Results are presented for angle-resolved secondary-electron emission normal to a W(110) surface for the special case of “band gap emission”, which arises when the energy of the emitted electron corresponds to an energy gap in the final states' bands located along a principal symmetry direction. Under such conditions, surface enhanced emission occurs, and scattering out of adsorbate-induced (i.e., “extrinsic”) surface states is resolved. Spectra are presented for the specific cases of O₂ and CO adsorption on W(110) which, together with parallel LEED studies, show that such effects only appear in the presence of ordered adsorbate monolayers, and are extremely sensitive to the surface unit cell dimensions of the adsorbate–substrate superstructure. The results are interpreted in terms of adsorbate surface resonances associated with excited two-dimensional surface Bloch states produced by the periodicity of the adsorbate layers.