Experimental study of surface states on the (110) faces of the noble metals

Using photoemission and inverse photoemission, we have studied several crystal-induced surface states of the (110) surfaces of the noble metals. We have observed surface states above the Fermi level at 2.1±0.2 and 5.5±0.3 eV at X¯ on Cu(110), at 2.25±0.2 and 4.75±0.3 eV at X¯ on Au(110), and at 0.2±0.2 eV at Y¯ on Au(110), as well as a state below the Fermi level at -0.1±0.1 eV at Y¯ on Ag(110). Combining these results with data from the literature we find that two surface states occur in the X¯ and Y¯ gaps on Cu(110), at Y¯ on Ag(110), and at X¯ on Au(110). The case of X¯ on Ag(110) requires further experimental investigation. At Y¯ on the reconstructed Au(110) surface, two surface bands are observed near the Fermi level, one below $E_F$ with photoemission and one above $E_F$ with inverse photoemission. We speculate that they may originate from a single surface band split by the reconstruction. The occurrence of multiple surface states is addressed with a model calculation for unreconstructed surfaces, which correctly predicts the experimentally observed results in each case where definitive experimental data exist.