The experimental mapping of the density of states requires knowledge of all kinematic variables. Conventional techniques, such as photoelectron spectroscopy, average over unobserved variables. The (e,2e) technique, where the scattered and ejected electrons are detected and correlated in time, completely specifies the kinematics. We have successfully applied the technique to thin (130-Å) aluminum (oxide) films with 7.5-keV incident electrons. The use of new position-sensitive detectors on each of the scattered- and ejected-electron-energy analyzers with parallel data collection gives about 2 orders of magnitude improvement in data-collection time over the previous method of sequentially scanning the energy spectrum over a slit. The coincidence binding-energy spectrum with 4.5 eV [full width at half maximum (FWHM)] resolution shows well-defined peaks at 13 and 30 eV which are probably related to the upper and lower valence bands, respectively, of aluminum oxide. This spectrum has been obtained at values of electron momenta from -3.9 to 3.9 A${̊}^{\mathrm{-}1}$ in increments of 0.77 A${̊}^{\mathrm{-}1}$ with a resolution of 1.1 A${̊}^{\mathrm{-}1}$ FWHM. Consideration of the peak dispersion as a function of momentum support the interpretation of an oxide, rather than metallic, origin of the peaks.

• Received 20 June 1988

DOI:https://doi.org/10.1103/PhysRevB.38.13371