We have already mentioned some incremental improvements in photoelectron spectroscopy, increased energy- and angle resolution through improvements in instrumentation. Another is the angle-scanned photoelectron spectroscopy pioneered by Aebi et al. (1994a,b), described in Chapter 5. Lindroos and Bansil (1996) recently pointed out additional advantages of this technique when coupled with suitable calculations. They found not only the usual features produced by direct transitions from initial states on the Fermi surface, but additional features from indirect transitions whose intensities are proportional to the one-dimensional densities of initial states for transitions at a fixed k∥. They illustrated these aspects with calculations for several surfaces of Cu. A re-examination of such spectra for Bi2212 is likely to occur soon. There are several other variations of photoelectron spectroscopy already tested at some level, some of which may play a role in future work on cuprates.

Photoelectron microscopy

The photoemission studies normally carried out collect electrons from an illuminated area usually no smaller than about 100 μm × several hundred μm. There may be lateral inhomogeneities on a scale smaller than this, so photoelectrons from special sites may be averaged with those from the rest of the area sampled. This may be particularly misleading in studies of reactions with overlayers which may take place preferentially at steps. Photoelectron microscopy has been developed over the past fifteen years or so, along with x-ray microscopy, which uses absorption differences for contrast.