A Review of (e,2e) Studies in Thin Films and Some Recent Results

Abstract

The form of the (e, Ze) cross section applicable to high energy symmetric transmission measurements in thin films is given in the plane wave impulse approximation. The difference expected for single crystal, non-single crystal and amorphous films is discussed. For certain kinematics, the measured (e, Ze) coincidence intensity can be related directly to the energy-momentum distribution of the target electron and a map of the occupied electron states can be obtained in principle. Recent experimental work in the authors' laboratory using about 100 A thin films of aluminium/aluminium oxide is discussed. Two peaks in the valence binding energy are seen at 13 and 31 eV whose angular correlations indicate that both are dispersionless and resemble P and S type atomic orbitals of oxygen. Neither peak resembles the free electron structure of aluminium. Improving the electron energy resolution from 4·5 to 1·5 eV while maintaining the momentum resolution constant at about 0·9 A-I did not significantly change the data which indicates the dominant effects of mUltiple scattering.

Measured energy - momentum densities of the valence band of aluminium

“…This measurement was substantially restricted by energy resolution (16 eV). Williams and Hayes [12] also used (e, 2e) spectroscopy to measure the structure of aluminium/aluminium oxide. Their experiment had an energy resolution of 1.5 eV and a momentum resolution of 0.45 au.…”

Direct Observation of Energy–Momentum Densities in Solids

“…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 samples has a coincidence count rate of about 4 Hz.…”