2π*level of chemisorbed co on ni(111): nearest-neighbor interactions versus bonding to the substrate

Using inverse photoemission we have studied the development of the ${2\pi }^{*}$ derived bands in chemisorbed CO on Ni(111) with increasing coverage. Up to 0.3 monolayers only one degenerate ${2\pi }^{*}$ derived state is observed at 1.7 eV above the Fermi energy. Increasing the coverage to 0.5 monolayer results in a shift by 2.7 eV to higher energies and a splitting of the ${2\pi }^{*}$ bands. Since the adsorption site remains the same we conclude that the position of the ${2\pi }^{*}$ band is determined largely by intermolecular interactions.