Angle-resolved photoemission investigation of the electronic structure of Be: Surface states

We have performed an angle-resolved photoemission investigation, using synchrotron radiation, of the surface electronic structure of Be(0001). At normal emission we observe a surface state in the ${\ensuremath{\Gamma}}_{3}^{+}$-${\ensuremath{\Gamma}}_{4}^{\mathrm{\ensuremath{-}}}$ band gap with a binding energy of 2.8\ifmmode\pm\else\textpm\fi{}0.1 eV. Away from \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} it disperses parabolically towards ${E}_{F}$ with an effective mass of ${m}^{\mathrm{*}}$/m\ensuremath{\sim}1.5. For \ensuremath{\Elzxh}\ensuremath{\omega}40 eV, the energy dependence of the photoexcitation cross section for this state shows rapid variations caused by changes in the local electromagnetic field at the surface. For \ensuremath{\Elzxh}\ensuremath{\omega}>40 eV, it shows only weak structure. This high-energy behavior is quite different from the large resonances observed for surface states on other metals and is associated with the short penetration depth of the Be surface state. The dispersion of this state is measured along \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\rightarrow}M\ifmmode\bar\else\textasciimacron\fi{} and \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\rightarrow}K\ifmmode\bar\else\textasciimacron\fi{} in the two-dimensional surface Brillouin zone. For a small range of ${k}_{\ensuremath{\parallel}}$ around M\ifmmode\bar\else\textasciimacron\fi{}, there is evidence for two surface states in the ${M}_{2}^{\mathrm{\ensuremath{-}}}$-${M}_{4}^{\mathrm{\ensuremath{-}}}$ gap with binding energies of 1.8\ifmmode\pm\else\textpm\fi{}0.1 eV and 3.0\ifmmode\pm\else\textpm\fi{}0.1 eV.