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https://ir.library.illinoisstate.edu/fpphys
Recent documents in Faculty publications – Physicsen-usSun, 25 Feb 2024 01:36:10 PST3600A Data-Driven Machine Learning Approach for Electron-Molecule Ionization Cross Sections
https://ir.library.illinoisstate.edu/fpphys/44
https://ir.library.illinoisstate.edu/fpphys/44Fri, 23 Feb 2024 16:36:21 PST
Despite their importance in a wide variety of applications, the estimation of ionization cross sections for large molecules continues to present challenges for both experiment and theory. Machine learning (ML) algorithms have been shown to be an effective mechanism for estimating cross section data for atomic targets and a select number of molecular targets. We present an efficient ML model for predicting ionization cross sections for a broad array of molecular targets. Our model is a 3-layer neural network that is trained using published experimental datasets. There is minimal input to the network, making it widely applicable. We show that with training on as few as 10 molecular datasets, the network is able to predict the experimental cross sections of additional molecules with an accuracy similar to experimental uncertainties in existing data. As the number of training molecular datasets increased, the network's predictions became more accurate and, in the worst case, were within 30% of accepted experimental values. In many cases, predictions were within 10% of accepted values. Using a network trained on datasets for 25 different molecules, we present predictions for an additional 27 molecules, including alkanes, alkenes, molecules with ring structures, and DNA nucleotide bases.
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Allison Harris et al.Decomposition-based recovery of absorbers in turbid media
https://ir.library.illinoisstate.edu/fpphys/43
https://ir.library.illinoisstate.edu/fpphys/43Tue, 22 Apr 2014 20:16:41 PDT
We suggest that the concept of the point-spread function traditionally used to predict the blurred image pattern of various light sources embedded inside turbid media can be generalized under certain conditions to predict also the presence and location of spatially localized absorbing inhomogeneities based on shadow point-spread functions associated with each localized absorber in the medium. The combined image obtained from several absorbers can then be decomposed approximately into the arithmetic sums of these individual shadow point-spread functions with suitable weights that can be obtained from multiple-regression analysis. This technique permits the reconstruction of the location of absorbers.
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S D. Campbell et al.Classical-quantum correspondence in electron-positron pair creation
https://ir.library.illinoisstate.edu/fpphys/42
https://ir.library.illinoisstate.edu/fpphys/42Tue, 22 Apr 2014 20:16:39 PDT
We examine the creation of electron-positron pairs in a very strong force field. Using numerical solutions to quantum field theory we calculate the spatial and momentum probability distributions for the created particles. A comparison with classical mechanical phase space calculations suggests that despite the fully relativistic and quantum mechanical nature of the matter creation process, most aspects can be reproduced accurately in terms of classical mechanics.
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N I. Chott et al.Nonlocal entanglement of coherent states, complementarity, and quantum erasure
https://ir.library.illinoisstate.edu/fpphys/41
https://ir.library.illinoisstate.edu/fpphys/41Tue, 22 Apr 2014 20:16:37 PDT
We describe a nonlocal method for generating entangled coherent states of a two-mode field wherein the field modes never meet. The proposed method is an extension of an earlier proposal [C. C. Gerry, Phys. Rev. A 59, 4095 (1999)] for the generation of superpositions of coherent states. A single photon injected into a Mach-Zehnder interferometer with cross-Kerr media in both arms coupling with two external fields in coherent states produces entangled coherent states upon detection at one of the output ports. We point out that our proposal can be alternatively viewed as a "which path" experiment, and in the case of only one external field, we describe the implementation of a quantum eraser.
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Christopher C. Gerry et al.Unitary and nonunitary approaches in quantum field theory
https://ir.library.illinoisstate.edu/fpphys/40
https://ir.library.illinoisstate.edu/fpphys/40Tue, 22 Apr 2014 20:16:35 PDT
We use a simplified essential state model to compare two quantum field theoretical approaches to study the creation of electron-positron pairs from the vacuum. In the unitary approach the system is characterized by a state with different numbers of particles that is described by occupation numbers and evolves with conserved norm. The nonunitary approach can predict the evolution of wave functions and density operators with a fixed number of particles but time-dependent norms. As an example to illustrate the differences between both approaches, we examine the degree of entanglement for the Klein paradox, which describes the creation of an electron-positron pair from vacuum in the presence of an initial electron. We demonstrate how the Pauli blocking by the initial electron comes at the expense of a gain in entanglement of this electron with the created electron as well as with the created positron.
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K D. Lamb et al.Empirical model for the electron-impact K-shell-ionization cross sections
https://ir.library.illinoisstate.edu/fpphys/39
https://ir.library.illinoisstate.edu/fpphys/39Tue, 22 Apr 2014 20:16:33 PDT
The total cross sections of electron-impact single-K-shell ionization of 14 atomic targets ranging from H to U (1 <= Z <= 92) are calculated using a modified version of the BELI formula [Bell , J. Phys. Chem. Ref. Data 12, 891 (1983)] by incorporating both ionic and relativistic corrections in it. The proposed modified Bell model with a single set of parameters is found to provide an excellent description of the experimental data in the reduced energy range 1 <= E/I-K <= 10(6) (E and I-K are, respectively, the incident energy and ionization potential) with a performance level at least as good as any of the existing methods and models.
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A K. F Haque et al.Computation of electron-impact K-shell ionization cross sections of atoms
https://ir.library.illinoisstate.edu/fpphys/38
https://ir.library.illinoisstate.edu/fpphys/38Tue, 08 Apr 2014 20:52:52 PDT
The total cross sections of electron impact single K-shell ionization of atomic targets, with a wide range of atomic numbers from Z=6-50, are evaluated in the energy range up to about 10 MeV employing the recently proposed modified version of the improved binary-encounter dipole (RQIBED) model [Uddin , Phys. Rev. A 70, 032706 (2004)], which incorporates the ionic and relativistic effects. The experimental cross sections for all targets are reproduced satisfactorily even in the relativistic energies using fixed generic values of the two parameters in the RQIBED model. The relativistic effect is found to be significant in all targets except for C, being profound in Ag and Sn.
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M A. Uddin et al.Timing of pair production in time-dependent force fields
https://ir.library.illinoisstate.edu/fpphys/37
https://ir.library.illinoisstate.edu/fpphys/37Tue, 08 Apr 2014 20:52:51 PDT
We examine the creation and annihilation dynamics for electron-positron pairs in a time-dependent but subcritical electric force using a simplified model system. Numerical and semianalytical solutions to computational quantum field theory show that despite the continuity of the quantum field operator in time, the actual number of created particles can change in a discontinuous way if the field changes abruptly. The number of permanently created particles after the pulse, however, increases continuously with the duration of the electric field pulse, suggesting a transition from an exclusive annihilation to a creation regime.
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Christopher C. Gerry et al.Electron-impact ionization of L-shell atomic species
https://ir.library.illinoisstate.edu/fpphys/36
https://ir.library.illinoisstate.edu/fpphys/36Tue, 08 Apr 2014 20:52:49 PDT
Electron impact ionization cross sections (EIICS) of 30 L-shell targets, with open- and closed-shell configurations in the isoelectronic sequences ranging from Li to Ne, are evaluated using the generalized parameters of our recent modification of BELL formula (MBELL) [Haque , Phys. Rev. A 73, 012708 (2006)]. Three sets of parameters, one each for the 1s, 2s, and 2p orbits, provide an excellent account of the experimental EIICS data of atomic targets, neutral and ionic, up to the atomic number Z=92 and incident energies up to about 250 MeV. In comparison with the quantum mechanical predictions, it is found that the present MBELL cross sections are in better agreement with the experimental results.
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Abul Kalam fazlul Haque et al.Interpretational difficulties in quantum field theory
https://ir.library.illinoisstate.edu/fpphys/35
https://ir.library.illinoisstate.edu/fpphys/35Tue, 08 Apr 2014 20:52:47 PDT
Based on space-time-resolved solutions to relativistic quantum field theory we illustrate interpretational difficulties in associating field-theoretical quantities with properties of particles. These difficulties are related to the fact that the definition of the spatial probability density of particles depends on the choice of the Hilbert subspace on which the field operator is projected. We illustrate these problems by analyzing pair-production probabilities and spatial densities for the electron-positron dynamics associated with a spatially localized subcritical potential that is turned on and off in time.
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P Krekora et al.Velocity half-sphere model for multiple scattering in a semi-infinite medium
https://ir.library.illinoisstate.edu/fpphys/34
https://ir.library.illinoisstate.edu/fpphys/34Tue, 08 Apr 2014 20:52:39 PDT
We show how the velocity half-sphere model [S. Menon, Q. Su, and R. Grobe, Phys. Rev. E 72, 041910 (2005)] recently introduced to predict the propagation of light for an infinite turbid medium can be extended to account for the emission of multiply scattered light for a geometry with a planar boundary. A comparison with exact solutions obtained from Monte Carlo simulations suggests that this approach can improve the predictions of the usual diffusion theory for both isotropic and highly forward scattering media with reflecting interfaces.
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S Menon et al.Critique of the Wigner tunneling speed and a proposed alternative
https://ir.library.illinoisstate.edu/fpphys/33
https://ir.library.illinoisstate.edu/fpphys/33Tue, 01 Apr 2014 20:28:46 PDT
In the context of superluminal propagation of wave packets through potential barriers, the tunneling speed is usually characterized by the Wigner velocity. We propose an alternative speed that takes into account the interference between the incoming and the reflected waves and leads to a better estimation of arrival time for a wave packet entering the tunneling region. This arrival time is derived by an extrapolation from inside the barrier. The analytical theory is based on the stationary phase approximation whose validity is justified by a comparison with the numerical solution of the time-dependent Dirac equation.
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P Krekora et al.Effects of relativity on the time-resolved tunneling of electron wave packets
https://ir.library.illinoisstate.edu/fpphys/32
https://ir.library.illinoisstate.edu/fpphys/32Tue, 01 Apr 2014 20:28:44 PDT
We solve numerically the time-dependent Dirac equation for a quantum wave packet tunneling through a potential barrier. We analyze the spatial probability distribution of the transmitted wave packet in the context of the possibility of effectively superluminal peak and front velocities of the electron during tunneling. Both the Dirac and Schrodinger theories predict superluminal tunneling speeds. However, in contrast to the Dirac theory the Schrodinger equation allows a possible violation of causality. Based on an analysis of the tunneling process in full temporal and spatial resolution, we introduce an instantaneous tunneling speed that can be computed inside the potential barrier.
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P Krekora et al.Dirac theory of ring-shaped electron distributions in atoms
https://ir.library.illinoisstate.edu/fpphys/31
https://ir.library.illinoisstate.edu/fpphys/31Tue, 01 Apr 2014 20:28:42 PDT
The time-dependent Dirac equation is solved numerically on a space-time grid for an atom in a strong static magnetic field and a laser field. The resonantly induced relativistic motion of the atomic electron leads to a ringlike spatial probability density similar to the features that have been recently predicted [Wagner, Su, and Grobe, Phys. Rev. Lett. 84, 3282 (2000)] based on a phase-space method. We further demonstrate that spin-orbit coupling for a fast-moving electron in such an atom becomes significant and the time dependence of the spin can dephase even if initially aligned parallel to the direction of the static magnetic field.
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P Krekora et al.Dephasing model for spatially extended atomic states in cyclotronlike resonances
https://ir.library.illinoisstate.edu/fpphys/30
https://ir.library.illinoisstate.edu/fpphys/30Tue, 01 Apr 2014 20:28:41 PDT
In recent work, the formation of ring-shaped electron distributions for hydrogen atoms in resonant static magnetic-laser fields has exclusively been associated with the impact of relativity. In this note we will generalize this statement and show that the nonlinearity associated with the nuclear binding force can trigger similarly shaped steady-state charge clouds in atoms under suitable conditions. The dephasing model, based on modeling the quantum-mechanical state by a classical ensemble of quasiparticles evolving with slightly different cyclotron periods, can recover features, in the two lowest-order resonances as well as the Coulomb-field-induced charge distributions.
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R E. Wagner et al.Electric-field-induced relativistic Larmor-frequency reduction
https://ir.library.illinoisstate.edu/fpphys/29
https://ir.library.illinoisstate.edu/fpphys/29Tue, 01 Apr 2014 20:28:39 PDT
Using the numerical solution to the time-dependent Dirac equation we show that the effect of relativity on the usual Larmor period for an electron in a magnetic field can be enhanced drastically if a suitably scaled and aligned static electric field is added to the interaction. This electric field does not change the electron's speed but leads to an elliptical spin precession due to relativity. This spin precession is accompanied by a position-dependent spin distribution.
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P Krekora et al.Transitions into the negative-energy Dirac continuum
https://ir.library.illinoisstate.edu/fpphys/28
https://ir.library.illinoisstate.edu/fpphys/28Tue, 01 Apr 2014 20:28:38 PDT
We compare the predictions of the single-particle Dirac equation with quantum field theory for an electron subjected to a space and time dependent field. We demonstrate analytically and numerically that a transition into the negative-energy subspace predicted by the single-particle Dirac equation is directly associated with the degree of suppression of pair-production as described by quantum field theory. We show that the portion of the mathematical wave function that populates the negative-energy states corresponds to the difference between the positron spatial density for systems with and without an electron initially present.
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P Krekora et al.Klein paradox with spin-resolved electrons and positrons
https://ir.library.illinoisstate.edu/fpphys/27
https://ir.library.illinoisstate.edu/fpphys/27Tue, 01 Apr 2014 20:28:37 PDT
Using numerical solutions to relativistic quantum field theory with space-time resolution, we illustrate how an incoming electron wave packet with a definite spin scatters off a supercritical potential step. We show that the production rate is reduced of only those electrons that have the same spin as the incoming electron is reduced. This spin-resolved result further clarifies the importance of the Pauli-exclusion principle for the Klein paradox.
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P krekora et al.Electron-impact ionization of hydrogen and lithiumlike systems
https://ir.library.illinoisstate.edu/fpphys/26
https://ir.library.illinoisstate.edu/fpphys/26Tue, 01 Apr 2014 20:28:35 PDT
The electron impact single ionization cross sections on a number of targets with atomic number Z=1-92 in the H and Li isoelectronic sequences are calculated using a modified version of the recently propounded relativisitic improved binary-encounter dipole (MRIBED) model [M. A. Uddin , Phys. Rev. A 70, 032706 (2004); 71, 032715 (2005)]. The modified RQIBED (MRIBED) model along with a Z-dependent factor in it is found remarkably successful in the applications to H- and Li-like systems and also valid for the ionization of a filled s orbit including the He-like targets.
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M A. Uddin et al.Space-time properties of a boson-dressed fermion for the Yukawa model
https://ir.library.illinoisstate.edu/fpphys/25
https://ir.library.illinoisstate.edu/fpphys/25Tue, 25 Mar 2014 21:21:22 PDT
We analyze the interaction of fermions and bosons through a one-dimensional Yukawa model. We numerically compute the energy eigenstates that represent a physical fermion, which is a superposition of bare fermionic and bosonic eigenstates of the uncoupled Hamiltonian. It turns out that even fast bare fermions require only low-momentum dressing bosons, which attach themselves to the fast fermion through quantum correlations. We compare the space-time evolution of a physical fermion with that of its bare counterpart and show the importance of using dressed observables. The time evolution of the center of mass as well as the wave packet's spatial width suggests that the physical particle has a lower mass than the sum of the masses of its bare constituents. The numerically predicted dressed mass agrees with that from lowest-order perturbation theory as well as with the renormalized mass obtained from the corresponding Feynman graphs. For a given momentum, this lower mass leads to a faster physical particle and a different relativistic spreading behavior of the wave packet.
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R E. Wagner et al.