[en] The present article highlights some recently found interchannel coupling effects that must be taken into account to explain an atomic photoabsorption process. The importance of interchannel coupling effects is illustrated in a few select cases of recent interest: (a) sustenance of interchannel coupling effects at photon energies high enough where the independent particle approximation was believed to hold good; (b) demonstration of the generality of a new spin-orbit activated correlation effect as an essentially many-body collective event; (c) recognition of the scope and limitations of the indispensable approximation techniques to account for each 'kind' of correlation effect in configuration interactions between 'discrete' and 'continuum' channels; and (d) the recognition in some recent studies on atomic mercury, that contrary to the earliest expectations, dynamical interchannel coupling between dipole and quadrupole channels is important as found in the studies of photoionization of atomic xenon as well

[en] Electron-impact collisions excite optically allowed as well as optically forbidden transitions. Details of optical oscillator strengths, calculated using the relativistic local-density central-potential (RC) method, are presented. The oscillator strengths have been calculated using the small-momentum-transfer limit (K → 0) of the electron-impact generalised oscillator strength. Results for the Cooper-minimum trajectories for some high-Z (82-90) atoms and for Kr 3p ionisation are reported. These have been found to be in fair agreement with results of previous calculations in which more involved techniques were used. The present results offer further support to the utility of the RC potential in atomic physics by providing first estimates of some important atomic properties, using a relativistic atomic potential which is computationally not very demanding. 19 refs., 2 figs

No abstract available

[en] A new scale of electronegativity based on X-ray spectroscopic data was earlier presented by Mande and others for the elements ₃Li to ₅₄Xe. The present paper reports an extension of this scale carried out for most of the heavier elements lying between ₅₅Cs and ₉₂U. The present approach is based on the physical interpretation of electronegativity of an atom as the attractive electrostatic force it exerts at a distance equal to its covalent radius. To estimate this force, experimental spectroscopic data have been employed. The merits of such an approach have been pointed out recently by Mullay and may be of interest in the context of the intimate relation found between electronegativity and superconducting transition temperature. (author). 26 refs., 1 tab., 2 figs

[en] Generalized oscillator strengths (GOS) for ionization of the 2s subshell of several atoms are reported in this paper. The atomic wavefunctions used in the present work have been obtained from a relativistic local density potential method known as the RC≡ method. The GOS has been obtained using the Born approximation for 2s subshell ionization of Be, C, O, Ne, Al, Ar, Fe and Zn. The GOS behaviour has been studied both as a function of momentum transfer and energy transfer to understand the Bethe surface properties. Besides the RC≡ potential, the non-relativistic Hartree-Slater (HS) potential has also been used in a separate calculation for comparison. There is in general reasonably close agreement between the results of the RC≡ and the HS calculations. (Author)

[en] Subshell photoionization parameters for photoionization from the 4f, 5p, and 5s subshells of atomic ytterbium have been calculated using the relativistic random-phase approximation (RRPA). Interference effects from interchannel coupling of 21 relativistic dipole channels have been included. Interesting differences in the results for 4f parameters relative to a previous eight-channel RRPA calculation have been observed and are attributed to strong effects coming from coupling with the 5p channels

[en] Near the ionisation threshold, the behaviour of the generalised oscillator strength (GOS) is sensitive to the choice of the model atomic potential. In electron-impact ionisation, 'delayed maxima' of the GOS often occur near the ionisation threshold due to centrifugal barrier effects. In the present work, the sensitivity of this important effect of the centrifugal barrier to the model atomic potential has been studied using two local density atomic potentials, namely, the nonrelativistic Hartree-Slater potential and the relativistic potential. Calculations have been done for electron-impact 2p ionisation of atomic oxygen, neon, aluminium, argon, iron and zinc. The GOS results for 2p ionisation of oxygen and aluminium are presented to illustrate the results. 16 refs., 4 figs

[en] The generalized oscillator strength (GOS) of an atom is an essential factor in the differential cross section for inelastic scattering of fast charged particles. Recently, Takayanagi et al. [Phys. Rev. A 41, 5948 (1990)] have obtained the GOS for the excitation of atomic krypton to the 4p⁵(²P_1/2)5s and 4p⁵(²P_3/2)5s states from inelastic-scattering measurements using electron-energy-loss spectroscopy. The present study was undertaken with the twofold objective to determine the results of theoretical computations of the scattering parameters of the above experiment and to examine the suitability of a recently developed relativistic local-density-potential method [M. Vijayakumar, N. Vaidehi, and M. S. Gopinathan, Phys. Rev. A 40, 6834 (1989)] to study atomic-collision processes. Calculations have been done to obtain GOS for electron-krypton and electron-xenon collisions in the squared-momentum-transfer range of 0.01 to 10 atomic units. The well-known theory of Bethe has been used for the determination of the GOS in the first-order Born approximation. The present results are in fair agreement with previous Hartree-Slater and Hartree-Fock calculations and with the experimental data available in the literature. Furthermore, results of the present calculations predict that the GOS goes through a minimum, similar to the ''Cooper minimum'' in the photoabsorption cross section, as a function of the value of the momentum transfer. Experiments at slightly higher values of momentum transfer are suggested to verify the position of this minimum

[en] Experimental and theoretical techniques are now becoming sophisticated enough to measure/predict the very short time delay in photoemission. Typically, this delay is in the attosecond regime, and its accurate determination is of great importance for calibration of the atomic clock. In the present work, we use the Wigner-Eisenbud Lime delay formalism to determine the time delay in some photoionization channels in strontium atom using the Relativistic Random Phase Approximation (RRPA) methodology developed by Johnson and Lin. (author)

[en] Position of the Eu-L₃ absorption edge has been studied in pure metal and in its trivalent halides, EuF₃, EuCl₃, EuBr₃, and EuI₃, employing a simple X-ray spectrometer with an LiF single crystal as the analyser. A linear relationship was established between the chemical shift and the effective charge on the absorbing rare earth atom. The chemical shifts have also been correlated to Moessbauer isomer shifts. The results have been discussed in terms of nature of chemical bonding, effective atomic charge on the absorbing atom and some other parameters relevant to the immediate local environment of the absorbing atom. (author)