Kate Nixon

Total and positronium formation cross sections for positron scattering from H2O and HCOOH

Total and positronium formation cross sections have been measured for positron scattering from H2O and HCOOH using a positron beam with an energy resolution of 60 meV (full-width at half-maximum (FWHM)). The energy range covered is 0.5–60 eV, including an investigation of the behavior of the onset of the positronium formation channel using measurements with a 50 meV energy step, the result of which shows no evidence of any channel coupling effects or scattering resonances for either molecule.

Low energy (e,2e) measurements of CH4 and neon in the perpendicular plane

Low energy experimental and theoretical triple differential cross sections for the highest occupied molecular orbital of methane (1t(2)) and for the 2p atomic orbital of neon are presented and compared. These targets are iso-electronic, each containing 10 electrons and the chosen orbital within each target has p-electron character. Observation of the differences and similarities of the cross sections for these two species hence gives insight into the different scattering mechanisms occurring for atomic and molecular targets. The experiments used perpendicular, symmetric kinematics with outgoing electron energies between 1.5 eV and 30 eV for CH(4) and 2.5 eV and 25 eV for neon. The experimental data from these targets are compared with theoretical predictions using a distorted-wave Born approximation. Reasonably good agreement is seen between the experiment and theory for neon while mixed results are observed for CH(4). This is most likely due to approximations of the target orientation made within the model.

An electron momentum spectroscopy and density functional theory study of the outer valence electronic structure of stella-2,6-dione

We report on the first electron momentum spectroscopy (EMS) study into the outer valence electronic structure of the ground electronic state for the organic molecule stella-2,6-dione (C8H8O2). Experimentally measured binding-energy spectra are compared against a He(Iα) photoelectron spectroscopy result, while our derived momentum distributions (MDs) are compared against corresponding results from the plane wave impulse approximation (PWIA) level calculations. These computations employed density functional theory (DFT) basis states at the triple zeta valence polarization (TZVP) level, with a range of exchange-correlation (XC) functionals. A detailed comparison between the experimental and PWIA DFT-XC/TZVP calculated MDs enabled us to evaluate the accuracy of the various functionals, the Becke–Perdew (BP) XC functional being found to provide the most accurate description here. The importance of the through-bond interaction to the molecular orbitals (MOs) of stella-2,6-dione is demonstrated using the orbital imaging capability of EMS. Finally we show that the molecular geometry of this molecule, as derived from BP/TZVP, is in quite good agreement with corresponding independent experimental data.

Low energy (e,2e) coincidence studies of NH3: results from experiment and theory.

Experimental and theoretical triple differential cross sections (TDCS) from ammonia are presented in the low energy regime with outgoing electron energies from 20 eV down to 1.5 eV. Ionization measurements from the 3a1, 1e1, and 2a1 molecular orbitals were taken in a coplanar geometry. Data from the 3a1 and 1e1 orbitals were also obtained in a perpendicular plane geometry. The data are compared to predictions from the distorted wave Born approximation and molecular-three-body distorted wave models. The cross sections for the 3a1 and 1e1 orbitals that have p-like character were found to be similar, and were different to that of the 2a1 orbital which has s-like character. These observations are not reproduced by theory, which predicts the structure of the TDCS for all orbitals should be similar. Comparisons are also made to results from experiment and theory for the iso-electronic targets neon and methane.

Electron impact excitation of the ã 3B1u electronic state in C2H4: An experimentally benchmarked system?

We report on differential and integral cross section measurements for the electron impact excitation of the lowest-lying triplet electronic state (ã (3)B(1u)) in ethylene (C(2)H(4)). The energy range of the present experiments was 9 eV-50 eV, with the angular range of the differential cross section measurements being 15°-90°. As the ground electronic state of C(2)H(4) is a (1)A(g) state, this singlet → triplet excitation process is expected to be dominated by exchange scattering. The present angular distributions are found to support that assertion. Comparison, where possible, with previous experimental results from the University of Fribourg group shows very good agreement, to within the uncertainties on the measured cross sections. Agreement with the available theories, however, is generally marginal with the theories typically overestimating the magnitude of the differential cross sections. Notwithstanding that, the shapes of the theoretical angular distributions were in fact found to be in good accord with the corresponding experimental results.

Modelling low energy electron interactions for biomedical uses of radiation

Current radiation based medical applications in the field of radiotherapy, radio-diagnostic and radiation protection require modelling single particle interactions at the molecular level. Due to their relevance in radiation damage to biological systems, special attention should be paid to include the effect of low energy secondary electrons. In this study we present a single track simulation procedure for photons and electrons which is based on reliable experimental and theoretical cross section data and the energy loss distribution functions derived from our experiments. The effect of including secondary electron interactions in this model will be discussed.

(e,2e) measurements from laser aligned Mg atoms

Electron impact ionization measurements will be presented from an aligned atomic target. Magnesium atoms were resonantly excited from the 3S ground state to the 3P state using linearly polarized 285nm radiation. The alignment of the excited state electron density could then be varied by changing the polarization direction of the exciting laser radiation. Data from an asymmetric, equal-energy kinematic (e,2e) experiment for the ground state and three excited state alignment angles will be shown.

Electron impact ionization and excitation studies of laser prepared atomic targets

A review of current work at Manchester is given, with emphasis on low energy electron impact ionization & excitation experiments from laser-prepared targets. Three different methods are considered: super-elastic scattering from targets prepared by laser radiation in an optical enhancement cavity, (e,2e) studies of laser-aligned atoms, and electron impact ionization studies from laser cooled atoms in a new type of atom trap - the AC-MOT. The status of each experimental programme is detailed, and new data presented. Future directions and techniques are then discussed.

Positron interactions and transport in biologically relevant molecules

We present new, high-resolution measurements of positron scattering from biologically relevant molecules, such as water and formic acid. The measurements include absolute determinations of total scattering and positronium formation and they have enabled us to assemble a set of cross sections for these molecules which can be used in an investigation of positron transport in these systems.

Electron momentum spectroscopy in the study of intermolecular interactions

This paper describes progress towards developing a new electron momentum spectrometer specifically designed for the purpose of studying weak, non-covalent intermolecular interactions. Electron momentum spectroscopy will be employed to measure the electronic wavefunction, with particular attention being paid to the electrons involved in the intermolecular interactions present within the target. This work requires a triple coincidence experiment; namely the detection of an (e,2e+ion) event. Results of characterisation experiments conducted with noble gases are given in order to demonstrate the performance of this new apparatus.