Anharmonic vibrational modes of chemisorbed H on the Rh(001) surface.

The potentials for H atoms in the vicinity of the 4-fold hallow chemisorption site on the Rh(001) surface at monolayer coverage is calculated using local density functional theory, and the linear augmented plane wave method. The potential is found to contain important anharmonic components, one that couples parallel and perpendicular motion, and another producing azimuthal anisotropy. Variational solutions are found for the ground and low-lying excited states of H and D in this potential. The fundamental symmetric and asymmetric stretch H vibrational excitations are found to have energies of 67 and 92 meV. The latter agrees with recent experimental results, and higher- lying experimental modes are interpreted as mixed excitations. Comparisons are made with spring-constant models, calculated potentials for H on Ni and Pd(001), and theories of Bloch states for H on Ni(001).