Atomic Structure

People Involved

Dr. Karim (Director)

Research Projects

Hollow Atoms:

Hollow atoms, atoms with inverted electron populations, are atoms with unique properties. Hollow atoms have been produced in ion-atom collision experiments and by multiphoton laser excitation of rare-gas atomic-clusters. Few- electron hollow atoms are also produced in hot fusion plasmas. The potential of hollow atoms as sources of x-ray lasers has generated enormous speculations in recent years. We perform ab intio calculations of x-ray and Auger transition rates, transition energies, fluorescence yields, and lifetimes of hollow atoms using Hartree-Fock and Dirac-Fock atomic models.

Electron and positron scattering from atoms:

The measured cross sections for electron and positron scattering from atoms provide a sensitive test for theoretical models. There has been an enormous number of theoretical and experimental investigations on this subject over the past several decades. The theoretical models which yield reasonable results remained, however, semi-empirical in nature with adjustable fitting parameters. We compute cross sections of electron and positron scattering from atoms employing parameter-free numerical methods.

Dielectronic recombination:

Dielectronic recombination is the dominant electron-ion recombination channel in low-density high-temperature plasmas. Knowledge of dielectronic recombination rates is necessary for the understanding of astrophysical plasmas, for the development of laboratory fusion plasmas, and in connection with x-ray laser research. We calculate rate coefficients of dielectronic recombinations using various atomic models.

Research Projects (cont.)

Rydberg atoms:

Novel and innovative laser excitation techniques have recently been developed to explore the properties of atoms wherein two electrons are raised to highly excited states. These so-called Rydberg-atoms promise to yield information on that regime where quantum mechanical behavior blends into that of classical mechanics. The fundamental question we are addressing is: How does nature link these seemingly contradictory ``classical" and``quantum" regimes?

Effects of Exchange Interaction on Autoionization:

Autoionization is the process by which an excited atom decays by emitting electrons. This is also known as the Auger effect or the radiationless transitions. All electrons being identical it is not possible to determine which atomic electron is being ejected. This indistinguishable nature of electrons gives rise to so-called `exchange' force. We investigate the effects of exchange interaction between the outgoing Auger electron and the bound electrons.

Continuum-Continuum Interaction in Autoionization:

An excited atom can often decay through a few competing decay channels. The electrostatic couplings among these various outgoing channels are known as continuum-continuum interactions. This inter-channel interaction can substantially alter the spectral distribution of the Auger electrons. In our calculation of certain low-energy Auger transitions, individual transition rates are found to be affected by as much as eighty four percent due to inter-channel couplings. We investigate the effects of continuum-continuum interactions on lifetimes of exotic atoms for which large discrepancies exist between theoretical and experimental transition rates.

Effects of Relaxation on Atomic Transition:

As an excited atom decays, electrons `relax' in the new environment. In calculating transition rates these `relaxation' effects are ignored to avoid certain mathematical pitfalls. According to Quantum Mechanical laws, the transition rates of atoms are proportional to certain integrals, called the matrix elements. In traditional calculations of matrix elements the same wave functions are used for both the initial and the final states, as if the waves are `frozen' in time. This can introduce grave errors in theoretical transition rates, specially for a multi-excited few-electron atom, where the electrons are expected to experience significant relaxation. We are currently investigating the effects of such relaxation on transition rates of hollow atoms.

Publications Since 2006

• M. A. Uddin, A. Haque, M. Mahbub, A. K. Basak, K. R. Karim, and F. B. Malik, "Modified Kolbensvedt model for electron impact K-shell ionization cross sections of atoms," European Physics Journal D 37, 361 (2006)
• A. Haque, M. A. Uddin, A. K. Basak, K. R. Karim, and B. C. Saha, " An empirical formula for the electron-impact K-shell cross sections," Physical Review A 73, 12708 (2006)
• A. Haque, M.A. Uddin, A.K. Basak, K.R. Karim, B.C. Saha, and F.B. Malik, "Electron impact ionization of L-shell species," Physical Review A 73, 052703 (2006)
• A. Haque, M.A. Uddin, A.K. Basak, K.R. Karim, B.C. Saha, and F.B. Malik, "Relativistic effects in electron impact ionization from the p-orbital," Physics Letters A 354, 449 (2006)
• K. R. Karim, "Electron momentum distribution in multiply ionized neon atoms," Journal of Quantitative Spectroscopy and Radiative Transfer 98, 68-80 (2006)
• H. Park and K. R. Karim, "Electron Momentum Distribution in Multiply ionized Atoms as a function of atomic number and the degree of ionization," Physica Scripta 73, 30-39 (2006)
• A. Haque, M. A. Uddin, A.K. Basak, K.R. Karim, B.C. Saha, and F.B. Malik,"Electron impact ionization of M-shell atoms," Physica Scripta 74, 377-383 (2006)
• M.A. Uddin, A. Haque, M. Billah, K.R. Karim, and A.K. Basak, "Electron-impact single ionization of mono- and di-positive ions," Pramana:Journal of Physics (India), 66, 119 (2006)