Browsing by Author "Choubisa, Rakesh"

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Atomic and Molecular Physics: Introduction to Advanced Topic
(Narosa Publishing House, 2012-01) Choubisa, Rakesh
An 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.
Coplanar (e, 3e) differential cross-section of He atom
(Springer, 2003-06) Choubisa, Rakesh
We present in this paper the results of our calculation of five-fold differential cross-section (FDCS) for (e,3e) process on He atom in low momentum transfer and high electron impact energy in shake-off mechanism. The formalism has been developed in Born approximation using plane waves, Byron and Joachain as well as Le Sech and correlated BBK-type wave functions respectively for incident and scattered, bound and ejected electrons. The angular distribution of FDCS of our calculation is presented in various modes of coplanar geometry and comparison is made with the available experimental data. We observe that the present calculation is able to reproduce the trend of the experimental data. However, it differs in magnitude from the experiment. The present theory does not predict four-peak structure insummed mutual angle mode for lower excess ejected electron energies. We also discuss the importance of momentum transfer, post-collision interaction (PCI) and ion participation in the (e,3e) process in constant θ12 mode
Double ionization of helium by twisted electron beam
(IOP, 2020-06) Choubisa, Rakesh
In this communication, we report our calculations of the five-fold differential cross-section (FDCS) for (e,3e) on the He atom using both the plane wave and the twisted incident electron beam. The formalism has been developed in the first Born approximation using the plane wave as well as the twisted electron wave for the incident electron. We describe plane wave, Le Sech, and 2 Coulomb wave with Gamow factor (2CG) respectively for the scattered, bound electrons of He atom and the ejected electrons. We compare the angular profile of the FDCS for different values of orbital angular momentum (OAM) (m) of the twisted electron with that of the plane wave. We present the angular profiles of the FDCS in the θ-variable and constant θ12 mode in the coplanar geometry. The effects of the opening angle (θp) of the twisted electron on the angular profile of the FDCS are also studied.
Dynamics of Twisted Electron Impact Ionization of CH4 and NH3 Molecule
(MDPI, 2023-05) Choubisa, Rakesh
Electron vortex beams (EVBs, also known as twisted electron beams) possess an intrinsic orbital angular momentum (OAM) with respect to their propagation direction. This intrinsic OAM represents a new degree of freedom that provides new insights into investigating the dynamics of electron impact ionization. In this communication, we present, in the first Born approximation (FBA), the angular profiles of the triple differential cross section (TDCS) for the (e, 2e) process on CH4 and NH3 molecular targets in the coplanar asymmetric geometry. We compare the TDCS of the EVB for different values of OAM number m with that of the plane wave. For a more realistic scenario, we investigate the average TDCS for macroscopic targets to explore the influence of the opening angle 𝜃𝑝 of the twisted electron beam on the TDCS. In addition, we also present the TDCS for the coherent superposition of two EVBs. The results demonstrate that the twisted (e, 2e) process retrieves the p-type character of the molecular orbitals, which is absent in the plane wave TDCS for the given kinematics. The results for the coherent superposition of two Bessel beams show the sensitivity of TDCS toward the OAM number m.
(e,2e) Impact Ionization Processes for Surface Science
(Springer, 2018-04) Choubisa, Rakesh
We 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.
Elastic scattering of twisted electrons by CO2 molecules at high energies
(IOP, 2025-09) Choubisa, Rakesh
The 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.
Electron impact single ionization of hydrogen molecule by twisted electron beam
(IOP, 2020-12) Choubisa, Rakesh
In this communication, we present the results of the five-fold differential cross-section (5DCS) and triple differential cross-section (TDCS) for the (e, 2e) process on molecular hydrogen (H2) by the plane wave and the twisted electron beam impact. The formalism is developed within the first Born approximation using the plane wave and the twisted wave for the incident electron beam. We describe the plane wave, Heitler–London type wave function, Coulomb wave for the scattered electron, the H2 molecular state, and the ejected electron. We compare the angular profiles of the 5DCS and TDCS for the different values of orbital angular momentum (OAM) number m of the twisted electron beam with that of the plane wave beam. We also present the 5DCS for different molecular orientations and study the effect of m on the 5DCS. We further investigate the influence of the twisted electron beam on the (e,2e) process on the H2 molecule from the perspective of the 'Young-type' interference of the scattered waves, emanating from the two atomic centers of the H2 molecule. We also study the TDCS for macroscopic H2 target to explore the effect of opening angle (θp) of the twisted electron beam on the TDCS. Our results clearly show the effect of the twisted electron's OAM number (m) and the opening angle (θp) on the 5DCS and TDCS of the molecular hydrogen.
Interference Effect in the Relativistic Inner Shell Ionization of Atoms by Electron Impact
(JSPF, 2006) Choubisa, Rakesh
We 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.
K-shell (e, 3e) processes with transversely spin-polarized relativistic electrons
(IOP, 2012) Choubisa, Rakesh
An 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.
Laser-assisted (e,2e) study with twisted electron beam on H-atom
(ARXIV, 2023-08) Choubisa, Rakesh
We study the laser-assisted twisted electron beam impact ionization of the hydrogen atom in coplanar asymmetric geometry. We develope the theoretical model in the first Born approximation. In the presence of the laser field, we treat the incident and scattered electrons as Volkov waves, the ejected electron, moving in the combined field of the laser and residual ion H + , is described by a Coulomb-Volkov wave function . In this communication, we compare the angular profile of triple differential cross-section (TDCS) for laser-assisted twisted electron beam incidence with the plane-wave, laser-assisted plane wave, and field-free twisted electron beam for (e,2e) processes, for different orbital angular momentum (OAM) number (m l ) values. We analyze the influence of the laser parameters (photon exchanged, intensity ) on the angular distribution of the TDCS. We study the (T DCS) av for macroscopic target to examine the effect of opening angle {\theta} p of the twisted electron beam on the angular profile of TDCS. Our results clearly show the impact of laser parameters (electric field ? and number of photon exchanged (l)) and twisted electron beam parameters (OAM number (m l ) and opening angle ({\theta} p )) on the angular distribution of TDCS.
Many body phenomenon within multiple scattering approach in PEELS
(IOP, 2020) Choubisa, Rakesh
Plasmon excitation within multiple scattering approach is being explored. The plasmon loss is described within the so-called quasi-boson approximation, originally proposed by Hedin and further extended by Fujikawa. We compare the different fluctuation potentials i.e. Plasmon-pole model, Bechstedt model and other available potentials based on beyond-RPA dielectric functions and use either the exact core state wave function or more accurate model than a simple delta function used previously for total energy loss in Aluminium targets
Model dielectric functions for fluctuation potential calculations in electron gas: A critical assessment
(APS, 2022-05) Choubisa, Rakesh
In this article, we report on a critical assessment of dielectric function calculations in electron gas through the comparison of different modeling methods. This work is motivated by the fact that the dielectric function is a key quantity in the multiple scattering description of plasmon features in various electron-based spectroscopies. Starting from the standard random phase approximation (RPA) expression, we move on to correlation-augmented RPA, then damped RPA models. Finally, we study the reconstruction of the dielectric function from its moments, using the Nevanlinna and memory function approaches. We find the memory function method to be the most effective, being highly flexible and customizable.
MsSpec-DFM (Dielectric function module): Towards a multiple scattering approach to plasmon description
(ARXIV, 2022-07) Choubisa, Rakesh
We present here the MsSpec Dielectric Function module (MsSpec-DFM), which generates dielectric functions in an electron gas or a liquid, either isolated or embedded into an environment. In addition to standard models such as the plasmon pole and the RPA, this module also provides more involved methods incorporating local field corrections (in order to account for correlations), Boltzmann-Vlasov hydrodynamical methods, the relaxation-damped Mermin and the diffusion-damped Hu-O'Connell methods, as well as moment-based methods using either a Nevanlinna function or a memory function. Ultimately, through the use of form factors, the MsSpec-DFM module will be able to address a wide range of materials such as metals, semiconductors, including inversion layers, hetero-structures, superconductors, quantum wells, quantum wires, quantum dots, Dirac materials such as graphene, and liquids.
A Multiple Scattering Approach to the EELS Cross-Section
(Springer, 2018-04) Choubisa, Rakesh
We derive a general spherical wave multiple scattering expression of the EELS cross-section. Contrarily to most of the previous theoretical frameworks, this approach treats all the electrons involved on an equal footing with respect to multiple scattering. The main point in our results is that the cross-section depends not only on Coulomb integrals on the absorbing atom, but also on contributions from all the other atoms. We show that these external contributions should be restricted to neighbouring atoms.
New geometry for the quasi-binary incident electron–centre of mass collision in (e, 3e) process on He
(IOP, 2008-01) Choubisa, Rakesh
We present in this paper the results of our calculation of five-fold differential cross sections (FDCS) in a new geometrical arrangement (constant theta-12-phi-12 mode) for the study of the quasi-binary incident electron–centre of mass collision in the (e, 3e) process on He in a better way. In this geometrical arrangement, we restrict the momentum of the centre of mass of the ejected electrons to lie in the scattering plane, defined by the momenta of the incident and scattered electrons, even though the momenta of the ejected electrons are not in the scattering plane. This geometrical mode has a greater potential to study the quasi-binary collision than the earlier study of FDCS (Choubisa et al 2003a Pramana J. Phys. 60 1187, 2003b J. Phys. B: At. Mol. Opt. Phys. 36 1731, Lahmam-Bennani et al 2002 J. Phys. B: At. Mol. Opt. Phys. 35 L215) in the coplanar constant theta-12 mode which has a limited application due to the restriction on the momenta of all the outgoing electrons to lie in the scattering plane whereas the new geometrical mode can be used for any direction of ejected electrons as long as the momentum of the centre of mass is in the scattering plane. We present our first and second Born results in the 3 C wave approach and compare our results with the available experimental data and the first Born CCC theory to check the consistency of our theoretical model and it is found to be satisfactory qualitatively. The results of our calculation of FDCS in the first as well as the second Born approximation on the He atom in the proposed geometrical mode reveal some interesting features about the quasi-binary incident electron–centre of mass collision, its dependence on the geometrical arrangement of the momenta of the ejected electrons as well as on the first- and the second-order projectile–target interaction. We are of the opinion that this geometrical arrangement may serve as a useful guideline for future (e, 3e) experimentations.
On the effects of Orbital Angular Momentum (OAM) and impact parameters on (e, 2e) process on atoms by twisted electron
(IOP, 2017) Choubisa, Rakesh
We 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.
On the relativistic electron impact K-shell (e, (3-1)e) differential cross sections for atoms
(IOP, 2015-01) Choubisa, Rakesh
In this paper, we present the results of our calculation of four-fold differential cross section (4DCS) for the K-shell electron impact double ionization (DI) on various atoms in the relativistic energy regime. The 4DCS is computed from the five-fold differential cross sections (5DCS) by integrating 5DCS over either the angles of one of the ejected electrons or the scattered electron (this process is referred as an (e, (3-1)e) process). Hence, it requires, possibly feasible double coincidence techniques to supplement our findings experimentally instead of the difficult triple coincidence technique, which is required for the complete description of the DI process. The angular profile of 5DCS is scanned with the angles of one of the ejected electrons for both the (e, (3-1)e) cases. A dominant peak in the momentum transfer direction is found when the scattered and one of the ejected electrons are detected in coincidence. The effects of nucleus (Z), energies of the ejected electrons, and the scattering angle on the 4DCS have been discussed. The angular correlation of the ejected electrons has also been discussed when only the ejected electrons are detected. We consider the spin aspects by scanning the singlet and triplet contributions in the angular profile of 4DCS. We also find that the interference term of the longitudinal and transverse interaction is responsible for the strengthening of the secondary peak. We observe a clear manifestation of the effect of nucleus on the spin asymmetry for behavior targets.
On the spin interplay of the ejected electrons in the relativistic electron impact K-shell double ionization of atoms
(IOP, 2013-09) Choubisa, Rakesh
A detailed analysis on the spin aspects of the ejected electrons is presented for the electron impact K-shell double ionization of Ca, Mo and Xe atoms. The five-fold differential cross sections have been seperated in terms of singlet–singlet and singlet–triplet transitions during the double ionization of the K-shell electrons of atoms. This type of study has led us to unravel the interesting spin interplay of the ejected electrons and their higher degree of dependence with their kinematical arrangements in the continuum state. Various geometrical arrangements are identified wherein the important relative contributions of the singlet and triplet terms are found. The singlet contribution for the back-to-back emission of the ejected electrons is found to be smaller for the Ca atom, however, on the other hand, it becomes the dominating term for the Xe atom and is isotropic in nature. We also observe that the parallel spin orientation of the ejected electrons is more favourable for the perpendicular emission of the ejected electrons (i.e., when θ1 = θq and θ2 = θq − 90°) and it remains the dominant term regardless of the energy sharing ratio of the ejected electrons. All of the singlet and triplet transitions depend on the emission direction of the ejected electrons as well as on their energies.
Quasi-binary incident electron—centre of mass collision in (e, 3e) process on He and He-like ions
(Springer, 2005-07) Choubisa, Rakesh
We 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.
Reply to the comment on 'New geometry for the quasi-binary incident electron–centre of mass collision in the (e, 3e) process on He'
(IOP, 2008-10) Choubisa, Rakesh
In this reply, we clarify our theoretical model used (the so-called 2CG model) in our recent paper (Choubisa and Sud 2008 J. Phys. B: At. Mol. Opt. Phys. 41 035202) and address the queries and comments raised by Dal Cappello and Ancarani. Recently, we published a paper (Choubisa and Sud 2008) in which we investigated new results in a new geometrical mode, the so-called constant θ12−φ12 mode. We claimed that we used 3C wave theory to calculate FDCS in the first as well as the second Born approximation using Le Sech and Bonham and Khol wavefunctions for the He atom. We also compared our results with other first-orderCCCtheory and experimental data and based on our comparison, we claimed that our calculation is able to reproduce mostly well-established results and hence the model is reliable enough to present our theoretical results in the proposed geometrical mode. We have also shown that we need to multiply FDCS by 3 and 0.54 factors respectively for Le Sech and Bonham and Khol wavefunctions to compare them with the absolute experimental data of Lahmam-Bennani