Science, Technology and Engineering

Centre for Materials and Surface Science

Toroidal Spectrometer Project - Band Structure Studies

The toroidal spectrometer collects intensity data covering 180 degrees of (polar) emission angle from a solid surface. To accommodate different requirements concerning the parity selection rules, the sample normal can be rotated away from the photon incidence direction by (say) 45 degrees without restricting the the range of polar acquisition angles.

The energy window may be thought of as a collection of narrow segments, each representing an angular distribution of intensity corresponding to the centre of that particular segment. It is convenient to operate the spectrometer so that the energy interval between segments is the same as the energy step used to increment data collection. Under these circumstances, each segment collects data as long as it remains within the energy window. During a typical band structure experiment, the spectrometer might be operated with 5 eV pass energy with the result that the energy window covers 0.4eV. If the energy step is chosen to be 30meV, the window will be divided in software into 12 equal segments and the data so collected will therefore be roughly equivalent to 12 "normal" experiments by a spectrometer without parallel energy detection.

Since the spectrometer energy and angular resolution is slightly degraded towards the edges of the energy window, the most precise experiments should be performed by selecting only the central segment. We anticipate that this will only be necessary in rare cases.

An example of the valence band structure of graphite, grown as a thin film on SiC, is provided below. Each spectrum was recorded in 20 minutes on U56/1 at BESSY in 2004.

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