Andrew James Murray

Low-energy symmetric coplanar and symmetric non-coplanar (e, 2e) studies from the 3a1 state of H2O

Experimental and theoretical results are presented for electron impact ionization of water in the energy regime from near threshold to intermediate energies. Results were taken in symmetric coplanar and non-coplanar geometries, with both equal and non-equal outgoing electron energies. The models approximate the random orientation of the target using a spherical averaging of the wavefunction prior to the collision, using sophisticated distorted wave Born calculations that include post-collisional interactions in first order and to all orders of perturbation theory. The calculations predict the data most accurately at the lowest energy studied (4 eV above threshold) in a coplanar symmetric geometry, whereas the comparison between theory and experiment is generally marginal for higher energies and for non-coplanar geometries.

Real-time computer-optimized electron coincidence spectrometer

An electron coincidence spectrometer utilizing real‐time optimization and control by a standard IBM 80286 personal computer is described. Details of the system hardware and software are presented together with a description of the optimization routine adopted for maintaining the tuning of the spectrometer and data acquisition. Data collected by the computer‐controlled spectrometer for (e,2e) coincidence experiments are also presented.

Results from symmetric and non-symmetric energy sharing (e, 2e) experiments in the perpendicular plane

Results are presented of (e, 2e) coincidence measurements on helium in the perpendicular plane, defined as the plane orthogonal to the incident electron trajectory. The incident energy is varied in the range from 10 to 80 eV above the ionization threshold and results are presented for both symmetric energy sharing, where the outgoing electrons have the same energy, and for non-symmetric sharing. The experimental apparatus used to collect these results is a fully computer controlled and computer optimized spectrometer able to access a wide range of geometries from the perpendicular to the coplanar geometry.

Coplanar symmetric and asymmetric electron impact ionization studies from the 1b1 state of H2O at low to intermediate impact energies

(e, 2e) ionization differential cross sections are presented for incident electron energies ranging from 15 eV to 95 eV above the ionization threshold of the 1b1 molecular state of H2O. Experimental results and theoretical analysis were derived for three energies in a coplanar symmetric geometry, and for three energies in an asymmetric geometry. The experimental data show a wide variation in the cross section over this range of energies, whereas the theoretical analysis carried out using a sophisticated molecular DWBA model, which includes the final state post collision interaction (PCI), shows best agreement at lower energies. The experimental techniques used to collect the data are described here as well as an improved theoretical approach using elastic scattering cross sections to evaluate the accuracy of the distorted waves utilized in the calculation of the ionization cross sections.

(e, 2e) ionization measurements from the 3σg and 2σu* states of N2—comparison between experiment and theoretical predictions of the effects of exchange, polarization and interference

Gao et al (2005 Phys. Rev. A 72 032721) have predicted a Youngs type interference effect in the fully differential cross sections for ionization of the 3σg state of N2 for highly asymmetric collisions with one electron detector fixed at very small scattering angles (1° or 10°). The purpose of this work was to look for this interference effect at a larger scattering angle. (e, 2e) ionization measurements have been conducted from the 3σg and 2σu* states of N2 in a coplanar asymmetric geometry, where one electron emerges in the forward direction and the correlated electron is measured as a function of scattering angle. Both final-state electrons have an energy of 30 eV, and the forward scattering angle was θa = 22° relative to the incident beam direction. The theoretical prediction is that there should be a strong interference peak near 180°. The measurements were carried out from the 3σg state over a range of scattering angles from θb ~ 10° to θb ~ 170° using a magnetic angle changing spectrometer. The present experimental results for 3σg find a normal binary peak plus another peak at back angles in the vicinity of 180°. Consequently, this work supports the possibility of a strong Youngs type interference effect for small fixed scattering angles.

Deep interference minima in non-coplanar triple differential cross sections for the electron-impact ionization of small atoms and molecules

The time-dependent close-coupling method and a distorted-wave approach are used to explore deep minima discovered in the non-coplanar triple differential cross sections for the electron-impact ionization of helium. This phenomenon has been well studied experimentally but so far has not been investigated by a non-perturbative theoretical approach. We find that our time-dependent calculations reproduce very well the experimental minima, and that the distorted-wave calculations also confirm this phenomenon. Further investigations reveal that the minima appear to be due to deep destructive interference between the partial wave contributions which make up the cross sections. We also show that similar minima may be found in triple differential cross sections arising from the electron-impact ionization of atomic and molecular hydrogen.

Low energy super-elastic scattering studies of calcium over the complete angular range using a magnetic angle changing device

Super-elastic scattering processes can be considered as the time reversal of electron–photon coincidence measurements, with the advantage that data are accumulated thousands of times faster. This allows a far more extensive and accurate study of electron excitation of atoms which can also be excited using laser radiation. The application of a newly invented magnetic angle changing (MAC) device to these experiments has allowed the complete scattering geometry to be accessed for the first time, and experimental methods adopted in these new experiments are discussed here. Data are presented for excitation of the 41P1 state of calcium by electron impact at scattering angles from near 0° to beyond 180°, with incident energies of 45 eV and 55 eV. The results are compared to the DWBA theory of Stauffer and colleagues, with generally excellent agreement.

Exploring the helium (e, 2e) differential cross section at 64.6 eV with symmetric scattering angles but nonsymmetric energies

Helium (e, 2e) measurements are presented for an incident energy of 64.6 eV over a wide range of scattering angles from the coplanar to the perpendicular plane geometry. These measurements were taken for symmetric scattering angles and a range of detection energies from symmetric energy sharing where each electron has an excess energy of 20 eV to asymmetric sharing where the electrons have energies of 5 eV and 35 eV. A deep minimum observed for an electron gun angle of 67.5 degrees was explored as a function of this energy sharing.

Superelastic electron scattering from laser excited rubidium at 20 eV incident energy

Superelastic electron scattering measurements are presented from rubidium atoms excited by laser radiation to the 52P states at around 780 nm. The incident energy of the electrons was 18.4 eV corresponding to 20 eV incident electrons for the excitation process 52S-52P. The measurements were conducted over a range of scattering angles from 5° through to 125°. A complete set of atomic collision parameters for the interaction process is presented together with the associated pseudo-Stokes parameters obtained from the measurements. A comparison with three sophisticated theoretical models indicates that none of the models completely describes the interaction process at this energy, and that further experimental and theoretical work is needed.

Laser probing of the electron-impact excited c(2p) 3Πu manifold of states in H2

A stepwise laser excitation method has been used to probe individual ro-vibrational levels in the molecular hydrogen metastable c(2p) 3Πu manifold of states. Metastable states produced by electron-impact excitation are subsequently excited by the absorption of a single UV laser photon, producing a complex triplet nd Rydberg spectrum observed via field ionization and autoionization. The spectrum has been analysed to determine quantum numbers of the states associated with the transitions. Excitation functions for individual ro-vibrational states in the c(2p) 3Πu manifold are then determined as a function of electron-impact energy.