Full two-electron calculations of antiproton collisions with molecular hydrogen

Total cross sections for single ionization and excitation of molecular hydrogen by antiproton impact are presented over a wide range of impact energy from 1 keV to 6.5 MeV. A nonpertubative time-dependent close-coupling method is applied to fully treat the correlated dynamics of the electrons. Good agreement is obtained between the present calculations and experimental measurements of single-ionization cross sections at high energies, whereas some discrepancies with the experiment are found around the maximum. The importance of the molecular geometry and a full two-electron description is demonstrated. The present findings provide benchmark results which might be useful for the development of molecular models.Comment: 4 pages, 3 figure

Theory of Ion-Atom Collisions for Stopping Power Calculations

This thesis is devoted to calculations of stopping power in ion-atom and ion-molecule collisions. A single-centre convergent close-coupling method is used to calculate the stopping power of simple atoms and molecules, including the water molecule, for antiprotons. A two-centre convergent close-coupling method, which includes electron capture, is used to calculate the stopping power of hydrogen for protons. The results presented in this thesis can be used for radiation-dose simulations in hadron therapy

Electron correlations in the antiproton energy-loss distribution in He

We present ab initio calculations of the electronic differential energy-transfer cross sections for antiprotons with energies between 3 keV and 1 MeV interacting with helium. By comparison with simulations employing the mean-field description based on the single-active electron approximation we are able to identify electron correlation effects in the stopping and straggling cross sections. Most remarkably, we find that straggling exceeds the celebrated Bohr straggling limit when correlated shake-up processes are included

Impact-parameter convergent close-coupling approach to antiproton-atom collisions

This thesis is devoted to extension of the convergent close-coupling (CCC) method to heavy projectiles and its application to the theoretical studies of antiproton scattering on the hydrogen and helium targets.In the Introduction (Chapter 1) the motivation for the study and the current status of antiproton scattering on hydrogen and helium are presented. Other theoretical methods that previously have been applied to these problems are reviewed and their limitations are indicated. The extension of the fully quantummechanical CCC method to ion-atom collisions is presented in Chapter 2. The derivations of the momentum-space coupled-channel Lippmann-Schwinger integral equations from the exact Schr¨odinger equation is given in detail. Transition matrix elements are derived in momentum-space. In Chapter 3 a direct method for solving multi-dimensional Lippmann-Schwinger integral equations without recourse to partial-wave expansion or any other transformation scheme will be described. A direct method has been applied to the antiproton-hydrogen as well as to the proton-hydrogen collisions. In Chapter 4 we solve the full multichannel problem by transforming the coupled-channel integral equations into the impactparameter representation. The scattering amplitude necessary to calculate the differential and total cross sections will be derived from the transition matrix elements. The results of the CCC calculations for antiproton scattering from atomic hydrogen and helium are presented and compared with available experimental data and the results of other calculations in Chapter 5 and Chapter 6, respectively. Finally, in Chapter 7, we draw conclusions arising from this work and indicate future directions for the research.Main results of this work • The convergent close-coupling method has been extended to heavy projectiles and applied to antiproton scattering on atomic hydrogen and helium. • For the first time, the relative motion of the heavy particles in antiproton collisions with atomic hydrogen and helium has been treated quantummechanically. • A direct method to solving the three-dimensional momentum-space coupledchannel Lippmann-Schwinger integral equations has been developed. • A scheme for transforming the three-dimensional Lippmann-Schwinger integral equations into the impact-parameter representation has been developed. The fully off-shell transition matrix elements in the impactparameter space have been derived. • For the first time, the fully quantum mechanical calculations of the cross sections for all the major channels of interest in antiproton collisions with hydrogen and helium have been performed over a wide range of scattering energies. • The total ionization cross sections for the H target has been calculated. The results are in excellent agreement with the available experiment. An overall agreement of the present results with the semiclassical calculations by other groups has practically confirmed the validity of the semiclassical approximation imposed on the relative heavy particle motion. • The total cross section for the He single ionization has been calculated using frozen-core (FC) and multi-configuration (MC) approximation for the target. As opposed to rather sophisticated and rigorous MC calculations the FC results agree with the experimental data at a wider energy range. • For the first time, based on the fully quantum-mechanical treatment of the problem the triple differential cross sections have been calculated for antiproton scattering on both H and He.• The p−H results for the various differential ionization cross sections agree reasonably well with the results of the semiclassical close-coupling and the continuum-distorted-wave-eikonal-initial-state (CDW-EIS) approaches, particularly at high energies. • The longitudinal ejected electron and recoil-ion momentum distributions for the single ionization of helium have been calculated. The results are in good agreement with the available experimental data

Large nuclear scattering effects in antiproton transmission through polymer and metal-coated foils

We simulate the deceleration and transmission of antiprotons with keV-scale kinetic energies through polymer foils using a molecular dynamics approach, which includes a model of nuclear stopping based on the attractive interaction potentials between antiprotons and target atoms calculated by quantum chemical methods. Antiprotons scatter into larger angles with higher cross sections than protons. This causes a significant fraction of antiprotons to annihilate in the foil instead of emerging with energies of a few keV, especially when coatings of materials with high atomic number are applied to the surfaces. The simulation results are in good agreement with data from two experiments that involved pulsed antiproton beams with incident energies between 63 keV and 122 keV that traverse polymer foils with thicknesses of approximate to 1.3 mu m and 1.8 mu m. The 25-nm-thick layers of Ag on the latter foil reduced the transmission of antiprotons. The results will be utilized to design the degrader foils in laser spectroscopy experiments of antiprotonic helium atoms and experiments involving Penning traps that are carried out at the ELENA facility of CERN.Peer reviewe

Angular momentum changing transitions in proton-Rydberg atom collisions

Collisions between electrically charged particles and neutral atoms are central for understanding the dynamics of neutral gases and plasmas in a variety of physical situaziones of terrestrial and astronomical interest. Specifically, redistribution of angular momentum states within the degenerate shell of highly excited Rydberg atoms occurs efficiently in distant collisions with ions. This process is crucial in establishing the validity of the local thermal equilibrium assumption and may also play a role in determining a precise ionization fraction in primordial recombination. We provide an accurate expression for the non-perturbative rate coefficient of collsions between protons and H(n_l) ending in a final state H(n_l'), with n being the principal quantum number and l,l' the initial and final angular momentum quantum numbers, respectively. The validity of this result is confirmed by results of classical trajectory Monte Carlo simulations. Previous results, obtained by Pengelly and Seaton only for dipole-allowed transitions, l--->l+-1, overestimate the l-changing collisional rate approximately by a factor of six, and the physical origin of this overestimation is discussed.Comment: 19 pages, 3 figure

Wave-Packet Convergent Close-Coupling Approach to Ion-Atom Collisions

We extend the two-centre wave-packet convergent-close approach to proton scattering on excited states of hydrogen, collisions of bare ions (He2+ and C6+) with hydrogen, and proton collisions with the helium atom and He+ ion. The wave-packet approach is used to discretise the continuum of the involved atoms. The total cross sections for direct scattering, electron capture and ionisation, and the differential ionisation cross sections are calculated and compared with experimental data and other calculations, where available

Quantum-Mechanical Approach to Collision-Induced Radiative Emissions

Charge exchange is a process that occurs in an atomic collision where an electron from one of the colliding particles is transferred to the other; typically from a neutral atom or molecule to an ion. Electrons transferred into an excited energy state then decay into a lower-energy state and emit photons during this process. This phenomenon of collision-induced radiative emissions is of great interest in astrophysics and experimental x-ray spectroscopy research since it helps understand the production of x-rays in astrophysical settings. On the theoretical side, obtaining a description of these radiative emissions involves numerical work since a closed-form solution is not possible. Using standard numerical approaches, one needs to rely on models and approximations, especially in collision problems involving many-electron systems. Consequently, results obtained in this way can be at odds with experimental observations and/or results from different theoretical methods. In this dissertation, the main method is the two-centre basis generator method performed within the independent electron model. It is a dynamical approach to solving atomic collision problems and has shown to be reliable in describing charge exchange and other electronic processes. This work gives an extensive view on the applicability of this approach in the context of collision-induced radiative emissions where present results from a variety of ion-atom and ion-molecule collisions are benchmarked with results from previous studies