Angle-resolved photoemission study of the orientation and electronic interactions of romanCO+K/Ru(001) in the low-coverage regime

We have studied the promoted system CO+K/Ru(001) in the regime of low CO and K coverages (${\mathrm{CTHETA}}_{\mathrm{K}}$,${\mathrm{CTHETA}}_{\mathrm{CO}\mathrm{\approxeq }0.1}$ monolayer) using the technique of angle-resolved ultraviolet photoelectron spectro- scopy (UPS) with a synchrotron light source. Contrary to the conclusion of previous electron-energy-loss spectroscopy and electron-stimulated-desorption ion angular distribution investigations and a recent molecular-orbital-cluster calculation, we find that the CO molecules are oriented normal or nearly normal to the Ru(001) surface in the presence of K. In particular, we are able to place an upper limit of 30° for the tilt of the CO molecules from the surface normal, based on several different UPS measurements. We have also measured the effects of varying the CO coverage on the binding energies and intensities of CO valence levels and the K 3p core level. The results of this investigation are found to be consistent with recent UPS studies of the high-coverage phases of K and CO coadsorbed on Cu(100) and Ru(001) surfaces. We extend the model proposed in connection with those results to the low-coverage phase studied here and suggest that both coverage regimes can be understood in one unified picture of alkali-metal promotion.