The lattice dynamics of GaO has been studied on the basis of the three body force shell model, which takes into account the effect of many body interactions in the lattice potential. The dispersion curves obtained by plottingω vsq agree fairly well with the experiments. It is concluded that the value of the molecular electronic polarizability of the solid must be must small than that determined experimentally which suggests that the interaction system in the solid may have a substantial covalent character.

The fractional ionic character of alkali and silver halide crystals is defined in terms of the deviations from the additivity rule for polarizabilities of ions. The electronic polarizabilities of ions are calculated using an empirical relationship according to which the electronic polarizability of an ion can be assumed to be directly proportional to the cube of its radius. The calculated ionicities indicate that the alkali halides are nearly or more than 90% ionic and silver halides are much less ionic which is also evident from the Phillips ionicity scale.

This paper addresses itself to the problem of thermoacoustic generation by an obliquely incident laser beam in a body of water. Far-field directivity pattern of thermoacoustic source (TS) has been studied analytically and numerically. Our study suggests the possibility of getting highly directional sound beams in a simple configuration.

The paper presents an analysis of ultrasonic wave generation in water by intensity modulated elliptical Gaussian laser beams. It is found that generated acoustic radiation is highly directional both in polar and azimuthal directions. An increase in the assymmetry of the transverse intensity distribution from the Gaussian dependence enhances the source directivity considerably. An obvious conclusion is that elliptical Gaussian beams are better choice in applications where it is desired to communicate in an approximately prescribed small solid angle.