multi-electron coincidences & angular resolved PES

The multi-coincidence apparatus.

The ten photoelectron analysers are grouped in two sets: seven analysers can rotate in the plane of polarization vector of the incidente radiation; while the other three can rotate around the same axis. This latter set enable measurements out of the polarization plane, to study non-dipole contributions to the photoionization cross-section.

The 800 mm I.D. multicoincidence end-station is lined with a 2 mm thick µmetal shield, additionally screened by three pairs of Helmholtz coils; the residual magnetic field is less than 10 mG. Two independently rotatable arrays of hemispherical electrostatic analysers are housed in the chamber. Seven spectrometers are mounted on a turntable that rotates about an axis in the direction of the incident beam; the polarisation vector of the radiation is in the same plane as the analysers. Three other spectrometers are mounted on a smaller turntable that rotates around the direction of the polarisation vector of the radiation. This latter array allows one to do photoionisation experiments out of the polarisation plane, to study, for example, non-dipole contributions to the photoionisation cross-section. All ten analysers are mounted at angular intervals of 30°. The chamber is illustrated in fig. 2, which shows the two sets of analysers of the spectrometers of the multicoincidence end-stations. Inset shows a detail of three analysers.
The spectrometers are composed of two four-element lenses (194 mm long) that focus the photoelectrons from the target region onto the entrance slits of the hemispherical deflector.
The lens system can be operated in two modes: a "low resolution" mode characterised by ΔE/Ek=102, where Ek is the kinetic energy of the photoelectrons, and an angular acceptance in the dispersion plane of ±3°. This mode is most suited for coincidence experiments. The second mode is a "high resolution" mode with ΔE/Ek =103, and an angular acceptance of ±0.5° used for non-coincidence photoelectron and Auger electron spectroscopies. The lens stack can work with retarding ratios of 2-5 and 5-100 in the low- and high-resolution modes, respectively. The mean radius of the hemispherical deflector is 33 mm and the gap 9.9 mm, giving a maximum time spread of 2 ns in the trajectories of 100 eV electrons in the dispersing element.