V D Rodríguez

Electron capture to the continuum by proton and positron impact

We describe recent experimental triple-differential cross sections of ionization by the impact of both protons and positrons by means of a single, kinematically exact theory. This model incorporates all the interactions in the final state on an equal footing and keeps an exact account of the three-body kinematics. We show that these provisions make it possible to evaluate any multiple-differential cross section for any given mass configuration, and analyse how it changes with the relative masses of the three particles in the final state. We analyse the differences in the electron cusp formation by both heavy and light projectile impact at the double-differential cross section level.

Cusp formation in the single-particle momentum distributions of three-body continuum states

We investigate the appearance of cusp structures in single-particle momentum distributions for processes leading to three-body continuum states. We discuss under which circumstances these structures are produced, and identify some of their properties by means of a final-state interaction theory. Finally, we show how they change with the relative mass ratios of the three particles in the final state.

CDW-EIS theoretical calculations of projectile deflection for single ionization in highly charged ion–atom collisions

Abstract We present continuum distorted wave-eikonal initial state (CDW-EIS) theoretical calculations for the projectile deflection in single ionization of helium by heavy-ion impact as a function of ionized electron energies. These calculations account for the helium passive electron shielding in the internuclear interaction improving standard CDW-EIS theory. The results are compared with recent experimental results by impact of 100 MeV/amu C 6+ and 3.6 MeV/amu Au 53+ . For highly charged projectiles there is a poor quantitative agreement between theory and experiment. However, this refined calculation does share some qualitative features with the data. In particular the variation of the effective charge of the residual He + ion from Z eff =1 to Z eff =2 when going from small to large projectile scattering angles is able to represent a shoulder observed in the double differential cross sections. Important qualitative differences are observed at the level of triple differential cross sections.

Projectile angular distribution in single ionization of helium by proton impact at high and intermediate energies

The CDW-EIS theoretical method is applied to explain recent measurements of helium single ionization by proton and deuteron impact. This theory can cope with the proton angular distribution data for a wide range of projectile energy from 100 keV to 6 MeV . At intermediate energies the CDW-EIS is shown to considerably improve the first Born approximation. The theoretical angular differential cross sections consider the internuclear interaction required for large scattering angles. A seminumerical technique for the introduction of the internuclear interaction is developed.

Longitudinal momentum distributions in ionization of helium by fast, highly charged projectiles

The longitudinal momentum distributions for the recoil ion, the electron and the projectile momentum transfer in single ionization of He by 3.6 MeV amu-1 Ni24+ impact are calculated using the CDW-EIS method within the independent electron approximation. Results are compared with the measurements and CTMC calculations of Moshammer et al. (1994). It is found that the CDW-EIS theory explains qualitatively the observed enhancement of the electron distribution in the forward direction, and the enhancement of the recoil-ion distribution in the backward direction. Our calculations show discrepancies with the measurement regarding the positions for the maxima of these distributions. A calculated electron energy spectrum is in good agreement with the measurement.

Above-threshold ionization of atoms by resonant XUV laser pulses

Above-threshold ionization of atoms by XUV short laser pulses with frequencies close to the resonant 1s?2p transition is investigated. We present a theory based on a variational expression using trial wavefunctions for the final and the initial states. For the former we use a Coulomb?Volkov wavefunction, and for the latter a close-coupling solution of the time-dependent Schr?dinger equation considering a few bound states. The close-coupling Coulomb?Volkov theory, fully accounting for the important 1s?2p transition, explains the photoelectron spectrum as well as the total ionization cross sections for the resonant case. We also compare the partial wave populations and angular distributions given by the theory with the numerical solutions of the time-dependent Schr?dinger equation.

Cusp formation and threshold effects in break-up collisions

We show how the single-particle double differential cross sections (DDCS) in breakup collisions are intertwined by dynamical constraints. In particular, we study the corresponding relations among threshold and cusp structures and identify some of their properties by means of a final-state interaction theory. We provide general expressions for the cusp and threshold structures that any theoretical description of the collision process has to fulfil. Finally, we show how these structures change with the relative mass ratios of the three particles in the final state.

Classical description of threshold effects in ion-atom ionization collisions

We employ a three-body classical trajectory Monte Carlo (CTMC) method to calculate the recoil-ion momentum distribution at its kinematic threshold in ion-atom ionization collisions. We analyse how this threshold is intertwined by dynamical constraints to the electron capture to the continuum cusp in the electron double differential cross section. We compare these calculations with those from a full quantum-mechanical description and explore how these structures depend on the interactions among the three particles in the final state.

Differential cross sections in antiproton and proton-helium collisions

Cross sections are presented for the first time for antiproton-helium collisions at an energy of 945 keV differential in longitudinal electron and recoil-ion momenta. The longitudinal momentum distributions for antiproton impact are compared with 1 MeV proton-helium collision. The electron and the recoil-ion momentum distributions for antiprotons agree with those for protons to within 10%. We did not observe a difference between antiproton impact and proton impact. A comparison with CDW and CTMC theories is presented.

Ion-atom excitation at high and intermediate energies

Exact post and prior impulse approximation cross sections are calculated for the excitation process H++H (1s to n=2). The results show a large post-prior discrepancy and a huge disagreement with the experiments, even at energies where the simple Born approximation already works. Following a previous work along this line, the authors show that the theory remarkably improves when the correct asymptotic conditions are observed by including eikonal phases. This new approximation, called the eikonal impulse approximation, presents a negligible prior-post discrepancy at high energies and the prior version produces very good agreement with the experiments. Departure of the total excitation cross section from the quadratic law on the projectile charge (saturation) is discussed and compared with the available data. Scaling laws, alignment factors and s-p coherences are reported and compared with previous results.