Direct Observation of Energy–Momentum Densities in Solids

Abstract

Electron momentum spectroscopy (EMS), based on kinematically complete observations of high energy electron impact ionisation events, directly observes energy-momentum dispersion laws and densities of electrons in solids. The valence electronic structure in the near surface region, up to a depth of about 20 A, is probed for thin free-standing films (about 100 A) by the multiparameter EMS spectrometer at Flinders University. The principles of the measurement are described and its application to the determination of energy-momentum densities in a range of amorphous, polycrystalline and crystalline materials is discussed. IntroductionElectron momentum spectroscopy (EMS) is the elucidation of the electronic structure of materials by the direct observation of the energy-momentum densities of the electrons determining the chemical and physical properties of the materials. The technique has proved extremely valuable in the understanding of the electronic structure of systems in the gaseous phase, i.e. atoms and molecules Weigold 1988, 1991). It has now been successfully extended to solids and surfaces, which are much more difficult, but in some respects much more interesting.Early EMS studies of solid targets had very poor energy and momentum resolution and very low count rates. Ritter et al. (1984) were the first to resolve structure in the valence states of a solid (evaporated carbon films), although they were not able to reveal much detail, since their energy resolution was still only about 6 e V. The coincidence count rates were of the order 0·02 Hz with a momentum resolution of around 0·3 a.u. Williams and Hayes (1990) reported some (e,2e) measurements on aluminium/aluminium oxide with improved energy resolution (1· 5 e V) but with a momentum resolution of 0·45 a. u. and similar low count rates. By comparison the present multi-parameter EMS spectrometer at the ESM Centre, Flinders University ) has an energy resolution of better than 1 e V, a momentum resolution of about O· 1 a. u. and for similar target

Electronic-structure investigation of oxidized aluminum films with electron-momentum spectroscopy

“…1 This was demonstrated by a series of (e,2e) experiments on solid targets [2][3][4][5][6][7][8][9][10] in the last few years. In these experiments, the (e,2e) spectrometer 11 of The Flinders University of South Australia was used.…”

Spectral momentum density of electrons in copper

“…Over the last ten years electron momentum spectroscopy ͑EMS͒ or (e,2e) spectroscopy has developed into a powerful technique to measure directly the electron spectralmomentum density of crystalline solids and, in particular, structurally disordered solids. 1,2 Electron momentum spectroscopy is based on the (e,2e) reaction. 3 In an (e,2e) experiment all kinematical parameters are accurately measured.…”

“…Elemental solids previously studied with EMS all have a valence band consisting of entirely s and p electrons. 1 The valence band of copper contains s, p, and d electrons. Also, an EMS study of the valence band of copper would build up a basis for EMS studies of transition metals that have partially filled d bands.…”