Toroidal Photoelectron Spectrometer

PDI is difficult to investigate experimentally because two outgoing particles need to be detected 'in coincidence' (i.e. time-correlated) to ensure the two electrons come from the same target atom/molecule. In this study the two ejected electrons are (independently) energy analysed during the detection process using electrostatic lenses which focus and steer the emitted electrons on to a 2-dimensional position-sensitive detector (see Fig.1). Novel toroidal analysers have been employed, which energy select the electrons while preserving the initial angle of emission. As a direct consequence of these focusing properties, the angular distribution of the photoelectrons are mapped directly onto the detectors and the resulting efficiency gains allow new experiments to be undertaken which otherwise would have been impossible. Experimental information is needed in all aspects of the PDI process; at Newcastle we have developed a spectrometer to investigate:

how the two electrons share the available energy,
the angular correlation of the ejected electrons, and
the relative importance of direct and indirect processes in PDI.

In addition to the above coincidence capability, photoelectron angular distributions from single ionisation processes can be imaged directly on a position-sensitive detector. This energy resolved, angle- dispersive capability is a powerful tool which can be used in more conventional photoelectron spectroscopy. For example, this system allows one to monitor changes the angular distribution while scanning the photon and/or photoelectron energy. Furthermore threshold photoelectron studies, in which electrons with energies between 0-10meV are detected with very high efficiency, have been performed on noble gases.

Figure 1: A schematic diagram showing the configuration of the two toroids along with lines indicating central trajectories of electrons with a selection of emission angles. The analysers are independent, i.e.: they are able to detect dissimilar electron energies and resolutions. The electron lenses are not shown for reasons of clarity. Electrons from the interaction region are accelerated to the entrance of the toroid. The axial symmetry results in the entrance lenses being slits on the curved surfaces of a series of coaxial cylinders. Only electrons of a specific energy are deflected through 142^o to pass through the exit slit of the toroid. The exit lenses, which consist of curved slits on the surfaces of a series of coaxial cones, accelerate the electrons onto their respective two-dimensional resistive anode encoders. The images on these detectors are circular arcs with the circle centre on the photon axis.

For further details see: Reddish et al (1997) Rev. Sci. Instrum. 68 p2685