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

Abstract

A new spectrometer is described for measuring the momentum distributions of scattered electrons arising from electron-atom and electron-molecule ionization experiments. It incorporates and builds on elements from a number of previous designs, namely, a source of polarized electrons and two high-efficiency electrostatic electron energy analyzers. The analyzers each comprise a seven-element retarding-electrostatic lens system, four toroidal-sector electrodes, and a fast position-and-time-sensitive two-dimensional delay-line detector. Results are presented for the electron-impact-induced ionization of helium and the elastic scattering of electrons from argon and helium which demonstrate that high levels of momentum resolution and data-collection efficiency are achieved. Problematic aspects regarding variations in collection efficiency over the accepted momentum phase space are addressed and a methodology for their correction presented. Principles behind the present design and previous designs for electrostatic analyzers based around electrodes of toroidal-sector geometry are discussed and a framework is provided for optimizing future devices.

Fully differential cross sections for electron-impact excitation-ionization of alignedD2

“…This measurement represents a stringent test of theory since there are two active electrons (four-body problem). To date, there have been a limited number of studies reported of the four-body problem for electron-impact excitation-ionization of atoms [10][11][12][13][14] and molecules [4,5,8,9]. For excitationionization of helium, although there was some qualitative shape agreement between experiment and theory, overall the agreement was not very good [5].…”

Toroidal spectrometer for signal detection in scanning ion/electron microscopes

“…Toroidal electron energy spectrometers have been used for many applications that involve the 2 emission of scattered charged particles, including photoemission studies, 3 electron momentum studies, 4 and BSE spectra inside the SEM. 5 These spectrometers typically have first-order focusing properties ͑second-order spherical aberrations͒ in the detection plane.…”

Fully Differential Molecular-Frame Measurements for the Electron-Impact Dissociative Ionization ofH2

“…The improved (e, 2e) binding energy resolution we achieve (2 eV, relative to 7 eV in [12,13]), enabled us to completely resolve transitions to the 2s g and 2p u states from those to the 1s g and 2p u molecular ion states, providing an enhanced level of state selectivity. Figure 2 shows a schematic of the apparatus (see [16]). Light from an 850 nm laser diode is intercepted by a fast acousto-optical shutter, driven at a frequency of 125 kHz by a pulse generator, to produce a train of 2-microsecondduration photon pulses.…”