Photoelectron imaging using an ellipsoidal display analyzer

We have built an ellipsoidal display analyzer (EDA) for angle-resolved phot oelectron spectroscopy and related techniques. The instrument is an improve d version of a design by Eastman [Nucl. Instrum. Methods 172, 327 (1980)] a nd measures the angle-resolved intensity distribution of photoelectrons at fixed energy I(theta,Phi)(E=const.). Such two-dimensional cuts through the Brillouin zone are recorded using a position-sensitive detector. The large acceptance angle (Delta theta =43 degrees in the polar direction and Delta Phi =360 degrees in the azimuthal direction) leads to a collection efficien cy which exceeds that of conventional hemispherical analyzers by a factor o f about 3000. Using ray-tracing calculations we analyze the electron optica l properties of the various analyzer components and optimize their arrangem ent. This minimizes distortions and aberrations in the recorded images and greatly improves the performance compared to previous realizations of this analyzer. We present examples demonstrating the performance of the analyzer and its versatility. Using a commercial He-discharge lamp we are able to m easure complete angular distribution patterns in less than 5 s. The energy and angular resolution are DeltaE(EDA)=85 meV and Delta theta =1.2 degrees, respectively. Complete stacks of such cuts through the Brillouin zone at d ifferent kinetic energies E can be acquired automatically using custom soft ware. The raw data are processed leading to a three-dimensional set (I(E-B, k(parallel to)) of photoelectron intensity versus binding energy E and wave vector k(parallel to). From this all relevant information, like the disper sion relations E-B(k(parallel to)) along arbitrary directions of the Brillo uin zone or Fermi-surface maps, can then be computed. An additional electro n gun enables low-energy electron diffraction, Auger electron spectroscopy, and electron energy-loss spectroscopy. Switching between electrons and pho tons as the excitation source is possible without any movement of the sampl e or analyzer. Because of the high acquisition speed it is possible to stud y the electronic structure of solids as a function of an external parameter (i.e., temperature) or to make animated movies showing, for example, the e volution of electronic states in reciprocal space. After installation of th is EDA at a synchrotron providing tunable photon energy, the full power of the instrument will come into play by adding techniques like constant final state or constant initial state spectroscopy, and x-ray photoelectron diff raction. (C) 2001 American Institute of Physics.