We have performed SH spectroscopy on a reconstructed Au(1 0 0) 5 x 20 surface. We found a broad SH intensity continuum for the SH photon energy from 2homega = 2.2 to 4.2 eV. Between the photon energies 2homega = 3.7 and 4.2 eV, the SH intensity in the s-in/p-out (s-polarized input/p-polarized output) polarization configuration is stronger than that in p-in/p-out polarization configuration, but the former signal disappeared when the surface was exposed to air. Surface electronic states are suggested to contribute to the SH response in this photon energy region. (C) 2004 Elsevier B.V. All rights reserved.