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High-resolution angle-resolved photoemission results are presented which allow us to determine the complete Fermi surfaces for the surface-localized electronic levels on the clean and hydrogencovered Mo(011) surfaces. Similar to previously presented data for W(011), we observe a total of three distinct closed hole orbits and one closed electron orbit. The hole orbits are elliptical and are centered on different projections of the same bulk Fermi-surface ellipsoid. They are located at the center and along each of the edges of the surface Brillouin zone. The surface electron pocket is closed but has a very complex shape which is somewhat different from the one observed on W(011). It orbits the projection of a bulk electron pocket which is traditionally called a jack, and is centered in the surface Brillouin zone. As was observed for W(011), these orbits are affected to different extents by hydrogen adsorption. The hole pockets are rapidly quenched by hydrogen, while the electron pocket grows in area until it merges with its image in the second Brillouin zone. At saturation there exist two hole pockets which are the remnants of the clean-surface electron pocket. These results are discussed in terms of the dynamical response of the surface. Electronic damping mechanisms for low-energy surface excitations are discussed. Some of the possible vibrational Kohn anomalies are enumerated.

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Angle-resolved photoemission measurements of the Fermi surfaces of several surface localized states of clean and hydrogen-covered W(l 10) are reported. Three hole orbits and one electron orbit have been characterized. The hole-orbit states are rapidly attenuated by hydrogen, while the electron-orbit states are shifted to higher binding energy resulting, initially, in an expansion of the Fermi surface, and ultimately, in its conversion to two hole orbits. These data represent the most detailed study of the behavior of the two-dimensional Fermi surface to date.

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