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Circular dichroism in angular distribution was observed for the valence band photoelectron from nonmagnetic and nonchiral material Si(001) with the incident photon energy in UPS region. The obtained two-dimensional angular distributions are to a certain extent consistent with an analysis of simple electronic dipole transition. Calculated angular distributions based on highly accurate one-step model showed better agreement considering photoelectron diffraction effect.

Physical review letters 83(26), 5551 - 5554 (1999). doi:10.1103/PhysRevLett.83.5551, Angle-resolved photoemission spectroscopy (ARPES) has been commonly applied to evaluate the shape of Fermi surfaces by employing simple criteria, such as maximum photoemission intensity at the Fermi level or discontinuity in the momentum distribution function, for the determination of the Fermi vector kF parallel to the surface. Here we show that these criteria may lead to large uncertainties, in particular for narrow band systems. We develop a reliable method for the determination of Fermi vectors employing high resolution ARPES at different temperatures. The relevance and accuracy of the method is discussed on data of the quasi-two-dimensional system TiTe2., Published by APS, College Park, Md.

A combined experimental and theoretical study of very-low-energy electron diffraction at the (0001) surface of 2H-NbSe2 is presented. Electron transmission spectra have been measured for energies up to 50 eV above the Fermi level with k|| varying along the GammaK line of the Brillouin zone. Ab initio calculations of the spectra have been performed with the extended linear augmented plane wave k-p method. The experimental spectra are interpreted in terms of three-dimensional one-electron band structure. Special attention is paid to the quasi-particle lifetimes: by comparing the broadening of the spectral structures in the experimental and calculated spectra the energy dependence of the optical potential Vi is determined. A sharp increase of Vi at 20 eV is detected, which is associated with a plasmon peak in the Im(-1/epsilon) function. Furthermore, the electron energy loss spectrum and the reflectivity of NbSe2 are calculated ab initio and compared with optical experiments. The obtained information on the dispersions and lifetimes of the unoccupied states is important for photoemission studies of the 3D band structure of the valence band., 17 pages, 11 Postscript figures, submitted to Phys. Rev. B