F. Bary Malik

ELECTRONIC PATHWAYS IN PHOTOACTIVATED REPAIR OF UV MUTATED DNA

An investigation of the physics, underlying the damage caused to DNA by UV radiation and its subsequent repair via a photoreactivation mechanism, is presented in this study. Electronic pathways, starting from the initial damage to the final repair process, are presented. UV radiation is absorbed to create a hole-excited thymine or other pyrimidine that subsequently is responsible for the formation of a dimer. The negative-ion of the cofactor riboflavin, FADH-, formed by the exposure of the photolyase protein to visible light, interacts with the hole-excited electronic orbital of the thymine dimer inducing a photon-less Auger transition, which restores the two thymines to the ground state, thereby detaching the lesion and repairing the DNA. Density functional theoretical calculations supporting the theory are presented. The mechanism involves the least amount of energy dissipation and is charge neutral. It also avoids radiation damage in the repair process. Recent experimental data are compatible with this theory.

Analyses of π±‐40Ca Elastic Scattering Data in the Delta Resonance Region using Inverse Scattering Theory and the Klein‐Gordon Equation

The elastic scattering cross sections for π+ by 40Ca have been analyzed, for the first time, using the Klein‐Gordon (KG) equation that incorporates the Coulomb interaction between the charged pions and targets explicitly for the incident energies of 163.3 and 180 MeV. The nuclear part of the potentials is determined using an inverse scattering theory as a guide. Our results are then compared to those where the Coulomb potential has not been explicitly included in the KG equation but its effect is studied by modifying the incident kinetic energy following the prescription of Stricker. Our calculations that include the Coulomb potential in the KG equation reproduce the results using the Stricker prescription for π+. The Stricker method is then used to calculate π− scattering. In all cases, the data have been well accounted for.

On the single- and double-ionization of He by protons and anti-protons

In this paper we present the calculations of: (a) the single-ionization cross section of He by protons and anti-protons in the incident energy range of about 10 keV - 50 MeV, (b) the double-ionization cross sections by both projectiles in the energy rage of about 50 keV - 20 MeV and (c) the ratio of double- to single-ionization in the energy range of 50 keV - 20 MeV. The calculations have been performed in the two-coupled channel plane wave Born approximation, which is essentially the Born approximation that includes the final channel interaction between two electrons, in addition to the usual projectile - electron potential. The projectile has been represented by plane waves, and a correlated Hylleraas type of wavefunction has been used for the initial ground state. For the single-ionization case the final state electronic wavefunction is taken to be an anti-symmetrized product of a hydrogen-like (ls) and a continuum Coulomb wavefunction. For the double-ionization case, the final state wavefunctions of the two ejected electrons are taken to be Coulomb functions for calculating the matrix elements of the electron - projectile interaction, whereas plane waves are used for calculating the matrix elements of the electron - electron interaction. The calculated results are in reasonable agreement with the data in all three cases. The single-ionization cross section has also been calculated in the Lewis - Merzbacher approximation to determine the range of incident energy where it might represent a suitable approximation.

Heavy-ion fusion cross-section data for systems with compound atomic mass between 18 and 80

Fusion cross-section data are presented as a function of center-of-mass energy for different reactions involving light and medium-light heavy-ion systems with compound atomic mass number between 18 and 80. The systems range from /sup 6/Li+/sup 12/C to /sup 40/Ca+/sup 40/Ca. The data are given for the region starting from slightly below the Coulomb barrier energy to about six to seven times its value, if they are available. This compilation is primarily to facilitate theoretical interpretation of experimental results. It also serves to give researchers an overview of the range and status of the data. Literature published to June 1983 has been surveyed.

On the pocket in the 238U-238U and 238U-248Cm potential-energy surfaces

Abstract The potential energy curves between 238 U and 238 U, and 248 U and 248 Cm are likely to exhibit pockets if an adiabatic situation occurs at the time of contact. They are calculated under this condition as well as in the sudden approximation with and without the consideration of deformation using an energy density formalism.

COHERENCE, ENERGY AND CHARGE TRANSFERS IN DE-EXCITATION PATHWAYS OF ELECTRONIC EXCITED STATE OF BIOMOLECULES IN PHOTOSYNTHESIS

The observed multiple de-excitation pathways of photo-absorbed electronic excited state in the peridinin–chlorophyll complex, involving both energy and charge transfers among its constituents, are analyzed using the bio-Auger (B-A) theory. It is also shown that the usually used Forster–Dexter theory, which does not allow for charge transfer, is a special case of B-A theory. The latter could, under appropriate circumstances, lead to excimers.

THE EXPRESSIONS FOR THE SUPER-ALLOWED BETA-DECAY RATES IN THE CORIOLIS COUPLING MODEL: APPLICATION TO THE 1d5/2 SHELL NUCLEI

The expressions for super-allowed beta-decay transition rates have been derived within the context of the Coriolis coupling model. The derived expressions, valid for the beta-decay between any two mirror nuclei, have been applied to calculate super-allowed beta-decay transition rates of 21Na, 23Mg, 25Al, and 27Si. Without the use of any hindrance factor, the calculated rates agree well with the data and also with the calculations done using the shell model with configuration admixture.

ELECTRON-ELECTRON INTERACTION IN THE NON-RELATIVISTIC LIMIT

The electron-electron potential in the one-photon exchange approximation with the omission of the spin-spin interaction, leads to the classical Coulomb interaction, but the inclusion of the latter results in the Moller interaction. Bethe and Fermi showed that the latter interaction leads to the Breit potential, if a few of the terms in the expansion of the retardation effect are considered. In this article, it is shown that the higher order terms omitted in the Bethe-Fermi treatment reduces to terms of the same order in Diracs alpha-matrices considered by Bethe and Fermi. This raises questions whether the Breit interaction is the appropriate first order correction to the Coulomb potential in the non-relativistic limit. It is pointed out that the nature of the interaction between two bound (1s) electron derived by Brown using the Schwinger formalism of the quantum electrodynamics but proposed empirically in 1929 by Gaunt could be a better correction to the Coulomb potential for bound electrons in atoms. The calculated energies using these matrix elements plus the vacuum polarization energies are in reasonable agreement with the data. For comparison, calculated energies using the Breit interaction plus vacuum polarization energies are also presented.

Energy Density Functional Formalism and Nuclear Fission

An energy-density functional theory is applied to deal with the nuclear fission process. The functional is found to reproduce observed nuclear masses using proper density distributions. The theory gives rise to an additional potential barrier between the saddle and scission points and accounts for observed half-lives with experimental total kinetic energies for 234U, 236U, 240Pu, 244Cm, 252Cf and 258Fm. It also accounts for the mass distributions. The theory accounts for the observed half-lives in the decay of 258Fm to symmetric modes. The theory can also be applied to light-ion radioactivity and a preliminary calculation provides a good understanding of 14C activity in the decay of 226Ra.

A Reaction Theory and the Relation between Widths and Energy Shifts

We present here a complete reaction theory that incorporates the Pauli principle explicitly and is devoid of projection operators or matching radii. We provide explicit expression for widths, reduced widths and energy shift and proper relations between them. For a series of nearby overlapping resonances we develop explicit expressions showing that they cannot be, in general, considered as a series of non-interacting isolated resonances. The analysis indicates that the location of resonance energies are affected in the complex-rotation method and examine the situation when this could be neglected. Finally we examine the importance of including the Pauli principle for the calculation of auto-ionization widths of He-like atomic system.