed by de Launay and Clark, Gazis and Wallis. The 2 models have been applied to silicon and germanium to investigate the phonon dispersion curves in the three symmetry directions of the Brillioun zone. The results are found to be qualitatively in good agreement with experiments.     

Self-focusing of electromagnetic waves in a magnetoplasma

The steady state self-focusing of a Gaussian electromagnetic beam in a magneto-plasma has been studied. On a short time scale, a non-linearity in the dielectric constant of a plasma appears due to the ponderomotive force. This force in the case of the extraordinary mode has opposite signs forω>ω _c andω<ω _c, whereω _c is the electron cyclotron frequency. The self-focusing due to this effect is predicted at frequencies except forω _c /2<ω<ω _c . The focusing of the ordinary mode is adversely affected by the magnetic field. On a larger time scale, the non-uniform heating of electrons by the beam and the resulting redistribution of the electron density is a source of non-linearity. This non-local non-linearity is several orders of magnitude higher than the ponderomotive non-linearity. We predict self-focusing of the extraordinary mode only above the gyroresonance (ω>ω _c ), while the ordinary mode can be focused at all frequencies.     

Cross focusing of two coaxial spatial modes of a single frequency of a laser beam in a nonlinear dielectric

The cross-focusing of two co-axial transverse modes of a laser beam viz. TEM₀₀ and TEM₀₁ modes of the same frequency in a non-linear dielectric has been studied. When the TEM₀₀ mode is stronger than the TEM₀₁ mode, both modes focus on the axis. In the opposite case, defocusing occurs at the axis and energy concentrates in a ring.     

On electromagnetic and gravitational waves in general relativity. I

In this paper the external field of a bounded source emitting electromagnetic and gravitational radiation has been considered. The field has been assumed to be axially symmetric as well as reflexion symmetric and that it tends to flatness at infinity. The form of the electromagnetic tensor which is in accordance with the classical harmonic electric dipole radiation has been assumed. It has been shown that the whole set of field equations falls into seven main equations together with three supplementary conditions. These field equations have been solved by multipole expansion method and the corresponding Initial Value Problem has been discussed.     

A. C. Electroluminescence of CdS : Sm and CdS : Cu,Sm EL phosphors

Electroluminescent CdS : Sm and CdS : Cu, Sm phosphors have been prepared and their EL characteristics are investigated. The brightness waves of these phosphors were measured at a sinusoidal alternating voltages with a frequency of 500 Hz, 1 kHz, 2 kHz and 5 kHz. One primary and one secondary peak have been observed in each half period of the applied sinusoidal field. Time averaged EL brightness has been found to follow the Alfrey-Taylor relationB= =B ₀ exp (–b/V ^1/2) over a wide range of frequencies. Variation of the constants of this relation with frequency of the applied field have been studied. Variation of current across the EL cell with the applied voltage has also been investigated. The EL emission spectra show peaks at 600 and 650 nm in CdS : Sm phosphor whereas in the case of CdS : Cu, Sm phosphor the emission peak occurs at 600 nm. The observed results have been interpreted in terms of acceleration collision theory of the electroluminescence.One of the authors (RKT) expresses his gratitude to the University Grants Commission, New Delhi, for a fellowship and the Principal, S. D. J. Post Graduate College, Chandesar Azamgarh, for study leave.