Condensed matter electron momentum spectrometer with parallel detection in energy and momentum

Abstract

An electron momentum spectrometer has been constructed which measures electron binding energies and momenta by fully determining the kinematics of the incident, scattered, and ejected electrons resulting from (e,2e) ionizing collisions in a thin solid foil. The spectrometer operates with incident beam energies of 20–30 keV in an asymmetric, non-coplanar scattering geometry. Bethe ridge kinematics are used which for 20 keV incident energy has scattered electron energies of 18.8 keV at a polar angle of θs=14°and azimuthal angles φs in the range from −18° to +18° and ejected electrons of 1.2 keV and θe=76°with φe=π±6°. The technique uses transmission through the target foil, but it is most sensitive to the surface from which the 1.2 keV electrons emerge, to a depth of about 2 nm. Scattered and ejected electron energies and azimuthal angles are detected in parallel using position sensitive detection, yielding true coincidence count rates of 6 Hz from a 5.5 nm thick evaporated carbon target and an incident beam current of around 100 nA. The energy resolution is approximately 1.3 eV and momentum resolution approximately 0.15 a0−1. The energy resolution could readily be improved by monochromating the incident electron beam.

Elastic electron scattering from methane at high momentum transfer

“…In front of the analyser is a five-element slit lens, used to decelerate the scattered electrons on the entrance plane of the analyser. The design of this slit lens is based on those used in solid state electron momentum spectrometers as described elsewhere (Storer et al 1994, Vos et al 2000. At the exit plane the electrons are detected by a set of channel plates, and the impact position of the electron is determined using a resistive anode as a position-sensitive detector (psd).…”

Invited Article: An improved double-toroidal spectrometer for gas phase (e,2e) studies

“…This is problematic as modern positionsensitive charged-particle detectors are generally flat. While previous analyzers have exploited the fact that, for small range of azimuthal angles ⌬, the conical-sector focal surface can be adequately approximated by a plane, 10,27 for larger values of ⌬ the planar approximation is inaccurate, leading to the introduction of significant imaging aberrations. As the volume of accepted momentum phase space is proportional to ⌬, limiting its value to reduce aberrations is not conducive to the objective of designing an efficient analyzer.…”

“…Thus, an analyzer constructed with this c parameter value and with a detector positioned at the polar angle of init + 142°will satisfy the requirements for simultaneous focusing in both azimuthal and axial planes. This design approach has been successfully adopted by a number of groups 10,27,34 to optimize momentum resolution.…”

“…8,9 More recently, third generation spectrometers have been developed whose analyzers incorporate two-dimensional positionsensitive detectors, enabling a range of emission angles and energies to be measured in parallel. [10][11][12][13][14][15][16] From the measured arrival positions of electrons at the detectors, their energies and emission angles are deduced.…”

Direct Observation of Energy–Momentum Densities in Solids

“…The spectrometer has been designed to allow ()s to be changed, which allows momentum components Py to be measured for non-zero pz. The use of two-dimensional position-sensitive detectors mounted on the exit planes of the hemispherical (fast electron) and toroidal (slow electron) electrostatic analysers allows the spectrometer to measure simultaneously over a predetermined range of energies Ef and Es , as well as azimuthal angles ¢f and ¢s (see Storer et al 1994 for details).…”