Rearrangement Collisions Research Articles

Formation of the Positronium Antihydride Molecules (H¯Ps\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\bar{\ extrm{H}}\ extrm{Ps}$$\\end{document}) in Low Energy, 5-Body Collisions of Antihydrogen Ion H¯+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\bar{\ extrm{H}}^+$$\\end{document} with Positronium Atoms Ps

In this paper we present the first pilot calculation of the elastic and inelastic cross sections for the 5-body scattering of antihydrogen ions with positronium atoms. These cross sections have not been calculated before and are not known experimentally. In particular, we focus on the collisional rearrangement reactions H¯++Ps→H¯Ps+e+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\bar{{ \\mathrm H}}^+ + \ extrm{Ps} \\rightarrow {\\bar{\ extrm{H}}}\ extrm{Ps} +{\\mathrm{e^+}}$$\\end{document} which deplete the H¯+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\bar{\ extrm{H}}^+$$\\end{document} ions and result in stable atomcules H¯Ps\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\bar{\ extrm{H}}\ extrm{Ps}$$\\end{document}. To better understand the mechanism of this rearrangement, we study the 3-dimensional, angle resolved positron densities of the 3- and 4-body fragments in the initial and final states of the rearrangement collision.

Rearrangement collision theory of phonon-driven exciton dissociation

Understanding the processes governing the dissociation of excitons to free charge carriers in semiconductors and insulators is of central importance for photovoltaic applications. Dyson's S-matrix formalism provides a framework for computing scattering rates between quasiparticle states derived from the same underlying Hamiltonian, often reducing to familiar Fermi's “golden rule” like expressions at first order. By presenting a rigorous formalism for multichannel scattering, we extend this approach to describe scattering between composite quasiparticles and, in particular, the process of exciton dissociation mediated by the electron-phonon interaction. Subsequently, we derive rigorous expressions for the exciton dissociation rate, a key quantity of interest in optoelectronic materials, which enforce correct energy conservation and may be readily used in calculations. We apply our formalism to a three-dimensional model system to compare temperature-dependent exciton rates obtained for different scattering channels. Published by the American Physical Society 2024

One-center close-coupling approach to two-center rearrangement collisions

Calculations of ionization and electron-capture cross sections in ion-atom collisions usually require solving the Schrödinger equation governing the collision system by expanding the total scattering wave function in a basis of target- and projectile-centered pseudostates. This approach leads to the two-center close-coupling equations which in some cases may become ill-conditioned due to the non-orthogonality of the underlying combined basis. Here we develop a technique which allows to accurately extract the necessary collision information, including that for the rearrangement channels, from the computationally more convenient one-center close-coupling equations which are built from only target-centered pseudostates. The robustness of the method is demonstrated by considering the proton-hydrogen scattering problem across a wide incident energy range. The developed method is then applied to study proton scattering on multielectron target of lithium. The obtained results for the 2s → 2p excitation and the total electron-capture cross sections are in good agreement with corresponding experimental data. It is concluded that the presented technique could be a simpler alternative when integrated cross sections are required.

Projectile angular-differential cross sections for transfer and transfer-excitation in α–He collisions

Total as well as projectile angular-differential cross sections for transfer and transfer-excitation have been investigated in collision of α-particles with helium atoms in the energy range 50–5,000 keV/amu. The four-body Coulomb–Born distorted wave formalism and boundary corrected continuum intermediate state approximation have been applied for this rearrangement collision. In both methods dynamic electron correlations have been taken through the complete perturbation potentials in post form. It is shown that for single electron-transfer, the ground state capture is dominant and the contribution from excited states to the total cross section is less important. It is also noted that the cross sections for transfer with target excitation channel have minor contribution to the total cross sections. We have also studied the total cross sections for single electron transfer from lithium atom by α-particle impact in the energy range from 50 to 3,000 keV/amu. Present computed results, both in prior and post forms of the transition amplitude for symmetric and asymmetric collisions, agree reasonably with the available theoretical and experimental results.

Application of exterior complex scaling to positron-hydrogen collisions including rearrangement

The application of an exterior complex scaling method to an atomic scattering problem with distinct rearrangement channels is reported. Calculations are performed for positron-hydrogen collisions in an s -wave model employing an electron-positron potential of V12 =- [8+ (r1 - r2) 2] -1/2, using the time-independent propagating exterior complex scaling method. This potential has the correct long-range Coulomb tail of the full problem and the results demonstrate that exterior-complex-scaling-based methods can accurately calculate scattering, ionization,and positronium formation cross sections in this three-body rearrangement collision.

Structural and energetic analysis of copper clusters: MD study of Cu n (n = 2-45)

Molecular dynamics simulations, via an empirical potential, have been performed in order to investigate geometries, growing patterns, structural stabilities, energetics, and magic sizes of copper clusters, Cun (n = 2-45). Possible optimal stable structures of the clusters have been generated through following rearrangement collision of the system in fusion regime. This process serves as an efficient alternative to the growing path identification and the optimization techniques. It has been found that copper clusters prefer to form three-dimensional compact structures in the determined configurations and the appearances of medium sizes are five fold symmetry on the spherical clusters. Moreover, relevant relations between atomic arrangements in the clusters and the magic sizes have been observed. Cu26 may be accepted as another putative magic size like Cu13.

Electron- and positron-impact atomic scattering calculations using propagating exterior complex scaling

Calculations are reported for four-body electron-helium collisions and positron-hydrogen collisions, in the S-wave model, using the time-independent propagating exterior complex scaling (PECS) method. The PECS S-wave calculations for three-body processes in electron-helium collisions compare favourably with previous convergent close-coupling (CCC) and time-dependent exterior complex scaling (ECS) calculations, and exhibit smooth cross section profiles. The PECS four-body double-excitation cross sections are significantly different from CCC calculations and highlight the need for an accurate representation of the resonant helium final-state wave functions when undertaking these calculations. Results are also presented for positron-hydrogen collisions in an S-wave model using an electron-positron potential of V12 = − (8 + (r1 − r2)2)−1/2. This model is representative of the full problem, and the results demonstrate that ECS-based methods can accurately calculate scattering, ionization and positronium formation cross sections in this three-body rearrangement collision.

Time-dependent approach to three-body rearrangement collisions: Application to the capture of heavy negatively charged particles by hydrogen atoms

We present a theoretical method for Coulomb three-body rearrangement collisions solving a Chew-Goldberger-type integral equation directly. The scattering boundary condition is automatically satisfied by adiabatically switching on the interaction between the projectile and hydrogen atom. Hence the outgoing wave function is obtained without the tedious procedure of adjusting the total wave function in the asymptotic region. All the dynamical information can be derived from the outgoing wave function obtained on pseudospectral grids numerically. Taking ${\ensuremath{\mu}}^{\ensuremath{-}}+\mathrm{H}(1s)$ and $\overline{p}+\mathrm{H}(1s)$ collisions as examples, we demonstrate the usefulness and powerfulness of the method and present the state-specified capture cross sections of heavy negatively charged particles by hydrogen atoms. The convergence and accuracy of the numerical procedure are examined with sufficient care.

MOLECULAR DYNAMICS STUDY OF Tin, Vn AND Crn CLUSTERS

Using a Morse type pair potential, molecular-dynamics simulations have been performed to investigate the atomic geometries, growing patterns, structural stabilities, energetics and magic sizes of Ti n, V n and Cr n (n = 2-50) clusters. Following rearrangement collision of the atom–cluster system in fusion process, and absorbing their energies step by step down to 0 K, possible optimal equilibrium geometries of the clusters have been generated to tackle the structural determination problem. This approach serves an efficient alternative to the growing path identification and the optimization techniques. It has been found that titanium, vanadium and chromium clusters prefer to form three-dimensional compact structures in the determined configurations and the appearances of medium sizes are, in general, five-fold symmetry on the spherical clusters. Moreover, relevant relations between atomic arrangements in the clusters and the magic sizes have been observed.

Nonradiative formation of the positron-helium triplet bound state

We have previously calculated the cross section for radiative formation of the interesting bound state consisting of a positron bound to helium, where the atomic electrons are in the triplet spin state. That process uses the metastable triplet helium system as target, and, as expected, it has a very small cross section. In this paper we examine a more probable process in which the state of interest is produced in an exchange rearrangement collision between a positronium atom and the singlet helium ground state: $\mathrm{Ps}+\mathrm{He}(^{1}S^{\mathrm{e}})\ensuremath{\rightarrow}\mathrm{Ps}{\mathrm{He}}^{+}(^{3}S^{\mathrm{e}})+{e}^{\ensuremath{-}}$. The present calculation is done in the plane-wave Born approximation, using simple initial and final wave functions and compares post and prior forms.

Molecular-dynamics study of possible packing sequence of medium size gold clusters: Au 2–Au 43

Growing pattern, structural stability, energetics and magic sizes of gold clusters, Au n ( n = 2 –43), have been investigated by using molecular-dynamics simulations. Starting from the dimer configuration, following rearrangement collision of the system in fusion process, and absorbing its energy step by step up to 0 K, possible stable structures of the clusters have been identified via an empirical model potential energy function. It has been found that gold clusters prefer to form three-dimensional compact structures and five-fold symmetry appears on the spherical medium clusters. This approach serves an efficient alternative to the growing path determination and the global optimization techniques.

Size evolution of structures and energetics of iron clusters (Fe n, n ≤ 36): Molecular dynamics studies using a Lennard–Jones type potential

Stable structures and energetics of iron clusters, Fe n ( n up to 36), have been investigated by performing molecular dynamics simulations. A Lennard–Jones type pair-potential energy function recently proposed for iron crystal studies [Mohri et al., J. Alloys Compd. 317 (2001) 13] has been used to describe the particle interactions in the simulations. The growing pattern of iron clusters is analyzed via rearrangement collision. The general trends in this pattern are discussed by comparing with recent quantum calculations. Finally, a preferable growth mechanism for Fe n clusters is determined.

Hydrogen-Antihydrogen Molecule and Its Properties

The metastability of the four-body system is discussed from the point of view of radiative rearrangement collisions of the hydrogen and antihydrogen. Such collisions lead to the formation of an intermediate molecular state which further decays into positronium and protonium or is destroyed via annihilation. We present calculations of the lifetime of the system and the branching ratios for Coulombic decay and annihilation.

Rearrangement collisions between gold clusters

Collision processes between two gold clusters are investigated using classical molecular dynamics in combination with embedded atom (EA) potentials, after checking the reliability of the EA results by contrasting them with first principles calculations. The Au projectiles considered are both single atoms (N = 1) and clusters of N = 2, 12, 13 and 14 atoms. The targets contain N = 12, 13 and 14 gold atoms. The collision energy E and impact parameter b are chosen within a range such that the three regimes we are interested in studying (fusion, scattering and fragmentation) are realized. The results of the collision processes are described and analyzed in detail, and compared with previous work.

Path integral approach to non-relativisticelectron charge transfer

A path integral approach has been generalized for thenon-relativistic electron charge transfer processes. The chargetransfer - the capture of an electron by an ion passing anotheratom, or more generally the problem of rearrangement collisions - is formulated in terms of influence functionals. It has beenshown that the electron charge transfer process can be treatedeither as an electron transition problem or as ion and atom elastic scattering in the effective potential field. Thefirst-order Born approximation for the electroncharge transfer reaction cross section has been reproduced to prove theadequacy of the path integral approach for this problem.

Stability of hydrogen-antihydrogen mixtures at low energies

The stability of antimatter in contact with matter has been investigated. The interaction between hydrogen and antihydrogen is considered as the prototype reaction. We have focused interest on the rates for proton-antiproton and/or electron-positron annihilation during hydrogen-antihydrogen collisions at low energies. In particular, we have concentrated on the calculation of the rates for the rearrangement reaction leading to formation of protonium and positronium, ending inevitably in particle-antiparticle annihilation. The cross section for the rearrangement collision has been calculated in a fully quantum-mechanical treatment. Additionally, we have calculated cross sections for direct annihilation during the collision process, which was found to be comparable to the rearrangement cross section. The elastic cross section and its low-energy limit, given by the scattering length, have been calculated, allowing, by comparison to the inelastic processes, a prediction for the efficiency of cooling antihydrogen via collisions with ultracold hydrogen atoms.

Time-dependent coupled-channel calculations of positronium-formation cross sections in positron-hydrogen collisions

We demonstrate the usefulness of the time-dependent coupled channel (TDCC) method in theoretical studies of atomic rearrangement collisions. In the TDCC method, coupled-channel partial differential equations for scattering wave functions are solved time dependently. We develop the Hankel-interpolation method, which is used to transform wave functions from an initial coordinate to a final coordinate system. By using this method, reaction cross sections of rearrangement collisions can be directly extracted from solutions of TDCC equations. We calculate positronium-formation cross sections in positron-hydrogen collisions in which a considerable number of theoretical calculations and precise experiments have been made. The present results for positron energies of 6.8--35 eV are in excellent agreement with the recent experiment of Zhou et al. [Phys. Rev. A 55, 361 (1997)].

Cumulative reaction probability and reaction eigenprobabilities from time-independent quantum scattering theory

The cumulative reaction probability (CRP) is a gross characteristic of rearrangement collision processes defining the reaction rate constant. This paper presents a complete development of the approach to the theory of CRP that we have recently proposed [Phys. Rev. Lett. 80, 41 (1998)]. In the core of this approach lies an alternative expression for CRP in terms of the outgoing wave Green's function which is formally equivalent to the Miller's definition of this quantity in terms of the scattering matrix [J. Chem. Phys. 62, 1899 (1975)] and to the Miller-Schwartz-Tromp formula [J. Chem. Phys. 79, 4889 (1983)], but is direct, in contrast to the former, and more suitable for practical calculations than the latter. Furthermore, our approach rests on solid grounds of time-independent quantum scattering theory and provides an appealing competitive alternative to the absorbing potential formulation given by Seideman and Miller [J. Chem. Phys. 96, 4412 (1992); 97, 2499 (1992)]. Ideologically, it is close to the approach considered earlier for a one-dimensional model by Manolopoulos and Light [Chem. Phys. Lett. 216, 18 (1993)], but is formulated from scratch for realistic systems with many degrees of freedom. The strongest point of our approach is that its final working formulas are expressed in terms of the Wigner-Eisenbud $\mathcal{R}$ matrix, so they can be easily implemented on the basis of many existing quantum scattering codes. All these features are discussed and illustrated by calculations of the CRP and reaction eigenprobabilities for two prototypical light atom transfer reactions in heavy-light-heavy triatomic systems in three dimensions for zero total angular momentum.

Thermotectonic history of the Bassian Rise, Australia: implications for the breakup of eastern Gondwana along Australia’s southeastern margins

During the Mesozoic breakup of eastern Gondwana, rifting occurred along both the Southern Ocean Rift (SOR) between Australia and Antarctica, and the Tasman Sea Rift (TSR) between Australia and the Lord Howe Rise. As a consequence of breakup, Tasmania, located at the southeastern tip of Australia, is now surrounded by rifted margins, including a failed branch of the SOR within Bass Strait to the north. Apatite fission track (AFT) results from the Furneaux Islands, located along the Bassian Rise at the eastern edge of Bass Strait, record two major episodes of rapid cooling/denudation since breakup began. The first occurred during the middle Cretaceous at ∼94±2 Ma and documents the timing of formation of the Bassian Rise. This was followed during the Early Tertiary by the second episode at some time between ∼65–45 Ma, possibly related to Pacific plate rearrangement and north Australian collision in New Guinea. Importantly, the AFT results indicate that formation of the Bassian Rise occurred during the middle Cretaceous at the same time as: (1) rifting along the TSR began, and (2) rifting along the SOR within Bass Strait ceased. This suggests a link between the timing of formation of the Bassian Rise and the initiation and cessation of major rifting events along the adjacent margins.

Multichannel Schwinger’s principle for rearrangement collisions: Positronium formation in positron-hydrogen collisions

Post and prior forms of the multichannel Schwinger's principle for rearrangement collisions are presented using discrete basis sets. An application is made to positronium formation in positron-hydrogen collisions at low energies in the range 6.8--30.0 eV. A total number of eight terms of a type of correlated basis functions involving inverse powers of half-odd integers is required to predict accurate results in conformity with the available variational and nonvariational values in the literature. Our findings indicate that destructive interference between partial-wave contributions to the scattering amplitude is responsible for the appearance of critical angles in positronium formation. Surface plots of the differential cross section display immensely rich structure. The total positronium formation cross sections agree nicely with the observed data of Zhou et al. [Phys. Rev. A 55, 361 (1997)] in the entire energy range.