We report high-resolution results from the well-known ''Shockley''-type surface state localized around the Ȳ point of the Cu͑110͒ surface Brillouin zone. Along ⌫Y the effective mass is m*/mϭ0.26Ϯ0.03 and the bottom of the dispersion parabola at Ȳ shifts linearly with sample temperature T as E 0 ϭϪ(510Ϯ15)meV ϩ(0.26Ϯ0.02 meV/K)T. We have also analyzed the photoemission linewidth ⌫ h of this surface state. Our analysis results in an upper limit for the imaginary part of the self-energy Im ⌺ϭ⌫ h /2р(16Ϯ3)meV, which corresponds to a photohole lifetime у(21Ϯ5)fs at Ȳ and T→0. On the basis of recent calculations of ⌫ h for Cu͑111͒ at ⌫ ͓Chulkov et al., Surf. Sci. ͑to be published͔͒ we interpret ⌫ h of Cu͑110͒ at Ȳ by inelastic hole-electron interactions, with about 50% contribution each from surface band ͑in-plane͒ and from threedimensional ͑coupling to substrate͒ decay channels. Finally we discuss the contribution of the Ȳ surface state to the surface optical properties as observed in both second harmonic generation and ͑one-photon͒ reflection anisotropy spectroscopy from Cu͑110͒.

The contributions of direct transitions ͗ f ͉A ជ •p ជ ͉i͘ and surface emission ͗ f ͉divA ជ ͉i͘ in angle-resolved photoelectron spectroscopy spectra can be varied considerably by changing the incidence angle of p-polarized light. Due to interference of both contributions line shape and asymmetry of certain peaks change dramatically as a function of light incidence direction. This effect has been studied in detail at an s,p band of copper and a photon energy of បϭ21.2 eV. Our experimental data can be explained quantitatively within a physically transparent model calculation, which supplies a criterion to distinguish between bulk direct and surface emission.

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