Theoretical and experimental study of the unoccupied electronic band structure of Ru(001) by electron reflection.

Abstract

Very-low-energy electron diffraction (VLEED) is used to study the unoccupied electronic states of Ru(001). Experimental and theoretical data are presented for reflection of electrons with energies of the specular beam between 7 and 32 eV and at different angles of incidence between 3\ifmmode^\circ\else\textdegree\fi{} and 28\ifmmode^\circ\else\textdegree\fi{} in both the \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}K\ifmmode\bar\else\textasciimacron\fi{} and \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}M\ifmmode\bar\else\textasciimacron\fi{} directions. Two sharp reflection minima at kinetic energies of 11.1 and 12.3 eV (relative to the vacuum level) corresponding to a final-state peak in Ru(001) angle-resolved photoemission and secondary-electron emission are observed near normal incidence. Theoretical VLEED intensity and band-structure calculations are carried out to confirm the origin of the experimental spectral features. An energy-dependent optical potential is shown to be sufficient to explain the observed narrow spectral structures. The observed minima can be reproduced excellently with the imaginary part of the optical potential equal to -0.6 eV at 11 eV above the vacuum level. The minimum at 12.3 eV can clearly be correlated with a high density of states in the volume band structure, whereas the other one is assigned to a Shockley-type surface state. The effects of several other parameters on the theoretical spectra are discussed.