• Volume 50, Issue 6

      June 1998,   pages  463-698

    • Editorial note

      Rajaram Nityananda H R Krishnamurthy

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    • Foreword

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    • Synchrotron radiation sources INDUS-1 and INDUS-2

      D D Bhawalkar G Singh R V Nandedkar

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      The synchrotron radiation sources, INDUS-1 and INDUS-2 are electron storage rings of 450 MeV and 2 GeV beam energies respectively. INDUS-1 is designed to produce VUV radiation whereas INDUS-2 will be mainly used to produce x-rays. INDUS-1 is presently undergoing commissioning whereas INDUS-2 is under construction. Both these rings have a common injector system comprising of a microtron and a synchrotron. Basic design features of these sources and their injector system are discussed in this paper. The radiation beamlines to be set up on these sources are also described.

    • Ion-molecule reactions at thermal energies

      S V K Kumar

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      Ion-molecule reactions is a generic word for reactions involving ions (both positive and negative), radicals and stable neutrals. In this presentation, use of the flowing afterglow technique to study ion-molecule reactions at thermal energies is demonstrated using the examples of positive ion-negative ion mutual neutralization of molecular nitrogen ion (N2+) with F and the reaction of atomic nitrogen with SFn (n=1 to 5) to form NF.

    • The power and beauty of (γ, 2e) experiments

      Volker Schmidt

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      The power and beauty of energy- and angle-resolved two-electron emission in the double photoionization of atoms is demonstrated, concentrating on the particular shapes of the angular correlation patterns of the triple differential cross section. The cases selected are direct double photoionization in helium and neon as well as sequential double photoionization in xenon, both for equal and unequal energies of the emitted electrons.

    • Atomic parity non-conservation: Present status and future prospects

      B P Das

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      The general features of parity nonconservation (PNC) in atoms arising from neutral weak currents and the nuclear anapole moment are discussed. The theoretical approaches used to calculate PNC observables are briefly mentioned. A brief review of the present status of atomic PNC is presented and its potential as a probe of physics beyond the standard model is highlighted.

    • Field optimized initial state based control of photodissociation

      K Vandana Deepa B Bairagi Peter Gross Manoj K Mishra

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      A new scheme for controlling photodissociation through preparation of a variationally optimized linear superposition of field free vibrational eigenstates is applied for selective control of IBr and HI dissociation. The dependence of photodissociation on various field parameters and initial conditions is examined to investigate the mechanistic basis of selective control. The parametric equations of motion approach for determining vibrational dynamics as a function of field parameters without having to solve the time dependent Schrödinger equation explicitly for each field parameter separately is outlined and its use to identify field characteristics which will provide the requisite population mix represented by the optimal linear superposition of vibrational states is advocated.

    • Calculation of highly excited eigenstates of chaotic quantum systems

      V B Sheorey M S Santhanam

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      Computational methods for the calculation of a large number of eigenstates of coupled oscillator system are developed and discussed. These calculations have enabled us to identify and investigate properties of an infinite set of states sharply localized in configuration space in this system. Some of the results and their significance are discussed. Extensions to three-dimensional systems are also briefly considered.

    • Towards a theory of dissociative recombination

      Asger Halkier Mark Roberson Jan Linderberg

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      Current experiments in storage rings at Aarhus and Stockholm reveal that ions such as CH3+, OH3+, OH2+ and CH5+ recombine and fragment into three parts more often than into just two. Analysis of the possible resonances between free electron and bound electron states for the ions require a detailed examination of the correlation effects as well as the coupling to nuclear degrees of freedom. The problem is well suited for the propagator approach. An analysis of the structure of the self-energy kernel shows the presence of possible resonances with degenerate electronic states which are unstable according to the Jahn-Teller theorem and provides channels for multiple fragmentation.

    • Total cross sections for electron scattering from well-known and exotic molecules

      K N Joshipura

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      In this review paper, scattering of intermediate to high energy electrons on well-known as well as exotic molecular targets is considered. The ‘additivity rule’ and its modifications for calculating various total cross sections are discussed against the background of an extensive experimental data. The theory succeeds at high impact energies (Ei>100 eV). Tentative upper and lower limits of e-molecule ionization cross sections are identified. Fitting formulas to represent total cross sections as functions of energy are also given.

    • A field theoretic approach to the atomic collision problems

      Sujata Bhattacharyya

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      It is an endeavour to make field theoretic approach available to the domain of electronic and atomic collision physics. The capacity of QED is demonstrated in explaining atomic collision phenomena in Coulomb gauge and depending on energy, in relativistic Lorentz gauge. Feynman diagrams are used to calculate bound state collision problems in atomic physics.

    • Positron scattering by atoms

      H R J Walters Ann A Kernoghan Mary T Mcalinden C P Campbell

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      We describe the present status of coupled-state calculations for positron scattering by ‘one-electron’ atoms. We show how pseudostates are used to represent the continuum channels. Illustrative results from positron scattering by atomic hydrogen and the alkali metals are presented.

    • Dissociative attachment of electrons to excited molecules

      E Krishnakumar

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      The development and the first results from an experiment to carry out dissociative attachment to excited molecules are discussed. A brief summary of the relevance and status of such measurements are given. Apart from measuring the absolute cross sections from excited and state selected SO2 molecule, we have been able to characterize the negative ion resonances using the excited state dissociative attachment. In addition, the state specificity of the electron attachment process has been used to derive information on the excited neutral state itself which has not been possible using optical spectroscopy. The applicability of this technique to other species are also discussed.

    • Photoionization of some closed shell atoms and ions

      H S Chakraborty P C Deshmukh S T Manson D W Lindle

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      Dipole-allowed single photoionization of some closed shell atoms and ions has been investigated in the relativistic random-phase approximation (RRPA). Application of relativisticmultichannel quantum defect theory (RMQDT) is made together with RRPA to analyse autoionizing resonances. Analysis points to the importance of interchannel coupling in high energy photoionization and reveals various degeneracies in relativistic atomic spectra to influence the low energy dynamics. Interesting threshold behavior in photoelectron spin polarization has been seen. Prospective future studies have been indicated.

    • Secondary ion mass spectrometry for quantitative surface and in-depth analysis of materials

      P Chakraborty

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      Secondary ion mass spectrometry (SIMS) is a technique based on the sputtering of material surfaces under primary ion bombardment. A fraction of the sputtered ions which largely originate from the top one or two atomic layers of the solid is extracted and passed into a mass spectrometer where they are separated according to their mass-to-charge ratios and subsequently detected. Because the sputter-yields of the individual species, coupled with their ionization probabilities, can be quite high and the mass spectrometers can be built with high efficiencies, the SIMS technique can provide an extremely high degree of surface sensitivity. Using a particular mode like static SIMS where a primary ion current is as low as 10−11 amp, the erosion rate of the surface can be kept as low as 1 Å per hour and one can obtain the chemical information of the uppermost atomic layer of the target. The other mode like dynamic SIMS where the primary ion current is much higher can be employed for depth profiling of any chemical species within the target matrix, providing a very sensitive tool (∼ 1 ppm down to ppb) for quantitative characterization of surfaces, thin films, superlattices, etc.

      The presence of molecular ions amongst the sputtered species makes this method particularly valuable in the study of molecular surfaces and molecular adsorbates. The range of peak-intensities in a typical SIMS spectrum spans about seven to eight orders of magnitude, showing its enormously high dynamic range; an advantage in addition to high sensitivity and high depth-resolution. Furthermore, the high sensitivity of SIMS to a very small amount of material implies that this technique is adaptable to microscopy, offering its imaging possibilities. By using this possibility in static SIMS or dynamic SIMS mode of analysis, one can obtain a two-dimensional (2D) surface mapping or a three-dimensional (3D) reconstruction of the elemental distribution, respectively within the target matrix.

      Secondary ion yields for elements can differ from matrix to matrix. These sensitivity variations pose serious limitations in quantifying SIMS data. Various methods like calibration curve approach, implantation standard method, use of relative sensitivity factor, etc. are presently employed for making quantitative SIMS analysis. The formation of secondary ions by ion bombardment of solids is relatively a complex process and theoretical research in this direction continues in understanding this process in general.

      The present paper briefly reviews the perspective of this subject in the field of materials analysis.

    • Ionization of atoms by charged particles

      Kanika Roy

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      Ionization of hydrogen atom by charged particle impact are studied at different collisional energies and the total and differential cross sections are calculated. In case of light particle impact the final-state wave function here considers all three two-body interactions on an equal footing and satisfies the exact Coulomb boundary conditions. The spin asymmetries are also found and the values are compared with other existing results. For heavy particle impact a final continuum state wave function which incorporates distortion due to the Coulomb fields of both the projectile and the target nuclei is employed. In this case the target hydrogen atom is considered in its ground as well as metastable 2s state. The results thus obtained are compared with the existing experimental findings as well as other theoretical predictions.

    • Metal-ion implantation in glasses: Physical and chemical aspects

      F Caccavale

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      A review of the state-of the-art of the research in the field of chemical interactions in silica and silicate glasses implanted with metal ions (e.g., Si, Ti, W, Ag, Cu, Cr) and N is presented in terms of new compounds formation. Moreover, under certain circumstances, the formation of nanometer-radius metal colloidal particles in a thin surface layer is observed. The chemical state of the implanted atoms is determined by X-ray photoelectron (XPS) and X-ray excited Auger-electron spectroscopies (XE-AES). Rutherford backscattering spectrometry (RBS) and secondary-ion mass spectrometry (SIMS) are used to determine the in-depth elemental distributions. Optical absorption measurements and transmission electron microscopy (TEM) are used to detect the presence of metallic clusters, as well as to determine their mean size and size distribution. A thermodynamics approach is used to explain the interaction between the implanted ion and the separate atomic species of the target glass and/or between the implanted ion and the target molecular species.

    • L-shell ionization of atoms and their subsequent decay by radiative and non-radiative transitions

      P Venugopala Rao

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      The study of the ionization of atoms resulting in vacancies in their inner shells and the subsequent decay of the atomic-vacancy states by x-ray and Auger transitions continue to be an active area of interest. A rapid survey of the theoretical efforts to calculate the transition probabilities involvingL-subshells in the high-Z atoms is presented. A complete review of theL1-subshell yields for single-vacancy atomic states obtained by various experimental techniques is included. The production of multiple vacancies in theL shell and the role of the spectator vacancies in the decay process is discussed. A detailed case study of determining experimentally the number of multiple vacancies produced, and the x-ray fluorescence yields during ionization by heavy-ion bombardment is presented. It is established that the effect of spectator vacancies is to increase the x-ray fluorescence yields substantially.

    • Electron-photon coincidence studies on electron impact excitation of lighter neutral atoms

      Rajesh Srivastava

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      Recently we applied non-relativistic distorted wave approximation theory to study electron impact excitations in lighter atoms (viz. hydrogen, helium and some alkalis). Excitations from the ground and (or) initially excited metastableS states to next upper excitedP andD states have been considered. Results for the differential cross-sections and electron-photon coincidence parameters are obtained. Here the theory and the calculation of various scattering parameters are described briefly and some selected results are presented and discussed.

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