• B M Deb

      Articles written in Pramana – Journal of Physics

    • CNDO/2 calculations on the structure and properties of chlorine difluoride

      B M Deb Geeta D Mahajan V S Vasan

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      According to the open-shell CNDO/2 calculations on ClF2, performed by using the computer programme developed by Pople, Beveridge and Dobosh, the molecule is linear and stable, with equilibrium bond length 1.507Å and binding energy −173.7 kcal/mole. The molecule has a tendency to dimerise and to disproportionate into ClF3 and ClF. The netd-orbital population in the monomer is 0.88. Bonding characteristics and other molecular properties are also discussed.

    • A link between the second-order Jahn-Teller effect and the highest occupied molecular orbital postulate for molecular shapes

      Anjuli S Bamzai B M Deb

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      For a closed-shell molecule, a connection is drawn between two recent models for molecular shapes, namely, those based on the second-order Jahn-Teller (SOJT) effect and the highest occupied molecular orbital (HOMO) postulate respectively. Two necessary and sufficient conditions are derived within the molecular orbital framework for the approximation inherent in the SOJT model to be valid. One of these conditions is akin to the HOMO postulate.

    • Origin of accidental degeneracy in ligand-field splittings of substituted octahedral complexes

      S K Bose B M Deb D Mukherjee

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      Accidental degeneracy seems to be the rule rather than an exception amongst thed orbital energies of substituted octahedral complexes ofd1 configuration. By using symmetry and physical arguments, in conjunction with first-order and second-order degenerate perturbation theory, it is shown that such accidental degeneracies arise in crystal-field theory due to the choice of an inflexible basis set of metal orbitals which neglects the polarisation of metal orbitals by the ligand charges.

    • Current trends in the theories of gas-surface interaction

      Harjinder Singh B M Deb

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      Studies on gas-surface dynamics have acquired considerable importance recently not only for their intrinsic scientific interest but also for their technological potential. This article first briefly describes various experimental techniques and a number of interesting recent observations resulting from these techniques. It then discusses certain important theoretical methodologies being extensively used nowadays. There arethree broad overlapping streams of theoretical works, viz classical, semi-classical and quantum-mechanical. There are alsothree basic problems in gas-surface interaction, viz (i) the interface presents a manybody problem; (ii) the solid surface is “rough”; (iii) the number of diffractive and inelastic channels is enormously large. The semi-classical approaches appear to dominate over the others in variety and quantity. But the sources of benchmark theoretical results are still the rigorous classical-trajectory and close-coupling quantum-mechanical calculations. The coming years are likely to witness not only increased numerical accuracy through refinements in semi-classical and quantum-mechanical approaches, but also certain special approximate methods designed to yield deeper physical insights into the nature of gas-surface interaction.

    • Current trends in the theories of gas-surface interaction

      Harjinder Singh B M Deb

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    • Local scaling transformation function and atomic shell structure in density functional theory

      B M Deb Bijoy Kr Dey

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      The method of local scaling transformation in density functional theory calculates a transformation function (TRF) in order to generate an optimized atomic N-electron wave function from a trial density and a reference density/wave function. The TRFsf(r) for several atomic systems are studied and it is observed that the number of minima in df(r)/dr equals the number of atomic shells, except whenρ=ρ0 andf=r.

    • Helium atom in intense and superintense laser fields: A new theoretical approach

      Bijoy Kr Dey B M Deb

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      A quantum hydrodynamical study is made of the dynamical changes of a helium atom interacting with lasers of two different intensities, but having the same frequency. Under the intense laser field, electron density oozes out of the helium atom by absorbing laser photons and getting promoted to higher excited states including the continuum. Under the superintense field, electron density partly moves away from the helium nucleus but remains in the “quasi-bound” dressed states along with the laser field, thus suppressing ionization.

    • One-dimensional multiple-well oscillators: A time-dependent quantum mechanical approach

      Neetu Gupta Amlan K Roy B M Deb

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      Time-dependent Schrödinger equation (TDSE) is solved numerically to calculate the ground- and first three excited-state energies, expectation values 〈x2j〉, j=1, 2 …, 6, and probability densities of quantum mechanical multiple-well oscillators. An imaginary-time evolution technique, coupled with the minimization of energy expectation value to reach a global minimum, subject to orthogonality constraint (for excited states) has been employed. Pseudodegeneracy in symmetric, deep multiple-well potentials, probability densities and the effect of an asymmetry parameter on pseudodegeneracy are discussed.

    • One dimensional multiple-well oscillators: A time-dependent quantum mechanical approach

      Neetu Gupta Amlan K Roy B M Deb

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    • Does the classically chaotic Henon-Heiles oscillator exhibit quantum chaos under intense laser fields?

      Neetu Gupta B M Deb

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      The quantum dynamics of an electron moving under the Henon-Heiles (HH) potential in the presence of external time-dependent (TD) laser fields of varying intensities have been studied by evolving in real time the unperturbed ground-state wave function φ (x, y, t) of the HH oscillator. The TD Schrödinger equation is solved numerically and the system is allowed to generate its own wave packet. Two kinds of sensitivities, namely, sensitivity to the initial quantum state and to the Hamiltonian, are examined. The threshold intensity of the laser field for an electron moving in the HH potential to reach its continuum is identified and in this region quantum chaos has been diagnosed through a combination of various dynamical signatures such as the autocorrelation function, quantum ‘phase-space’ volume, ‘phase-space’ trajectory, distance function and overlap integral (akin to quantum fidelity or Loschmidt echo), in terms of the sensitivity towards an initial state characterized by a mixture of quantum states (wave packet) brought about by small changes in the Hamiltonian, rather than a ‘pure’ quantum state (a single eigenstate). The similarity between the HH potential and atoms/molecules in intense laser fields is also analyzed.

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