The longitudinal relaxation timeT₁ and the transverse relaxation timeT₃ for the inversion levels of the NH₃ molecule have been calculated for NH₃ — NH₃, NH₃ — N₂ and NH₃ — H₂ collisions. The ratioT₂/T₁ lies between 1 and 2 which agrees well with the experiment. The phase shift effect onT₂ and on the collisioninduced line shift δω under the framework of the Mehrotra-Boggs theory have also been evaluated. The change in the line intensity ΔI/I, in four-level double resonance experiments was also calculated, and the significance of higher order interaction terms has been pointed out.

We present differential and integrated cross-sections for elastic scattering of positrons by helium and lithium at intermediate and high energies, calculated using the two-potential eikonal approach. The present calculations are compared with the available theoretical results.

A new oscillator model has been proposed by introducing some modifications in the Morse potential function. Its efficacy is tested by taking a number of electronic states of diatomic molecules. For comparison the Hulbert-Hirschfelder model potential is also used. A new approximation method to find the vibrational eigenfunctions suitable for the new oscillator model has been reported. Langer’s method has been used to determine the wavefunctions. Franck-Condon factors andr-centroids are reported for the observed bands ofD¹Π —X¹Σ system of SnO molecule.

A new method, that systematically combines results of random matrix theory and the usual statistical mechanics, is described to study thermodynamic properties of disordered systems. Two exactly solvable models are examined in this formulation to illustrate the usefulness of this method for systems described by random as well as non-random Hamiltonian.

Theoretical and experimental investigations of metal-jets, produced by interaction of a plane shock wave with a conical cavity at free surface of metals like aluminium, copper and mild steel, have been carried out and an attempt has been made to correlate the jet velocity with shock parameters. The velocity of the jet, obtained from the cavity of different half angles have been mostly measured with oscillographic technique. The experimental results agree with theory within experimental variations. The velocity of the jet increases monotonically with the particle velocity but decreases with increase in the angle of the cavity.

The second order pseudopotential theory suggests the possibility of a break-up of the total energy of simple metals into a purely volume-dependent part and an effective central pairwise interaction between ions. In the present paper finite contributions for these two parts of the energy have been extracted in a form convenient for calculation. Using the local Heine-Abarenkov model potential, a reliable effective ion-ion interaction is generated and the volume-dependent energy is calculated for Al. The relative contributions of the effective interaction and the volume-dependent energy term to various metallic properties are also calculated. The importance of volume dependence on the effective interaction is also discussed.

The interactomic force constants upto eighth neighbour are derived from the effective interaction and it is found that the force constants beyond the third neighbour are negligibly small. This result is also confirmed by the calculation of dispersion curves with force constants obtained from the effective interaction upto the third neighbour which is found to reproduce the results of the full pseudopotential calculations. The force constants obtained are also used to study some finite temperature properties of Al in the quasi-harmonic approximation and the limitations of the theory are pointed out.

Diffusex-ray scattering in the region between the Bragg peaks has been calculated for self-interstitials in zinc. The required lattice distortions are calculated by the Kanzaki method and the scattering contributions from all equivalent orientations of a defect configuration are averaged. It is shown that a measurement in the (110) plane can clearly establish the stable interstitial configuration.

Electric field gradient tensors at the bridging chlorine sites in niobium pentachloride dimer have been evaluated using point-charge model with de Wette’s planewise summation method for rapid convergence of the lattice sums. The results are discussed based on the experimental results and the existing theory on dimeric molecules.

The aim of this paper is to study the scattering of the two ground state¹⁶O nuclei, using classical microscopic approach. We have studied fusion cross-section (σ_F) for various incident energies, its energy variation with time, and also other aspects such as shape deformation in head-on collision, life-time of resonance-scattering etc. Our calculations indicate that for projectile energies of the order of 1–2 MeV per nucleon (low energies) classical microscopic calculations for heavy ion reactions seem to give satisfactory description. However, at very low energies (less than 1 MeV/nucleon) appreciable deviations are seen which may indicate the breakdown of classical approximations.

A general method of dealing with the photocount distribution is proposed when dead time effects are to be taken into account. The clipped correlations of photocounts are investigated in detail and pointwise bounds for the correlation functions are explicitly provided when the dead times are not necessarily small compared to the sample time.

An attempt is made to determine the response of CR-39 and cellulose nitrate plastic track detectors subjected to thermal neutrons. The α-particles are produced from (n, α) reactions in lithium tetraborate convertor placed in contact with different plastics and are recorded in the detectors. The corrected track density gives a fluence sensitivity and dose sensitivity of the order of 10⁻⁴ tracks per neutron and 10² tracks/cm² mrem respectively. A linear relationship is observed between track density and neutron fluence.