We present an approach that takes into account lattice dynamical effects in calculating the optical response of semiconductors by averaging over several perturbed configurations of a supercell extracted from molecular dynamic simulations. The validity of this approach is confirmed by comparing our results for the dielectric function of bulk GaAs in the range of 0 - 700 K with our and other highly accurate ellipsometry measurements. The results of our approach resolve the serious discrepancy in energy and line shape between the latest optical models, all of which neglect lattice dynamics, and experiment

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The total energy distributions ͑TEDs͒ in field emission ͑FE͒ and photofield emission ͑PFE͒ and the work functions have been measured at room temperature for Ba adsorbed on W͑100͒, W͑110͒, and W͑111͒ in the range of coverage from 0 to 1 ML ͑monolayer͒. We observe two initial state peaks and three final state peaks on W͑100͒ / Ba, six initial state peaks and one final state peak on W͑110͒ / Ba, and two initial state peaks and two final state peaks on W͑111͒ / Ba. We extend the full-potential linear augmented plane wave method for the electronic structures of periodic lattices to calculate the emission current in FE and surface PFE at a metaladsorbate-vacuum interface. Our calculations account for the energies of all of the initial state features observed experimentally in FE and surface PFE from clean W͑100͒, in PFE and angle-resolved inverse photoemission spectroscopy from W͑100͒ / Ba at 1 ML, and for all of the peaks observed in FE, surface PFE, and photoemission from W͑110͒ / Ba at 0.6 ML and from W͑111͒ /Ba at 1 ML. The dz 2 -like surface states of the Swanson hump ͓L. W. Swanson and L. C. Crouser, Phys. Rev. Lett. 16, 389 ͑1966͔͒ of clean W͑100͒ hybridize with s-like states of the c͑2 ϫ 2͒ overlayer and are shifted by −1.60 eV to yield a prominent peak in PFE. An isolated Ba c͑2 ϫ 2͒ layer is found to be weakly metallic; the metallicity is greatly enhanced when it is adsorbed on a W͑100͒ substrate. The TEDs in PFE from the atomically less dense overlayer W͑110͒ /Ba ͑2 ϫ 2͒ are dominated by substrate-overlayer interactions, while those from the atomically denser overlayer W͑111͒ /Ba ͑1 ϫ 1͒ are dominated by interactions within the overlayer. Our results yield evidence that above 1 / 3 ML Ba coverage on W͑111͒, which corresponds to a commensurate ͑3 1/2 ϫ 3 1/2 ͒R30°overlayer, the interstitial sites fill in randomly to form a commensurate ͑1 ϫ 1͒ overlayer at 1 ML coverage.