We have developed a third-neighbor tight-binding model, with spin-orbit coupling included, to treat the electronic properties of Bi and Sb. This model successfully reproduces the features near the Fermi surface that will be most important in semimetal-semiconductor device structures, including (a) the small overlap of valence and conduction bands, (b)

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In this paper we introduce a method for realistic simulations of nonadiabatic processes, including the interaction of light with matter. Calculations of the response of C 60 and carbon chains to laser pulses demonstrate that even rather subtle features are correctly described. For example, in C 60 the pentagonal-pinch mode is dominant at low fluence, the breathing mode is dominant at high fluence, and dimers are preferentially emitted during photofragmentation. In carbon chains, on the other hand, trimers tend to be broken off. After collisional fragmentation, the remnants of a C 60 molecule tend to reform their bonds, yielding new 5, 6, or 7 membered rings.

Detailed simulations are reported for the dynamics of electrons and nuclei during the cis to trans photoisomerization of stilbene. Our method, which employs a semiclassical description of both the nuclear motion and the radiation field, is described in the text. After excitation of electrons from the HOMO to the LUMO by a femtosecond-scale laser pulse, two principal avoided crossings are observed between the HOMO and LUMO levels, each of which leads to substantial depopulation of the LUMO. Based on our results and those of other groups, we propose that the first such HOMO-LUMO coupling can lead to the formation of 4a,4b-dihydrophenanthrene (DHP). The second coupling, on the other hand, leads to the formation of trans-stilbene. It is found that pyramidalization of the two carbon atoms of the vinyl group is involved significantly in both couplings, and that rotation of the two phenyl rings, together with their interaction, plays an important role in the first coupling. The occurrence along the same trajectory of two couplings, one leading to DHP and the other leading to isomerization, is a significant observation, and one which indicates that further theoretical and experimental investigations would be of considerable interest.

Motivated by recent experiments, we have performed simulations which show in detail how the electrons and ions in GaAs respond to fast intense laser pulses ͑with durations of order 100 fs and intensities of order 1Ϫ10 TW/cm 2). The method of tight-binding electron-ion dynamics is used, in which an arbitrarily strong radiation field is included through a time-dependent Peierls substitution. The population of excited electrons, the atomic displacements, the atomic pair-correlation function, the band structure, and the imaginary part of the dielectric function are all calculated as functions of time, during and after application of each pulse. Above a threshold intensity, which results in promotion of about 10% of the electrons to the conduction band, the lattice is destabilized and the band gap collapses to zero. This is most clearly revealed in the dielectric function ⑀(), which exhibits metallic behavior and loses its structural features after 100-200 fs. ͓S0163-1829͑98͒01843-8͔