Research Interests & Accomplishments:
My major research interests concern the following fields,
oriented chronologically start-timewise:
In what
follows are presented some aspects of central importance from my
research contributions.
[A] In energetic ion-atom electron capture process a clever treatment of residual "Coulomb-tail" at asymptotic limit ought to be considered to produce faithful cross section. Over the last decade, the boundary-corrected first Born (B1B) approximation has been employed to study electron capture in a variety of ion-atom collisions and it is found that the B1B approximation explains the total experimental electron capture cross sections well starting from the intermediate energy region. In previous studies, the B1B amplitudes were evaluated by performing one-dimensional integration numerically and a further quadrature was necessary for computing the total cross sections. We have evaluated in closed form the B1B amplitude and cross section for the 1s --> 1s capture.
[B] We have developed a method for efficiently computing the scattering amplitude in the Coulomb-Glauber (CG) approximation for arbitrary nlm --> n'l'm' excitation of hydrogen-like ions induced by the collisions of structureless charged particles. Studies by previous authors almost always hinge on the technique of successive parametric differentiation, a technique that, in general, becomes tedious and impractical as the principal quantum number n increases. On the other hand, the most attractive feature of our method is that it is absolutely devoid of parametric differentiation, as all detailed information about the initial and final state is supplied through the form factor. What emerges through this development is a new and elegant one-dimension integral representation of both the Glauber and CG amplitude which can be used for efficient calculations of cross sections for arbitrary nlm --> n'l'm' excitation of hydrogen-like systems. Also, we have investigated the Z-dependence of the scattering amplitude in the CG approximation and gaven a closed form expression of the CG amplitude in the limit of infinite Z. We have further examined the behavior of the CG amplitude for large and small momentum transfer.
The major applicational importance of this work comes from the following context. Emission lines from the atoms and ions excited through electron impact are observed in abundance from various astrophysical objects. Of particular importance are the lines due to hydrogenic systems, namely, HI, HeII, CVI, NeX, CaXX, SeXIV, FeXXVI etc. Spectral analysis of the observed data utilizes the rates of transitions between a number of levels of the model atomic systems over a wide range of electron temperature. However, the situation in the context of both laboratory and theoretical results vis-a-vis the demand of data in astrophysical applications is "far-from-ideal". Using our analytic results, we have developed an extensive computer code which is capable of calculating data applicable to astrophysical spectral-diagnostics; numerical process is considerably cost-effective (in terms of real time)--an advantage resulting essentially from the theoretical methodology adopted. As examples, we have computed the rates for electron impact excitations among the angular momentum sublevels of principal quantum numbers n=1,2,3,4,5,6,7 and 8 for HI, and the ions HeII and CVI over a wide range of electron temperature. Two distinct advantages of this technique are: (i) results involving excited states are relatively accurate as important atomic levels have approximately same energy and (ii) the Glauber and hence CG methods are calculationally simpler as against the close-coupling methods that encounter severe convergence problem progressively for higher levels.
[C] Study of the interaction of electromagnetic wave with atom is an excellent "laboratory" to understand the effects of electron correlation. Theoretical investigation of photoabsorption by atoms and ions has seen a resurgence of interest of late with the availability of a vastly superior computing power to tackle problems of great complexity employing sophisticated methodologies. Further, significant proliferation of experimental data-base, owing to advance generation synchrotron light sources worldwide, has been re-emphasizing the need of theoretical work for extensive analysis of measurements. Single-photoabsorption studies are valuable to understand correlation dynamics unambegiously since the interaction between the photon field and the target electrons are so week, along with the fact that the photon disappears in the process. Further, photoabsorption studies, particularly involving ions, have crucial importance in numerous astrophysical applications. In this program we have learned some new physics on correlation in atoms and its rather sensitive dependence on changing nuclear force. New effects of relativistic and non-dipolar interactions are also discovered. Some crucial results are highlighted below.
Photoelectron angular distributions provide information on dynamical processes which is often not extractable from total subshell cross sections. In the relativistic-random-phase approximation (RRPA), which treats both electron correlation and relativistic effect on an equal footing, we have calculated angular distribution of 2p electrons of Ne. Excellent agreement over a broad energy range with very recent measurements is found. Accuracy of RRPA methodology, thus emphasized, is a strong impetus to further our calculations by including magnetic dipole and electric quadrupole effects, effects that, though in conventional wisdom are important in the high energy regime, are found to be significant even at relatively low energies. We have also shown that the independent particle model breaks down for the photoionization of both inner and outer nl(l>0) electrons of all atoms, at high enough energy, owing to interchannel interactions with the nearby ns photoionization channels. We illustrate the effect for Ne 2p in the 1 keV photon energy range through a comparison of theory and experiment. Further, a recent combined experimental and theoretical study of Ar valence photoionization illustrates the discovery of even broader lack of validity of the independent particle approximation. This newly observed breakdown demonstrates generally that IPA is valid only in very restricted domains. These restrictions are expected to be relevant throughout the Periodic Table, with consequences for a wide variety of applications. In the framework of RRPA we systematically calculated over the entire Ne isoelectronic sequence the cross sections, angular distribution asymmetry parameter and photoelectron spin polarization parameters for threshold photoionization of 2p electrons. While the effect of characteristic centrifugal barrier potential is non-vanishing for lower members of the sequence, for ions with relatively high Z interesting features are noticed near 2p_{1/2} ionization threshold. For a group of ions, namely, Si4+, Ar8+, Ti12+, Co17+, As23$, Zr30+, In39+, Pr49+ and W64+, the results near their respective 2p_{1/2} threshold exhibit certain structures, that for Si4+ being the most dramatic. For each ion of the above group this effect essentially originates from a ``perturbation'' caused by an excited state corresponding to a 2s --> np transition which, although occurs in the discrete spectrum of 2p_{1/2} --> ns,nd channels, lie close enough to the 2p_{1/2} ionization threshold to influence the continuum. Excellent laboratory evidence of this phenomenon has also been obtained. This result indicates that evolution of photo dynamics with changing nuclear charge is not at all smooth or monotonics and, thereby, very strongly suggests against any interpolation (or extrapolation) method to extract ionic properties.
Analysis of 2p_{1/2} --> ns_{1/2},nd_{3/2} autoionizing resonances in Ne, Na+, and Mg2+ is carried out in the framework of relativistic multichannel quantum-defect theory (RMQDT) with the RRPA employed to calculate ab\,initio the quantum defect parameters. Resonances are shown to evolve in a highly complex manner along the sequence owing to non-trivial Z-dependence of spectroscopic properties. The generality of this phenomena over the entire Periodic Table is established. Studies of the 2p --> ns,nd autoionizing resonances have also been completed recently for Mg, Al+, and Si2+. Similar feature as in Ne sequence is found. Besides, for neutral Mg a breakdown of LS-coupling is noted. This apparently counter-intuitive feature is traced to some near-degeneracies in the non-relativistic energy levels such that the spin-orbit Hamiltonian causes considerable mixings, mixings which vanish when there are no degeneracies.
The spin polarization properties of the photoelectrons can provide more informations about ionization dynamics than what we can extract from cross section and angular distribution. These new informations may at times potentially defy our common sense. This is because spin polarization parameters has different functional dependence on the interfering photo-fragmentation channel amplitudes. As an example, our recent study on Be 1s --> np autoionizing resonances in 2s spin polarization parameters has shown strong triplet peak, while that is found too tiny at the level of cross sections. If this points to relativistic effects in an atom as light as Be, then the result is certainly counter-intutive! In this context, the theoretical understanding of spin polarization characters of photoelectrons indeed becomes particularly important. The importance of theoretical study is further augmented as the most convenient technique to produce spin polarized photoelectrons is the photoionization of atoms. The calculation performed previously considered, in general, photoelectrons from neutral atoms and, therefore, it remained to be seen more completely how the nuclear charge influences the polarization of photoelectrons. For Ne isoelectronic sequence, we have very recently carried out calculations of spin polarization parameters (both resonant and non-resonant) of 2p photoelectrons.
[D] In the structure physics of atomic-clusters the so-called jellium model allows the valence electrons to move as an "electron-gas" in the non-local mean-field potential of residual ions with relatively sharp edge. At low energies, the escape cross-section of the photoelectron can be characterized by the well-known plasmon peak resulting from electron collective dynamics. The cluster potential, on which the photo cross section behavior relies sensitively, has so far been considered spherically symmetric in all theoretical studies.