EQUILIBRIUM ADSORPTION OF NH3 ON GE(001), GE(113) AND GE(111)

Photoelectron spectra of NH3 adsorbed molecularly on different orienta tions of Ge were taken under adsorption-desorption equilibrium conditi ons, i.e. during exposure at different pressures in the temperature ra nge 90-400 K. Using the NH3-1e orbital intensity as measure for the co verage theta, isosteric heats of adsorption theta H-ad (theta) were de duced for coverages up to 4 monolayers (ML). On Ge(001) and Ge(113), s trongly bound alpha-states with a dipole moment of 1.8 D per molecule and Delta H-ad-values between 80 and 130 kJ/mole were observed which s aturate at 0.5 and 0.2 ML, respectively. They are attributed to adsorp tion of one molecule per reconstruction dimer. The Ge(111) surface doe s not contain dimers and no strongly bound state was found. Further ad sorption is weaker (Delta H-ad between 45 and 24 kJ/mole, depending on orientation and coverage) and induces much smaller dipole moments. Th e involvement of inverted (N-end up) adsorption is likely. A break of Delta H-ad is observed when 2 ML are completed but even beyond, normal 3D condensation is not yet observed. It occurs after deposition of mo re than 4 ML and an irreversible structural transition of the whole la yer.