The energy-resolved electron-momentum density of fullerene has been measured. Clear differences are found from the energy-momentum densities of either diamond or graphite. The energy-momentum density of fullerene can be described as being composed of a band and a split band. The observed spectral momentum densities are compared to the calculated orbitals of a C 60 molecule in momentum space. Good agreement is found. A simple classification of the orbitals is proposed that explains elegantly the calculated and observed structures. The splitting of the band can be interpreted as a consequence of the curvature of the carbon network forming the fullerene molecule.

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Electron-momentum spectroscopy based on the (e,2e) reaction has been used to observe the energy-momentum density of valence electrons in the ͓110͔ direction for an ultrathin, free-standing film of crystalline silicon. An asymmetric scattering geometry is used in which the incident, scattered and ejected electron energies are 20.8, 19.6, and 1.2 keV, respectively. The measurement is complicated by the possibility of diffraction of the free electrons. The theory of the reaction including diffraction is summarized and applied to experiments with different target orientations. The orientation is determined from an independent electron diffraction experiment. Very good agreement between theory and experiment is observed. ͓S0163-1829͑98͒01220-X͔

The spectral momentum density of the valence band of arc evaporated amorphous carbon has been measured by (e, 2e) spectroscopy with significantly improved energy resolution relative to earlier studies. The valence band has been studied over a range of momenta from 0 to 1.6 a.u. with a resolution of 0.15 a.u. and over a range of binding energies from 9 eV above to 35 eV below the Fermi energy with a resolution of 1.5 eV. As seen in earlier studies, two major peaks are observed in the spectral momentum density which previously have been associated with cr and vr bands in graphite. A third feature in the spectra, a weak shoulder approximately 4 eV below the Fermi energy, is observed. A heuristic model is introduced based on the assumption that the spectral momentum density of evaporated amorphous carbon is an angular average of the spectral momentum density of graphite. The behavior of the strongest feature in the experimental spectra is described well by this model, but the other two features, which are in the energy range of the graphitic x, o2, and cr3 bands, are poorly represented by the model. It is suggested that the poor agreement is due to rehybridization of these graphitic bands.