The efficient detection of "zero" energy electrons can be achieved using the field-penetration technique¹ , which is demonstrated in figure 1. The figure shows how trajectories of 0.001eV electrons emitted over 4-pi steradians from a point source can be focused and collimated by the weak electric field from an ‘extractor’ electrode that penetrates through the 0 Volt aperture. The solid angle of extracted, faster electrons is significantly smaller than for these "threshold" electrons and rapidly diminishes with electron energy. This highly-efficient, energy selective extraction allows one to perform 'threshold electron spectroscopy’.
 

Figure 1. Diagram of the electric field contours and electron trajectories of "threshold" electrons in the field-penetration technique.

We have operated the toroidal analyser spectrometer in the threshold detection mode for spectroscopic studies of the rare gases^2,3,4. Below are two figures (and references) that illustrate the senstivity of the threshold detection technique. As the energy selectivity, which depends on the electric field geometry, can give <5meV resolution, the threshold peaks in the figures have widths that are dominated by the spread in the photon beam energy. This technique is not only an accurate photon energy calibration method, but provides useful insight into electron correlation processes associated with ionic and (neutral) resonance state formation.

Figure 2. The threshold photoelectron spectrum of Xenon⁴ in the 23.5-29.5eV range with a measured overall resolution of ~13meV (FWHM). The system of assignment lines shows schematically the building of 'satellite' state configurations, starting with the three Xe⁺⁺ 5p⁴ core configurations and attaching the excited electron to one of the free orbitals. Virtually all the J-components are observed and identified.

Figure 3.  The threshold photoelectron spectrum of helium³ showing the characteristic ‘cusp’ feature at the double ionisation threshold energy, which Fano interpreted as the ‘manifestation of the formation and decay of electron correlations.’ The insert shows the improved statistical accuracy when a number of spectra taken over the shorter region of 0.6 eV around the double ionisation  threshold are added to the longer spectrum. The solid curve has an energy dependence given by the Wannier exponent both above and below the threshold energy, convoluted with a Gaussian profile of 70meV (FWHM) as the resolution is dominated by that of the photon beam.
 

¹ Cvejanovic and Read  J. Phys. B.: At. Mol. Opt. Phys. (1974) 7 p1180

² Cvejanovic et al J. Phys. B.: At. Mol. Opt. Phys. (1994) 27  p5661

³ Cvejanovic et al J. Phys. B.: At. Mol. Opt. Phys. (1995) 28  L707

⁴ Slattery et al J. Phys. B: At. Mol. Opt. Phys. (2000) 33 4833