I McGregor

Energy dependence of characteristic and bremsstrahlung cross sections of argon induced by electron bombardment at low energies

The energy dependence of characteristic x-ray and bremsstrahlung cross sections of argon has been investigated by electron impact. The incident electron energy was varied between 2.5 keV and 12 keV. From the data, absolute cross sections for Ar-K ionisation are obtained. The experimental data agree reasonably with available theoretical calculations.

Threshold behaviour of Ar K and Xe L3 ionisation by electron impact

Ionisation of the Ar K and Xe L3 shells has been investigated by electron impact with energies from about 10 eV to 1 keV above threshold. The experimental data are compared with the theoretical predictions by Wannier for ionisation near threshold and with more recent plane-wave Born and Coulomb-Born exchange calculations.

Angular distribution of X-radiation following electron bombardment of free atoms

Measurements of the angular distribution of X-radiation produced by electron bombardment of free atoms (argon, krypton, xenon) at incident electron energies between 3 keV and 15 keV are presented. The observed anisotropy or polarisation of characteristic X-radiation (L alpha 1,2 transition in xenon) is small (a few per cent) and near threshold approaches the expected value for Q0>>Q1. The measured anisotropies of bremsstrahlung radiation may be reproduced reasonably well by a modified Sommerfeld formula if the velocity beta is chosen to be half of the incident electron velocity beta 0. The measured anisotropy may be described by a parameter P, which is compared with the degree of polarisation obtained from theoretical calculations.

Electron-photon angular correlations from electron impact excitation of heavy rare-gas atoms

Electron-photon angular correlations have been measured for electron impact excitation of the first excited 1P and 3P states of krypton and of the first excited 3P state of xenon. In the case of these heavy atoms two independent measurements were necessary to determine the excitation parameters lambda and chi for these states.

Some consequences of atomic orientation in electron-atom scattering by consideration of attractive and repulsive forces

Discusses some consequences of the classical grazing model of Kohmoto and Fano (1981) to the problem of atomic orientation by electron collisions. The authors introduce amplitudes for repulsive and attractive scattering interactions and related them to the magnetic sublevel; amplitudes f0 and f1 of the coherent 1S to 1P excitation process. An important consequence of the grazing model is a left-right asymmetry in the scattering of electrons having excited the atom with a given definite orientation. It follows from parity invariance that the circular polarisation of the 1P to 1S radiation emitted perpendicular to the scattering plane is identical to the left-right intensity asymmetry of the scattered electrons measured in coincidence with right- and left-hand circularly polarised light. The magnitudes and the phase difference of the amplitudes for attractive and repulsive interactions of the coherent 1S to 1P excitation can be calculated from the knowledge of the lambda and chi parameters.

Determination of a complete set of parameters (λ,χ,Δ and ε) incorporating spin-orbit effects from an electron-photon coincidence experiment on the excitation of the 63P1 state of mercury by electron impact

Linear and circular polarisation measurements have been carried out for photons from the 63P1-61S0 transition in mercury, which were detected in delayed coincidence with electrons having excited the 63P1 state. These measurements have been used to determine values for the parameters lambda , chi and the spin-orbit phase angles Delta and epsilon for the 61S0 to 63P1 to 61S0 excitation/de-excitation process in mercury.

Hyperfine effect on the polarisation of the 63P1-61S0 intercombination line of mercury excited by electron impact

An isotope cell was used to eliminate the effect of hyperfine structure on the polarisation of radiation, near threshold, for the 63P1-61S0 line of mercury (253.7 nm) excited by electron impact. These results are compared with the polarisation of the radiation when hyperfine structure is present.

Angular Distribution and Polarization of x-Ray Radiation by Electron Impact on Free Atoms

Angular distributions of characteristic x-ray line radiation and bremsstrahlung photons, produced in collisions between electrons and krypton and xenon atoms, have been measured in the energy range from 5 to 15 keV. The x-rays have been detected by a Si(Li) detector with an energy resolution of 200 eV. While the bremsstrahlung photons display a highly anisotropic angular distribution, the combined Lα1,2 characteristic line radiation of xenon only shows a small anisotropy. The polarization of the Lα1,2 radiation which results from this anisotropy approaches the theoretically predicted threshold polarization of the characteristic Lα1,2 x-ray radiation of xenon. Bremsstrahlung anisotropies of Xe and Kr are compared with theoretical predictions.

L Subshell Ionization Cross Sections of Free Xenon Atoms by Electron Impact at Low Energies

The ionization of the L1, L2 and L3 subshells of free xenon atoms has been investigated by electron impact with energies of 6 keV to 14 keV, corresponding to values of a few times (1.2 – 3) the threshold energy. The measurements have been performed by measuring characteristic x-rays resulting from the decay of inner-shell vacancies. Xenon has been chosen as a target because it allows measurements on free atoms and its energy-level splittings are large enough to allow the use of a Si(Li) x-ray detector.

On the Z-Dependence of Bremsstrahlung Cross Sections

Electrons colliding with neutral or ionized atoms give rise to the emission of a continuous x-ray spectrum (bremsstrahlung). The origin of this spectrum is due to the slowing-down of incident electrons in the fields of nuclei. Classically, the acceleration of an electron moving along a parabolic path in a purely Coulombic field is given by a = Ze2/(mr2), with Ze the electric charge of the nucleus, m the electron mass and r the distance between electron and nucleus. According to electrodynamics this acceleration is accompanied by the emission of electromagnetic radiation, from which after integrating over the outgoing electron directions the classical result of Kramers1 for the bremsstrahlung cross section da/dk is obtained.