On the splitting of the CO 1 π level in the chemisorption system CO/Ni(111): substrate or lateral interaction?

LCGTO Xα model cluster calculations have been carried out to rationalize the shape of the CO 1 π band of the chemisorption system CO/Ni(111) observed in angular resolved photoemission. The splitting induced by substrate interaction at twofold bridging sites (0.4 eV) is much smaller than the value deduced from experiment (1.2 eV). From calculations on (CO)_n clusters lateral adsorbate interaction is estimated to cause a sizable broadening of the 1 π band (1.2eV, in satisfactory agreement with experiment), but essentially no splitting (≲0.1 eV). Therefore, rehybridisation due to local interaction does not seem to suffice as an explanation for the observed shape of the CO 1 π band.