Identification of a Surface State on the (111) Terraces of Stepped Cu(211)

Energy distribution measurements of field-emitted electrons reveal a free-electron-type behavior for clean Ir(100) and (110) surfaces. Adsorption of CO on Ir(100) causes the appearance of a structure at 0.6 eV below the Fermi level $E_{\mathrm{F}}$ which is identified with a chemisorption level created by "back donation" of metallic $d$ electrons into the CO $2{\pi }^{*}$ level. Adsorption of atomic oxygen on Ir(100) and (110) leads to the formation of a maximum at 1.0 eV below $E_{\mathrm{F}}$ which is ascribed to a state derived from one of the O $2p$ levels.