N. L. S. Martin

Complex Fano asymmetry parameters for helium L = 0, 1, 2 autoionizing levels excited by electron impact

Theoretical Fano q-parameters for He autoionizing states excited by electron impact are presented, as extracted from first- and second-order hybrid distorted-wave Born + R-matrix calculations. It is found that the q-parameters for (2s2)1S, (2p2)1D, and (2s2p)1P from the first-order calculations are essentially real quantities, while those from the second-order calculations are complex but with a real part similar to the first-order values. These findings are interpreted in terms of the relative phases of the first- and second-order amplitudes.

Investigation of complex ionization amplitudes in cadmium by ([ital e],2[ital e]) spectroscopy

High-resolution ([ital e],2[ital e]) energy spectra are presented which enable the isolation of interference effects between [ital J]=0,1,2 multipoles in electron-impact ionization of cadmium. It is found that both resonant and nonresonant processes are important. Relative magnitudes and phases of ionization amplitudes are obtained which disagree with plane-wave Born approximation calculations.

High-energy electron-helium scattering in a Nd:YAG laser field

We report measurements of the scattering of electrons by helium atoms in the presence of 1.17 eV photons from a Nd:YAG laser. The incident energy of the electrons was in the range 50-350 eV, and the polarization of the laser was arranged to be parallel to electrons scattered through 135 deg. Energy-shifted peaks corresponding both to one- and two-photon emission were observed. Calculations using the Kroll-Watson approximation are perfectly consistent with the data.

Coherent excitation of cadmium autoionizing energy levels: observation by (e, 2e) spectroscopy

The authors have observed interference effects in (e, 2e) ejected-electron angular distributions from the Cd(4d95s25p) 1P1 autoionizing level. Analysis of the data shows that the interference terms are too large to be accounted for in terms of a direct ionization background, but may be explained by the existence of an overlapping even-parity autoionizing level and possibly an additional odd-parity J=3 level.

Experimental and theoretical momentum transfer dependence of the He (e, 2e) cross section

The relative triple differential cross section for the electron impact ionization of helium has been measured and calculated for an incident energy of 488 eV. This is sufficiently highly asymmetric geometry that exchange processes may be neglected and the slow and fast outgoing electrons identified as ejected and scattered electrons, respectively. For two fixed ejected electron directions ±90°, and ejected electron energy 34.5 eV, coplanar (e, 2e) measurements have been made for a range of scattering angles between ±30°, corresponding to a momentum transfer range 0.38 → 3 au. The data are presented directly as scattered electron angular distributions and in the manipulated form of their sum, difference and the ratio difference / sum. These are compared to four types of theoretical calculations. Excellent agreement is found between experiment and a calculation that includes significant post collision interaction effects. All four calculations predict an almost identical oscillatory feature in the difference and ratio of the ±90° cross sections, which is consistent with the experimental data and is due to the fact that the cross section passes through zero close to, but not exactly at, 0° scattering.

Spin polarization of photoelectrons from zinc atoms in the autoionization region

We have measured the energy dependence of the dynamical spin-polarization parameters for photoelectrons from zinc atoms in the wavelength region from 100 nm to 115 nm. The pronounced structure near the resonances found in previously measured absorption spectra also occurs in the spin-polarization parameters. From the experimental results the dipole transition matrix elements including the relative phase shift difference have been determined. The data are also compared with an existing R-matrix calculation and a new calculation employing semi-empirical Slater - Condon parameters.

Experimental and theoretical momentum transfer dependence of the He (e, 2e) cross section for incident electron energies 150 eV and 488 eV

The relative triple differential cross section for the electron impact ionization of helium has been measured and calculated for incident energies of 150 eV and 488 eV, and an ejected electron energy 34.5 eV. At both incident energies coplanar (e, 2e) experiments were carried out for a fixed pair of ejected directions +90°, − 90° and a range of scattering angles −30° → +30°, and also for a fixed pair of ejected directions +75°, − 105° and a range of scattering angles −34° → +18°. The kinematics of these experiments cover a momentum transfer range 0.38 → 3 au. The data are presented directly as pairs of (e, 2e) scattered electron angular distributions, and in the manipulated form of their sum, difference and the ratio difference/sum. These are compared with up to five types of theoretical calculations. Good, but not perfect, agreement is found between experiment and calculations that include significant post-collision interaction effects. All calculations predict an almost identical oscillatory feature, in the difference and ratio of the cross sections, which is observed in the experimental data for incident energy 150 eV and is due to the fact that the cross section passes through zero close to, but not exactly at, 0° scattering.

A measurement of the Cd photoelectron asymmetry parameter using (e,2e) spectroscopy

In an (e,2e) simulation of photoelectron spectroscopy the authors have measured the parity unfavoured spectrum of the cadmium 4d95s25p J=1 autoionizing region. This result is combined with their earlier measurement of the parity favoured spectrum to yield the spectral behaviour of the photoelectron asymmetry parameter. The results are in fair agreement with the true photoelectron data of Jimenez-Mier et al. (1989).

The 4d95s5p2 autoionizing levels of cadmium

Three previously unknown optically forbidden autoionizing levels of cadmium have been observed between 18 eV and 20 eV above the ground state neutral. They appear as strong transitions in electron impact energy-loss spectra taken at 150 eV incident electron beam energy. Ab initio calculations have been carried out that support the assignment of these levels to Cd 4d95s5p2 J=2.

Effects of polarization direction on laser-assisted free–free scattering

This work will detail the effects of laser polarization direction (relative to the momentum transfer direction) on laser-assisted free–free scattering. Such processes play a role in the gas breakdown that occurs in electric discharges as well as providing a method for the laser heating of a plasma (Musa et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 175201, Mason 1993 Rep. Prog. Phys. 56 1275). Experimental results will be presented for electron-helium scattering in the presence of an Nd:YAG laser field ( eV) where the polarization direction was varied in a plane that is perpendicular to the scattering plane. To date, all of our experimental results are well described by the Kroll–Watson approximation (KWA) (Kroll and Watson 1973 Phys. Rev. A 8 804). The good agreement between our experiments and calculations using the KWA includes the case where the polarization is perpendicular to the momentum transfer direction, for which the KWA predicts vanishing cross section; other workers have found that the KWA tends to be inaccurate for cases where it predicts small cross sections (e.g. Musa et al 2010 J. Phys. B: At. Mol. Opt. Phys. 43 175201). We also present simulations of the effects that multiple scattering might have on experimental measurements. In particular, we examine conditions that are expected to be similar to those of the experiments reported by Wallbank and Holmes (Wallbank and Holmes 1993 Phys. Rev. A 48 R2515).