Science, Technology and Engineering

Centre for Materials and Surface Science

Toroidal Spectrometer Project - Principle of Operation

Photoelectron spectroscopy measures the number of electrons coming from a sample as a function of their kinetic energy. In addition, it is sometimes of importance to know the exit direction of the photoelectrons.

Following an idea by Riley and Leckey, the toroidal electron analyzer is designed to simultaneously analyze the kinetic energy of electrons leaving the sample at any polar angle between ± 90 degree at a given azimuthal angle . This reduces measurement time in comparison to a conventional angle resolved photoelectron spectrometer, which can often only measure photoelectrons for one direction at a time. Changing the azimuthal angle of emission in the toroidal anayzer is accomplished by rotating the sample around its surface normal.

After leaving the sample, the electrons will enter the space between the toroidal electrodes. Just like in a hemisperical analyzer, fast electrons with high kinetic energies will have trajectories closer to the outer toroid and slow electrons will stay closer to the inner toroid. Very slow and very fast electrons will actually hit the toroids.

After leaving this section of the analyzer, the electrons do not pass through an exit slit. Instead they are focussed by an electrostatic lens onto a two-dimensional detector (MCP assembly read out by CCD camera). This allows allows us to monitor a range of kinetic energies (±4% of pass energy) at the same time. This additional multichannel detection capability in energy further reduces the measurement time or increases signal to noise ratio at a constant measurement time.

An image taken from the detector is shown above. Bright areas indicate high countrates. The analyzer voltages were set so that the center of the energy window coincides with the the Fermi energy. The detector image clearly shows the s-bands of Cu₃Au(111) and the well known Shockley surface state, visible in two Brillouin zones. The transition from bright to dark marks the Fermi energy. The fact that intensity is also seen at "forbidden" angles (between bands) is due to imperfections of the crystal surface leading to diffuse scattering of the photoelectrons with a contribution from phonon scattering and indirect transitions. The energy window,in many situations, may be considered as a collection of separate angular distributions each corresponding to a slightly different kinetic energy. This enables the operator to decide which energy "slice" to select in a particular situation or (as may well be tthe case in an XPD experiment) to integrate intensity at each angle value so as to encompass an entire core level profile.

Energy distribution measurements

A photoelectron energy distribution curve (EDC) is acquired by taking many detector images at different kinetic energies, extracting the intensities as a function energy (radius in image) and angle, and storing these values in an array. In fact this array is three-dimensional, because we store the information for every energy channel separately.

An example of such a EDC is shown below. This was one of the first measurements taken with the toroidal analyzer. It shows the s-bands and the Shockley surface state of Cu3Au(111). The very intense d-bands are not shown for clarity. One can see s-bands of the 1st Brillouin zone and folded bands of the next Brillouin zone(i.e. bands visible due to the effect of the presence of other G vectors).

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