J.H. McGuire

Electron capture from H2 targets by H+ and He2+ ions. Dependence of the cross sections on the orientation of the molecular axis

We study the electron capture process from molecular hydrogen targets by proton and alpha particle impact at high collision velocities. The dependence of the differential cross sections on the orientation of the internuclear axis of H2 is evaluated within a two effective center approximation for capture to selective final states of the projectile. In particular, the molecular B1B model is applied in the calculations. The molecular orientation effects observed are due to the interference between the scattering amplitudes from the two centers of the target.

Analytic description of population transfer in a degenerate n-level atom

Analytic expressions have been found for transition probabilities in a degenerate n-level atom interacting with a strong external field that gives a common time dependence to all of the transition matrix elements. Except for solving a simple nth-order equation to determine eigenvalues of dressed states, the method is entirely analytic. These expressions may be used to control electron populations in degenerate n-level atoms. Examples are given for n = 2 and 3.

Spatial and temporal correlation in dynamic, multi-electron quantum systems

Cross sections for ionization with excitation and for double excitation in helium are evaluated in a full second Born calculation. These full second Born calculations are compared to calculations in the independent electron approximation, where spatial correlation between the electrons is removed. Comparison is also made to calculations in the independent time approximation, where time correlation between the electrons is removed. The two-electron transitions considered here are caused by interactions with incident protons and electrons with velocities ranging between 2 and 10 au. Good agreement is found between our full calculations and experiment, except for the lowest velocities, where higher Born terms are expected to be significant. Spatial electron correlation, arising from internal electron-electron interactions, and time correlation, arising from time ordering of the external interactions, can both give rise to observable effects. Our method may be used for photon impact.

Time ordering in atomic collisions

We present a general treatment for the effect of time ordering in the many-electron processes induced by charged-particle impact. We show that time ordering is essential in order to obtain odd power Z terms in the cross section which is necessary to get different values for positively and negatively charged projectiles. A second-order calculation is performed for the double ionization of helium. The electron - electron interaction is taken into account only by the shake-off term. The interference between the shake-off and second-order term is responsible only for a part of the dependence of the cross sections on the sign of the charge of the projectile.

INTERFERENCE EFFECTS IN COLLISIONS BETWEEN HEAVY IONS AND MOLECULAR TARGETS

Abstract The transfer-excitation process in O8++H2 collisions is studied at high impact energies using an independent event model. The transfer-excitation probability is expressed as a product of a capture probability from H2 (PC), leaving the H2+ in its ground state, and an excitation probability (PE) of the residual target to a dissociative state. PC is evaluated in the Continuum Distorted Wave-Eikonal Final State (CDW-EFS) model and PE is calculated in the first Born approximation (FBA). The differential cross-sections are studied as a function of the molecular orientation and the interference patterns obtained are compared with the available experimental data.

Influence of the molecular alignment on electron capture from H2 by O8+ projectiles

Abstract The single electron capture process by impact of O8+ on H2 targets is studied. The Continuum Distorted Wave-Eikonal Final State (CDW-EFS) model, is used within a two independent center approximation. The influence of the molecular orientation on the single electron capture process is analyzed. Differential and total cross sections are calculated and compared with previous Oppenheimer-Brinkman-Kramers (OBK) results and experimental data.

Complete population transfer in a degenerate three-state atom

We have found conditions required to achieve complete population transfer, via coherent population trapping, from an initial state to a designated final state at a designated time in a degenerate three-state atom, where transitions are caused by an external interaction. Complete population transfer from an initially occupied state one to a designated state two occurs under two conditions. First, there is a constraint on the ratios of the transition matrix elements of the external interaction. Second, there is a constraint on the action integral over the interaction, or area, corresponding to the phase shift induced by the external interaction. Both conditions may be expressed in terms of simple odd integers. Some specific examples are discussed.

Time ordering in kicked qubits

We examine time ordering effects in strongly, suddenly perturbed two-state quantum systems (kicked qubits) by comparing results with time ordering to results without time ordering. Simple analytic expressions are given for state occupation amplitudes and probabilities for singly and multiply kicked qubits. We investigate the limit of no time ordering, which can differ in different representations.

Population control of 2s-2p transitions in hydrogen

We consider the time evolution of the occupation probabilities for the 2s-2p transition in a hydrogen atom interacting with an external field V(t). A two-state model and a dipole approximation are used. In the case of degenerate energy levels an analytical solution of the time-dependent Schroedinger equation for the probability amplitudes exists. The form of the solution allows one to choose the ratio of the field amplitude to its frequency that leads to temporal trapping of electrons in specific states. The analytic solution is valid when the separation of the energy levels is small compared to the energy of the interacting radiation.

Time ordering in multi-electron dynamics

Time ordering of interactions in dynamic quantum multi-electron systems provides a constraint that interconnects the time evolution of different electrons. In energy space, time ordering appears as the principal value contribution from the Green function, which corresponds to the asymptotic condition that specifies whether the system has outgoing (or possibly incoming) scattered waves. We report evidence of effects of time correlation found by comparing calculations to recent spectropolarimetric data.