ELECTRONIC AND GEOMETRIC STRUCTURE OF NH3 ON GE(001) UNDER EQUILIBRIUM ADSORPTION CONDITIONS

The adsorption of NH3 on the Ge(001) surface is studied by angle-resol ved UV-photoelectron spectroscopy using synchrotron radiation and by h igh-resolution low-energy electron diffraction (LEED) measurements. On the clean surface, the phase transition from 2x1 to c(4x2) is observe d over a temperature range from 400 to 220 K. Measurements during NH3 admission at varying temperature in adsorption-desorption equilibrium yield a sequential occupation of several states. Up to 1/2 ML (one mol ecule per reconstruction dimer), NH3 is bound strongly on the dimer do wn atoms with the Ge-N axis perpendicular to the surface (alpha state) . The adsorption occurs via a mobile precursor state. The alpha state is connected with a (2x2) superstructure. Yet, the transition c(4x2)-- >(2x2) starts already at very low coverages (0.01 ML) and is completed for about 0.04 ML. We propose that a long-range attractive interactio n causes alpha-NH3-island formation and that a short-range repulsive i nteraction is responsible for a change to 2x2 within the islands by a dimer flip of every second dimer row. At the island edges, this dimer flip continues over the clean part of the surface thus changing its st ructure to 2x2 long before 1/2 ML saturation. Beyond 1/2 ML, the struc ture changes again. A good-quality 2x1 structure is seen up to 4 ML in dicating pseudomorphic growth up to this coverage. Beyond 4 ML, the ad sorbate structure changes irreversibly to NH3-ice without any LEED pat tern.