• Volume 15, Issue 1

      July 1980,   pages  1-115

    • Time-resolved laser saturation spectroscopy. Theory of the free induction decay of two-level saturation resonances

      Sunil Kumar Martial Ducloy

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      We give the theory of the free induction decay of two-level saturation resonances in the time-resolved laser saturation spectroscopy. The saturating and probe fields may be either co- or counter-propagating. The change-signal line-shape is analysed in detail in two cases of practical importance, namely when the saturating field is either strong or weak. The time evolution of the change-signal exhibits many interesting features which include Ramsey-type fringes, oscillatory decay, narrow resonances, line-narrowing etc. It is shown that by analysing the decay of the change-signal in time one can distinguish (i) various laser interaction processes such as population effects, coherence effects, three-photon Raman-type process, dynamic Stark splitting, power-broadening etc., and (ii) various relaxation mechanisms such as phase-changing collisions, etc.

    • Crucial role of the magnetic field in the evolution of life

      U R Rao

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      The formation of a steady ozone layer in the earth’s atmosphere is the most significant event in the evolutionary cycle of the earth which, in turn, has been responsible for the development of life with an oxygen metabolism. In addition to protecting biological life from exposure to ultraviolet radiation the ozone layer has also been responsible for maintaining the water and oxygen balance in the atmosphere. It is argued that the magnetic field of the earth is really responsible for the formation of this steady ozone layer in the earth’s atmosphere. Because of the earth’s magnetic field and associated trapped charge particle belts and the magnetosphere, the earth’s atmosphere does not directly interact with the interplanetary space. Without such a shielding, the free oxygen atoms could have been depleted considerably causing a severe depletion in the ozone concentration to start with. The impact of charged particles from galactic and solar cosmic rays over the entire earth’s atmosphere and the consequent production of NOx would have given rise to a major ozone sink, if earth were devoid of a magnetic field. The net result would have been the absence of a steady ozone layer and the absence of life with an oxygen metabolism, as in the case of the atmospheres of Venus and Mars, if the earth did not have a magnetic field.

    • On the joint eigenvalue distribution for the matrix ensembles with non zero mean

      Nazakat Ullah

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      Exact distributions are given for the two-dimensional case when the mean of the off-diagonal element is non-zero. The joint eigenvalue distribution for theN dimensional case, derived using the volume element in the space ofN ×N orthogonal matrices, is checked by rederiving the exact results forN=2. The smooth nature of theN-dimensional joint distribution supports the claim of the method of moments that the single eigenvalue distribution is a smooth function of the ratio of mean-to-mean square deviation.

    • Relativistic star clusters with high central redshift

      M C Durgapal P S Rawat R Banerji

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      Collisionless star clusters in dynamical equilibrium are of current interest in general relativity and astrophysics. A step-function distribution is chosen for star clusters. The corresponding equation of state is analogous to a Fermi-gas equation. These clusters are found to be pulsationally unstable for a central redshift ofZc ⩾ 0·54. Further, a model of clusters is developed in which the core has an extremely relativistic equation of state. These structures are unstable forZc ⩾ 2·55 when we use Chandrasekhar’s technique to study their pulsational stability.

    • Ab-initio effective potentials in crystal Hartree-Fock theory

      Sambhu Nath Datta

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      Ab initio effective potential technique of Ewiget al is applicable directly to crystal Hartree-Fock formalism provided the effective potentials are suitably defined. Corresponding to every atom or every molecule by constructing a crystal one can assign a unit species configuration. The effective potentials in the crystal can be expressed as functions of lattice parameters in terms of integrals over the orbitals of this unit species. These expressions are in a more exact form than those usually employed in molecular calculations.

    • Resistance changes in thin metallic films under ion bombardment

      S B Ogale S V Ghaisas A S Ogale V N Bhoraskar M R Bhiday

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      Resistance changes in thin films of copper, aluminium and bismuth have been studied under the bombardment of nitrogen, carbon and argon ions. Variations in resistance with implantation dose have been observed upto doses of ∼ 3 × 1017 ions/cm2 for ion energies in the range 40 to 120 keV. The results are discussed in terms of desorption of gases from the film and a composite action of sputter removal of the film and its structural changes upon ion bombardment. A simple theoretical model is discussed which can qualitatively explain the experimental observations.

    • Electrical second harmonic generation by TGSe and TGS-Se in the autostabilized state

      S H Chavan R N Patil

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      Second harmonic generation and TANDEL effect studies on ferroelectric triglycine selenate and triglycine sulphate-Selenate crystals near the Curie temperature indicate that the generated second harmonic is linear for low biasing fields with a zero off-set, while it decreases sharply at higher biasing fields. In the autostabilized state, the TANDEL elements adjust their impedance against the variation of a.c. field. The experiments on annealed crystals establish that the zero offset is due to the internal bias that owes its origin to the defect structure.

    • Decay characteristics of SrS:Nd phosphors

      M G Patil R D Lawangar

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      The phosphorescence decay of a series of strontium sulphide microcrystalline phosphors prepared with varying amounts of neodymium as an activator has been studied at room temperature. The decay obeys the relationI =I0t−b withb lying between 0·35 and 0·98. The trap depths have been evaluated by peeling off logI-t curves. The results show that the distribution of trap levels is likely to be quasi-uniform and the process of retrapping during luminescence is negligible.

    • Structure function of pion and its compositeness

      M K Parida

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      Using recent data on pion structure function and a rigorous inequality obtained recently using unitarity analyticity and Bjorken scaling a numerical upper bound on the wave-function-renormalisation constant of pion is computed. By the (somewhat drastic) act of neglecting the sea, it is shown that the bare part of the pion can be no more than 38%.

    • Simple regge pole model for the reactionπpη’n

      Mohammad Saleem M Iftikhar Ahmad

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      The most recently measured differential cross-section data forπp → η’n has been fitted by using a simple Regge pole model with phenomenological residue functions. It has also been observed that this inelastic process has the scaling property.

    • A curiosity concerning the role of coherent states in quantum field theory

      Abhay Ashtekar Anne Magnon-Ashtekar

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      In the usual Fock quantisation of fields in Minkowski space-time, one has the result that the expectation value of the quantum Hamiltonian in any coherent state equals the energy of the classical field at which the state is peaked. It is shown that this property can be used tocharacterise the usual Fock representation. It is also pointed out that the entire analysis goes through for a substantially more general class of systems including, in particular, Bose fields in arbitrary stationary space-times.

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