Equilibrium adsorption of NH3 on Ge(001), (113) and (111)

Abstract Photoelectron spectra of NH 3 adsorbed molecularly on different orientations of Ge were taken under adsorption-desorption equilibrium conditions, i.e. during exposure at different pressures in the temperature range 90–400 K. Using the NH 3 -1e orbital intensity as measure for the coverage θ, isosteric heats of adsorption ΔH ad (θ) were deduced for coverages up to 4 monolayers (ML). On Ge(001) and Ge(113), strongly bound α-states with a dipole moment of 1.8 D per molecule and ΔH ad -values between 80 and 130 kJ/mole were observed which saturate at 0.5 and 0.2 ML, respectively. They are attributed to adsorption of one molecule per reconstruction dimer. The Ge(111) surface does not contain dimers and no strongly bound state was found. Further adsorption is weaker ( ΔH ad between 45 and 24 kJ/mole, depending on orientation and coverage) and induces much smaller dipole moments. The involvement of inverted (N-end up) adsorption is likely. A break of ΔH ad is observed when 2 ML are completed but even beyond, normal 3D condensation is not yet observed. It occurs after deposition of more than 4 ML and an irreversible structural transition of the whole layer.