Emil Y. Sidky

QUANTUM MECHANICAL CALCULATION OF EJECTED ELECTRON SPECTRA FOR ION-ATOM COLLISIONS

We present a quantum mechanical method for calculating the ejected electron probability distribution in ion-atom collisions. The time-dependent Schr?dinger equation, formulated with a classical straight-line trajectory for the internuclear motion, is integrated directly on a two-centre momentum space grid. The time-dependent wavefunction at large internuclear separation is analysed to extract the ejected momentum distribution as well as excitation and charge transfer amplitudes. Calculations have been performed at collision velocities of 1, 2 and 5 au for proton and antiproton collisions with atomic hydrogen, at a few impact parameters. Excitation and charge transfer probabilities are also calculated to check against results obtained from the close-coupling method. The ejected electron momentum distributions are shown.

Long lived CH2+ and CD2+ dications

Abstract A search for long lived CH 2+ and CD 2+ dications formed in fast charge stripping collisions of CH + and CD + on Ar was conducted. An experimental method based on the detection of the H (or D) fragments of the dication was developed, in order to eliminate possible confusion with 13 C 2+ for the first and 14 N 2+ for the latter. The flight time of these dications through the apparatus is about 70 ns, well below the 3 μs time associated with earlier observations of CH 2+ . Our measurements indicate that no long lived states of either of these dications are formed in fast charge stripping collisions. However, this result does not exclude the possibility that long lived states, like the excited A 2 Σ + metastable state, are populated in slow charge stripping collisions.

Charge transfer in collisions

By means of a crossed-beams technique we have measured absolute cross sections for the charge-transfer reaction by coincident detection of the product ions and for CM energies between 4 and 200 keV. Estimates of angular differential cross sections are made from measured scattering distributions of reaction products at CM energies of 5.15 and 15.86 keV. The total cross sections for charge transfer are calculated by close coupling of a two-centre atomic basis.

Studies of charge exchange in symmetric ion-ion collisions

We have measured the total cross sections for single charge exchange in He2+-He+, Ne2+-Ne+ and Ar2+-Ar+ collisions at centre-of-mass energies of 1.8-14.8, 1.8-10.8 and 2.8-7.3 keV, respectively, using an intersecting beam technique. The results for He2+-He+ collisions are compared with existing measurements and calculations. For Ne2+-Ne+ and Ar2+-Ar+ the measured cross sections are compared with results from new close-coupling calculations based on a one-electron model potential description of the collision system.

Charge transfer in collisions of H2+ ions with He2+ and Ar2+

Using the crossed-beams technique, we have measured absolute total cross sections for electron capture from H + molecular ions by He 2+ and Ar 2+ at relative velocities vrel = 0.7–1.3 au. With He 2+ a distinct maximum around vrel = 1a u is observed, which can be attributed to the large Q-value for the dominant channel of this reaction. Theoretical calculations using an atomic model for ion–molecule collisions are in very good agreement with the experimental data. Charge changing collisions between ions are of fundamental interest as an ideal testing ground for theory [1–4] as well as for their application in plasma physics, accelerator and fusion research [5]. In contrast to ion–atom collisions, however, the study of ion–ion collisions is still in its infancy and, furthermore, most studies focused on collisions between atomic ions. A few experiments (e.g. [6, 7]) have involved collisions of molecular ions with negative atomic ions. On the other hand, in the last few years charge transfer between highly charged ions and neutral molecules has attracted increasing interest as it gives detailed insight into both the electronic processes involved during the collision and the dynamics of the molecular fragmentation afterwards. H2 molecules especially, have received particular attention [8–10] due to the relative simplicity of their electronic structure [11]. However, despite its simplicity the neutral H2 molecule is still a two-electron system with all its inherent theoretical difficulties. Using the well established technique of crossed ion–ion beams [12], we have measured, for the first time, total cross sections for charge transfer between two positive ions, where one collision partner is a molecular ion. In particular, we have studied the following collision systems

The role of the potential saddle in He2+ + H impact ionization

We compute the full three-dimensional momentum space distribution of ejected electrons resulting from alpha particle impact ionization of hydrogen. At low impact velocities the transverse momentum distributions are shown to exhibit strong oscillations with energy. For the longitudinal component, the momentum distribution peaks near the target at high energies, shifts towards the projectile centre at intermediate energies and then back towards the target at low energies. The shift of the longitudinal momentum distribution towards the target at low energies gives the experimental signature of the importance of potential saddle for impact ionization at low energies.

Intermediate energy ionization of helium by proton impact

We have investigated impact ionization of He by protons at energies between 20 and 100 keV. Momentum spectra of the ejected electrons were measured for experimentally determined vector impact parameters using cold target recoil ion momentum spectroscopy techniques. At the lowest impact energy, the electron momenta lie close to the saddle point and as the energy increases they slowly move towards the target centre. The measurements are compared with the results of a theoretical calculation carried out using a two-centre momentum space discretization method. Qualitative agreement with the experiment is seen, and systematic disagreements between experiment and theory are discussed.

Electron capture and ionization for ion - Rydberg atom collisions in a magnetic field

Within the classical trajectory Monte Carlo (CTMC) model, we calculate electron transfer and ionization cross sections for singly charged ions colliding with Rydberg atoms in the presence of a laboratory-strength magnetic field of 4 tesla. A new method for generating a stationary microcanonical ensemble for a quasi-integrable initial-state Hamiltonian is presented. The calculated cross sections show signatures of electron capture and ionization mechanisms for the field-free case, e.g. multiple swaps and saddle-point electrons; their structure as well as their magnitude, however, are strongly modified by the presence of the magnetic field.