V H Ponce

Two-centre effects in ionization by ion impact

When an atom is ionized by ion impact, the electron is ejected into a final continuum state of a two-centre potential due to the Coulomb fields of the projectile and ionized atom. The related effects on the electron yield or energy and angular distributions are referred to as two-centre electron emission (TCEE). The present report is devoted to a discussion of experimental and theoretical evidence of this TCEE. The use of heavy ions or antiprotons as projectiles allows to unravel these effects by monitoring the two centre potential. On the theoretical side, the continuum distorted wave-eikonal initial state theory (CDW-EIS) accounts for the TCEE thus allowing a detailed interpretation of the experimental findings.

A theoretical model for ionisation in ion-atom collisions. Application for the impact of multicharged projectiles on helium

The continuum-distorted-wave-eikonal-initial-state model is extended to describe single-electron ionisation by impact of bare projectiles on multielectronic targets. Applications are given for collisions between multicharged ions and helium. Double differential, single differential and total cross sections are calculated. Experimental data and present theoretical results show deviations from the square of the projectile charge dependence predicted by the first Born approximation.

Beyond the quantum adiabatic approximation: Adiabatic perturbation theory

Aside from interpretation, Quantum Mechanics (QM) is undoubtedly one of the most successful and use- ful theories of modern Physics. Its practical impor- tance is evidenced at microscopic and nano scales where Schrodingers Equation (SE) dictates the evolution of the systems state, i.e., its wave function, from which all the properties of the system can be calculated and confronted against experimental data. However, SE can only be ex- actly solved for a few problems. Indeed, there are many reasons that make the solution of such a differential equa- tion a difficult task, such as the large number of degrees of freedom associated with the system one wants to study. Another reason, the one we want to address in this pa- per, is related to an important property of the systems Hamiltonian: its time dependence. For time independent Hamiltonians the solution to SE can be cast as an eigenvalue/eigenvector problem. This allows us to solve SE in many cases exactly, in particular when we deal with systems described by finite dimen- sional Hilbert spaces. For time dependent Hamiltonians, on the other hand, things are more mathematically in- volved. Even for a two-level system (a qubit) we do not, in general, obtain a closed-form solution given an arbi- trary time dependent Hamiltonian, although a general statement can be made for slowly varying Hamiltonians. If a systems Hamiltonian H changes slowly during the course of time, say from t = 0 to t = T, and the system is prepared in an eigenstate of H at t = 0, it will re-

Electron emission from multielectronic atoms by ion impact at intermediate and high energies

The continuum-distorted-wave-eikonal-initial-state model is extended to describe single ionisation from an arbitrary initial state represented by a linear combination of Slater-type orbitals. Applications are given for ionisation of neon by protons with impact energies ranging from 100 keV to 5 MeV. Double-differential cross sections are calculated for ionisation from each subshell. These cross sections summed over all subshells are compared with available experimental data.

Ionisation of the first excited state of hydrogen by bare ions at intermediate and high velocities

The continuum distorted-wave-eikonal initial-state model is extended to study ionisation of hydrogen atoms in arbitrary initial states by multiply charged ion impact. These results are used to study total cross sections as a function of the impact energy for ionisation of the first excited stage by proton impact. The dependence on the projectile charge is discussed and compared with results for the ionisation of the ground state and with predictions from the first Born approximation.

Ionisation of helium by antiproton and proton impact

Single-electron ionisation of helium atoms by impact of antiprotons and protons is studied. The continuum-distorted-wave-eikonal-initial-state model is employed. Calculations of double differential cross sections, as a function of the final electron energy and for fixed electron-ejection angles, are given. A dip in the energetic region of electrons moving with low momentum with respect to the projectile is analysed. The importance of describing the dynamics of the electron in the simultaneous fields of the projectile and of the residual target is demonstrated.

Single-electron ionisation of helium by anti-proton and proton impact

Single-electron ionisation of He by 200 keV and 500 keV anti-proton and proton impact is studied. Single differential cross sections as a function of the energy and angle of the ejected electron and total cross sections are calculated with the continuum distorted-wave-eikonal initial-state model. Present results are compared with calculations performed with the classical trajectory Monte Carlo method and with experimental data for proton impact. A very good agreement is obtained with experiments and predicted differences between anti-proton and proton results are confirmed but of smaller magnitude.

The CDW‐EIS Method

The electron ionization in ion‐atom collisions is studied by using the Continuum Distorted Wave‐Eikonal Initial State model. Some relevant aspects of this theory are reviewed. Two‐center effects are analyzed.

Exchange contribution to binary electron emission in collisions between multiply charged hydrogenic projectiles and light targets

The emission of electrons from light targets bombarded by hydrogenic projectiles is approximated by the elastic scattering of electrons by these ions. The electron exchange transition amplitude is considered in a first-order distorted-wave approximation, and contributions attached to the non-orthogonality of continuum and bound electron states are fully incorporated into the calculation. A close agreement with measured cross sections is obtained.