Coupling Between Adsorbate Vibrations and an Electronic Surface State

We report direct angle-resolved photoemission measurements of the coupling between the symmetric stretch vibrational mode of adsorbed hydrogen and a surface band on W(110). This coupling is manifested by the surface band being split into two branches at a binding energy comparable to the vibrational mode energy, as confirmed by observation of a dramatic hydrogen/deuterium isotope effect. The electron-phonon coupling parameter $\lambda$ is found to be significantly larger than that for bulk W, and to be closely related to the degree of surface localization of the surface state wave function.