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Total energies and electron affinities of selected atoms are calculated in a local-spin-density scheme where the exchange is treated exactly. A much improved overall description of the electronic structure is obtained with respect to the usual Hartree–Fock and local-spin-density approximations. The joint use of the exact expression for the exchange and of the local approximation for the correlation gives much better total energies and allows to avoid the failures of the above two schemes in the calculation of negative-ion binding energies. The accuracy for total energies of first and second row atoms is better than 2 eV, while the typical error for electron affinities is 0.2 eV.

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Purposely disordered GaAs/Al Ga& As superlattices, followed by an enlarged well, were grown by molecular-beam epitaxy. Selective-excitation photoluminescence measurements were performed to study the energy dependence of the motion of resonantly photocreated excitons in disordered superlattices. We report the existence of different energy regions in the superlattice bands, corresponding to extended and spatially confined (localized) states induced by disorder. The temperature dependence of the photoluminescence spectra suggests the existence of activation energies for the localized states. This re'sects itself by an increase in the propagation rate of excitons by increasing the temperature. We compare qualitatively the experimental findings with a calculation of the superlattice miniband spectrum.

We succeeded in observing the two-photon absorption of germanium for both the direct and the indirect gaps, using the Vanderbilt free-electron laser. The two-photon absorption for the indirect gap was found to be 3 orders of magnitude smaller than the direct gap. The indirect-gap threshold position indicates that the two-photon absorption is assisted by an LO phonon, providing a long-delayed positive test of the Bassani-Hassan predictions.