• Ashok Pimpale

      Articles written in Pramana – Journal of Physics

    • Plasmon excitation in the x ray absorption process

      Ashok Pimpale Chintamani Mande

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      A simple approach is developed to study the excitation of a plasmon due to valence electronic correlations in the x ray absorption process. Assuming a complete separation between the core and the valence electrons, we introduce the plasmons in the system Hamiltonian externally according to the Bohm-Pines collective formalism and carry out the corresponding canonical transformations to separate the x ray photonplasmon-electron term. According to our calculations, the relative intensity of the plasmon process with respect to the main absorption, which represents single particle excitations is 0·12 times the interelectronic separation measured in Bohr radii. The theoretical estimates of the plasmon intensity agree satisfactorily with the experimental values for the Mn K and the Re LIII absorption discontinuities.

      The shape of the plasmon absorption band resembles the main absorption edge, if the wave vector dependence of the plasmon frequency is neglected. The intensity of the second plasmon harmonic is about 3 per cent of the first plasmon satellite spectrum. The electron plasmon interaction modifies the shape of the plasmon spectrum.

    • Raman effect in the x-ray region

      Ashok Pimpale Chintamani Mande

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      This paper presents a brief review of x-ray Raman scattering and some of our calculations on Raman scattered line shapes from light elements. We summarise the history of the Raman process in the x-ray region and present a detailed theory of the Raman scattering from an atomic many-electron system. Actual calculations of the Raman cross-section using this theory in single-particle approximation are given. The process of internal resonance Raman scattering is also discussed in the same formulation. The Raman cross-section is compared with the cross-sections of other x-ray scattering processes.

    • An automated laboratory EXAFS spectrometer of Johansson type: indigenous development and testing

      S K Deshpande S M Chaudhari Ashok Pimpale A S Nigavekar S B Ogale V G Bhide

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      An automated linear laboratory EXAFS spectrometer of the Johansson type has been indigenously developed. Only two translational motions are required to achieve the necessary Rowland circle configuration for the (fixed) X-ray source, the dispersing and focusing bent crystal and the receiving slit. With the available crystals the spectral region from 5 to 25 keV can be scanned. The linear motions of the crystal and receiving slit including the detector assembly are achieved by employing software-controlled DC motors and utilizing optical encoders for position sensing. The appropriate rotation of the crystal is achieved by the geometry of the instrument. There is a facility to place the sample alternately in the path of the X-ray beam and out of the path to record both the incident X-ray intensityI0 and the transmitted intensityI employing the scintillation detector. An arrangement with a two-window proportional detector before the sample to measureI0 and the scintillation detector to recordI is also developed; in this case it is not necessary to oscillate the sample. Fast electronic circuits are employed to minimize counting errors. The instrument is user-friendly and it is operated through a menu-driven IBM compatible PC. EXAFS spectra of high resolution have been recorded using the spectrometer and employing the Si(111) reflecting planes; the X-ray source being a Rigaku 12 kW rotating anode with Cu target. We describe the spectrometer and discuss its performance with a few representative spectra.

    • Ray tracing studies of a complete plane grating monochromator beam line

      V S Edlabadkar Ashok Pimpale

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      Optical design of a plane grating monochromator beam line for X-ray spectroscopic studies to be installed on INDUS-1 is studied using ray tracing technique. The main components of the beam line are pre- and post-elliptical mirrors and the plane mirror-grating dispersing system. The ideal positions of the optical components are decided by using our analytical formulation of the Riemer’s kinetic principle for reflecting synchrotron radiation onto the same spot of the dispersing grating. The program is developed indigenously and can be used on a PC. The tangent error and microroughness of the mirrors is explicitly accounted for in the program. The wavelength dependent absorption of radiation at the different reflecting surfaces is also included for calculations of the optical throughput. The dependence of the final image line shape and resolution on various beam line parameters is calculated. The results are useful in deciding the tolerances of the various beam line components and their positions.

    • Quantum motion over a finite one-dimensional domain: With and without dissipation

      Ashok Pimpale

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      Quantum motion of a single particle over a finite one-dimensional spatial domain is considered for the generalized four parameter infinity of boundary conditions (GBC) of Carreauet al [1]. The boundary conditions permit complex eigenfunctions with nonzero current for discrete states. Explicit expressions are obtained for the eigenvalues and eigenfunctions. It is shown that these states go over to plane waves in the limit of the spatial domain becoming very large. Dissipation is introduced through Schrödinger-Langevin (SL) equation. The space and time parts of the SL equation are separated and the time part is solved exactly. The space part is converted to nonlinear ordinary differential equation. This is solved perturbatively consistent with the GBC. Various special cases are considered for illustrative purposes.

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