Department of Physics
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Item Triple differential cross-section for Laser-assisted (e,2e) process on H_2O molecule by plane and twisted electrons Impact(2025-04) Choubisa, RakeshTwisted electron-impact single ionization ((e,2e) process) of water molecule is theoretically studied in the presence of an external laser field. Calculations have been performed in the framework of the first Born approximation in coplanar asymmetric geometry. The wave functions for the fast (incident/scattered) electron and the ejected electron are described by the Volkov and Coulomb Volkov wave functions respectively which take into account the interaction of the laser field with incident/scattered and ejected electrons respectively. We calculate the triple differential cross section (TDCS) corresponding to the laser-assisted (e,2e) process for different orientations of the linearly polarised laser field. We describe the molecular state of by the linear combination of atomic orbitals (self-consistent field LCAO method). The angular profile of the TDCS for plane wave electrons is strongly modified by the laser field compared to that of without laser field. However, for the twisted electrons with laser field the angular distribution is slightly changed from those of without laser field. We study the angular distribution of TDCS for laser field polarization parallel to the incident momentum ( ), parallel to momentum transfer ( ) and perpendicular to the momentum transfer ( ) and we observe that the has the highest magnitude of TDCS. For the orientations and , the laser-assisted plane wave studies shows oscillatory nature of TDCS but for the orientation we observe only recoil peak for character orbitals and dual peak, a recoil and binary peaks for the character orbital.Item Elastic scattering of twisted electrons by CO2 molecules at high energies(IOP, 2025-09) Choubisa, RakeshThe elastic scattering of a beam of twisted electrons (Bessel beam) by CO2 molecules is studied theoretically at high energies. The molecule’s structure is optimized using coupled cluster theory and density functional theory with correlation-consistent and Pople basis sets. Coulomb potentials are used for the interactions in the static approximation. The differential and total scattering cross-sections are computed in the first Born approximation. All cross-sections are orientation-averaged using a passive rotational averaging technique. The scattering is studied by the impact of the twisted beam with topological charges in the range ml = 1 and ml = 20. The cross sections are, in addition, averaged over the target’s impact parameters, which accounts for the cross sections of a large distribution of CO2 molecules. Finally, the molecule’s total cross-section by plane waves and twisted beams is reported. The proposed methodology can be applied to study any polyatomic molecule, regardless of its structure.Item Twisted electron impact elastic cross sections of polyatomic molecules: All active electron multicentered approach(2024-07) Choubisa, RakeshThe methodology and computation of absolute elastic differential and integral (total) cross-sections of polyatomic molecules by twisted electron beam (Bessel Beam) impact is presented with CO2 as an example. The target molecule is modeled using multicentered wavefunctions with the correlation consistent quadruple zeta basis set cc-pVQZ and optimized by the post-Hartree-Fock Coupled Cluster CCSD method. The electron density is obtained as a function of spatial coordinates and is used to calculate the elastic form factor while considering the active role of all electrons in the molecule. Coulomb direct potentials are used for the interactions of the incident beam with all electrons and nuclei in the First Born Approximation. Orientation averaging of the differential cross-section is performed to mimic experimental situations. The cross sections are calculated at intermediate energies ranging from 300eV to 1keV. The proposed methodology could be applied to any polyatomic molecule. The interactions of Bessel beams (Twisted Electron Beam) with topological charges ml = 1, 2, and 3 are analyzed. The average over-impact parameters of the differential cross-sections representing the beam's interaction with a large number of uniformly transversely distributed molecules are also studied and presented. Finally, the molecules' Integral elastic cross-section (ICS) by plane waves and twisted beams is calculated and presented.Item Up-down asymmetry in the relativistic (e, 2e) processes for K-shell ionization of Cu, Ag and Au atoms(OSTI, 2003-08) Choubisa, RakeshWe present in this communication a theoretical demonstration of up-down asymmetry in the relativistic (e, 2e) process for K-shell ionization of Cu, Ag and Au atoms. The theoretical formalism has been developed in plane wave Born approximation and in this approximation the triple differential cross section (TDCS) has been expressed in terms of a product of kinematical factors and atomic structure functions. The up-down asymmetry in the relativistic (e, 2e) process on K-shell of atoms has been shown to depend on the interference between the transition charge and component of the transition current perpendicular to the scattering plane. Further, the up-down asymmetry has been shown to depend on incident electron energy, atomic number of the target and scattering electron angle.Item Quasi-binary incident electron—centre of mass collision in (e, 3e) process on He and He-like ions(Springer, 2005-07) Choubisa, RakeshWe present in this communication the results of our first Born calculation in the three-Coulomb (3C) wave approach for the (e, 3e) process on He and He-like ions at an incident electron energy 5599 eV in the coplanar constant θ12 as well as out-of-plane constant φ12 modes. These two geometrical modes are such that the quasi-binary collision between the incident electron and centre of mass of the ejected electrons is in the scattering plane. The theoretical formalism has been developed using plane waves, Le Sech wave function and approximated BBK-type wave function respectively for the incident and scattered, bound and ejected electrons to calculate five-fold differential crosssection (FDCS) of the (e, 3e) process. We emphasize on the similarities and dissimilarities (asymmetries) in the angular profile of the FDCS in two modes as well as the effects of post-collision interaction (between the ejected electrons) and nuclear chargeZ on the angular profile of the FDCS. We observe that with the increment of nuclear charge the two quasi-binary collisions approach towards identical behaviour at larger mutual angles and thus bringing less asymmetry in FDCS for higherZ target.Item Interference Effect in the Relativistic Inner Shell Ionization of Atoms by Electron Impact(JSPF, 2006) Choubisa, RakeshWe present in this paper, the results of our calculation of right-left asymmetry in the relativistic (e, 2e) processes for inner K-shell ionization of atoms. The calculation has been performed in plane wave Born approximation (PWBA) using one photon exchange approximation. The triple differential cross section (TDCS), in plane wave Born approximation (PWBA) can be factorized into products of electron kinematic factors and atomic structure functions [Donnely (1984)]. The right left asymmetry in the relativistic (e, 2e) process on the K-shell of the atoms has been shown to depend on the interference between the transition charge and component of the transition current in the scattering plane. Further, we discuss the dependence of right-left asymmetry on the incident electron energy, atomic number of the target and scattering angle.Item Atomic and Molecular Physics: Introduction to Advanced Topic(Narosa Publishing House, 2012-01) Choubisa, RakeshAn integral route in the acquisition of knowledge in the field of theoretical Atomic Collision Physics. The book offers a broad yet comprehensive introduction to advanced topics of Atomic and Molecular Collision Physics, covering all relevant calculations, systems, collisions, data and diagnostics.Item K-shell (e, 3e) processes with transversely spin-polarized relativistic electrons(IOP, 2012) Choubisa, RakeshAn ab-initio calculation of five fold differential cross section (FDCS) and transverse spin asymmetry in FDCS for the K-shell double ionization of atoms by transversely polarized incident electron is proposed.Item On the effects of Orbital Angular Momentum (OAM) and impact parameters on (e, 2e) process on atoms by twisted electron(IOP, 2017) Choubisa, RakeshWe present our theoretical study of (e, 2e) process on atoms by the twisted electron, with special emphasis given on the effects of OAM and impact parameters on TDCS.Item (e,2e) Impact Ionization Processes for Surface Science(Springer, 2018-04) Choubisa, RakeshWe present a scattering theoretic approach to the calculation of the cross-section of (e,2e) impact spectroscopy where all the electrons involved are treated within the real space multiple scattering framework. This approach is particularly suited to the reflection geometry at low kinetic energies, with the ejection of a core-level electron. In this case, we expect (e,2e) spectroscopy can be turned into an extremely sensitive surface structure probe.