Jerry Peacher

Theoretical calculation of fully differential cross sections for electron-impact ionization of hydrogen molecules

We have recently proposed the orientation averaged molecular orbital (OAMO) approximation for calculating fully differential cross sections (FDCS) for electron-impact ionization of molecules averaged over all molecular orientations. Orientation averaged FDCS were calculated for electron-impact ionization of nitrogen molecules using the distorted wave impulse approximation (DWIA) and the molecular three-body distorted wave (M3DW) approximation. In this paper, we use the same methods to examine the FDCS for ionization of hydrogen molecules. It is found that the DWIA yields reasonable results for high-energy incident electrons. While the DWIA breaks down for low-energy electrons, the M3DW gives reasonable results down to incident-electron energies around 35 eV.

Radiative lifetime of the B2Σ− state of CH

The radiative lifetimes of the N′=3 to 15 rotational states of the v′=0 level of the B  2Σ− state of CH were measured. Emission lines were observed to N′=15, but no lines above this rotational state were observed. This result agrees with previous studies. The lifetimes of all rotational states were between 300 and 400 ns. Previous measurements have indicated that the N′=15 level might have a lifetime as short as 100 ns.

Four-body charge transfer processes in proton–helium collisions

Recent advancements in experimental techniques now allow for the study of fully differential cross sections (FDCS) for four-body collisions. The simplest four-body problem is a charged particle collision with a helium atom, in which both atomic electrons change state. This type of collision can result in many different outcomes, such as double excitation, excitation ionization, double ionization, transfer excitation, transfer ionization and double charge transfer. In this paper, we compare absolute experimental proton–helium FDCS for transfer excitation with the fully quantum mechanical 4BTTE (four-body transfer with target excitation) model. This model was previously used to study TTE for proton energies between 25 and 75 keV and reasonable agreement was found with the experimental data for large scattering angles, but not small angles. Since this is a first-order model, which contains contributions from all higher order terms, one would expect improved agreement with increasing energy and the purpose of this work was to look at higher energies. We found that the agreement with the magnitude of the experimental data became worse with increasing energy while the agreement with the shape of the data was reasonably good. Consequently, we conclude that the model contains the physical effects that determine the shape but not the magnitude of the cross section.

Post collision interactions in fully differential cross sections for four-body charge transfer processes

Recently, experimental fully differential cross sections have been reported for proton?helium collisions. In this work we will examine single capture, transfer with target excitation and double capture for proton?helium collisions. For single capture, the proton captures one electron from helium and leaves the other electron in the ground state. For the transfer-excitation case, the target is excited to arbitrary excited states. In the case of double capture, the proton captures both of the electrons from helium and leaves the collision as an H??ion. We introduce a fully quantum mechanical four-body model that includes the final state post collision interactions between all two-particle pairs exactly. In the initial state, the two-particle interactions are included asymptotically by using an Eikonal wavefunction. The results of this model are in better agreement with experimental data than previous quantum mechanical calculations.

Fully differential cross sections for C6+ single ionization of helium

We have examined the fully differential cross section (FDCS) for single ionization of helium by a 2 MeV amu−1 C6+ ion. The FDCS is presented for a variety of momentum transfers and ejected electron energies. The theoretical model we use, labelled 3DW-EIS (three-body distorted wave—eikonal initial state), treats the collision as a three-body problem (projectile, active electron, residual ion). In the final state, each two-particle pair is treated exactly and the initial state is an eikonal state which contains the proper asymptotic forms of the projectile–target ion and projectile–electron interactions. Most importantly, the final state of the ejected electron is treated as a distorted wave calculated numerically from the static Hartree–Fock potential for the ion. Our theoretical results are compared with both absolute experimental measurements and previous theoretical calculations. It is shown that the 3DW-EIS results are in good agreement with experiment for all cases except large momentum transfer and low ejected-electron energies.

Rotational and predissociation lifetimes of the A 2 ∑ + state of OD*

The lifetime and predissociation probabilities of various rotational levels of the A2∑+ state of OD were measured. A strong predissociation at N′ = 35 was observed. This predissociation could be explained by the level crossing of the A2∑+ state with the repulsive 4∑− state which are mixed by a weak spin–orbit interaction.

LATTICE, TIME-DEPENDENT APPROACH FOR ELECTRON-HYDROGEN SCATTERING

A time-dependent approach for treating electron-hydrogen scattering is reported that utilizes a fully correlated two-electron wavefunction represented on a three-dimensional lattice using the basis-spline collocation method. The lattice, time-dependent approach obviates the need for consideration of the three-body Coulomb boundary conditions, avoids the use of severe approximations such as those of perturbation theory for slow collisions, and provides a relatively dense representation of the one- and two-electron continua. Probabilities for excitation and ionization are computed by projection onto lattice eigenstates of the H atom. Partial cross sections for excitation and ionization are obtained and compared with results of other theoretical methods for the 1S and 3S channels.

Projectile interactions in theoretical triple differential cross sections for simultaneous excitation?ionization of helium

The importance of projectile interactions in triple differential cross sections (TDCS) is explored for the problem of simultaneous excitation–ionization of helium by electron impact using a new approach that we call the four-body distorted wave model (4DW). The 4DW model includes all projectile interactions, namely initial- and final-state projectile–target interactions, and the post-collision interaction between the two continuum electrons. Results are presented for an incident electron energy of 500 eV and are compared to experimental data, as well as a second-order R-matrix theory and the first Born approximation. Results for absolute TDCS ratios of ionization without excitation to excitation–ionization are also presented.

Highly charged particle impact ionization of He

Absolute experimental measurements have been reported for the fully differential cross section (FDCS) for single ionization of helium by 3.6 MeV/u AuQ+ (Q = 24, 53) ions. These absolute measurements are not in very good agreement with CDW-EIS (continuum distorted wave-eikonal initial state) calculations which are normally in excellent agreement with doubly differential cross-section measurements. We have recently introduced the 3DW-EIS approach which is a fully quantum mechanical approach. In addition, we have used a Hartree–Fock distorted wave for the outgoing electron instead of approximating the ejected electron as a Coulomb wave with an effective charge. For the case of low-energy C6+ ionization of helium, the 3DW-EIS model was in better agreement with absolute experimental data than the CDW-EIS approximation. The purpose of this paper is to investigate the validity of the 3DW-EIS model for more highly charged ions. Our theoretical results for the single ionization of helium by highly charged gold particles are in poor agreement with experimental results. The experiment displays unique forward peak structures in the FDCS that were not seen in the theoretical approach. We consider possible mechanisms to account for this forward peak structure.

A convenient formalism for Auger and autoionization of overlapping resonances

In our recent work we applied the multiple overlapping autoionizing resonance formalism of Davis and Feldkamp to electron–cadmium ionization. We also extended their formalism to include the scattered electrons angular dependence in order to calculate the triple-differential cross section. In this paper we generalize our formalism in order to apply it to any process involving overlapping resonances. We show that the effect of the interaction between different resonances can be treated as a correction factor to the isolated resonance treatment of the problem. We have applied our approach to four physically significant cases, each consisting of two overlapping resonances. Our results indicate that even in the case where the resonances are only somewhat overlapping, the correction factor makes a considerable contribution. In the case where one of the resonances is very close to the other one, the deviation from the isolated resonance treatment is significant.