Theory of H bonding on vibration on Ru(0001).

Self-consistent Linear Augmented Plane Wave total energy calculations have been carried out for a 1x1 H monolayer on the Ru(0001) surface. They give an H-Ru layer separation of 1.06angstroms and vibrational frequencies of 140meV for the symmetric stretch mode and 100meV for the asymmetric stretch mode. These compare very well with measured values of 138 and 105meV, but reverse the original mode assignments which appeared to be consistent with a nearest neighbor harmonic force model. Our results are qualitatively inconsistent with such a model. Another simple model, the effective medium theory, also fails to account for our results in the absence of significant covalent correction terms.