COADSORPTION AND REACTION OF NH3 WITH NO, O AND OH ON GE SURFACES

The coadsorption and reaction of NH3 and NO on the (001) and (111) sur faces of Ge has been studied by valence-band and core-level photoelect ron spectroscopy. On Ge(001), NH3 alone adsorbs molecularly at 100-130 K, first on the dimer-down atoms with the 2a1, 1e and 3a1 orbitals at - 23.0, - 11.9 and - 5.6 eV below E(v), then also on the up-atoms. Co adsorbed NO binds molecularly to the up-atoms and causes displacement of NH3 from these sites. A strongly polarized NH3-NO complex, stabiliz ed by hydrogen bonding and the antiparallel orientation of the molecul ar dipoles, is formed with the NH3 orbitals shifted to - 19.9, - 10.0 and - 5.4 eV. Upon annealing to 270-300 K, this complex reacts by spli tting off the N atom from NO. Coadsorbed OH and NH2 are formed, probab ly again stabilized by hydrogen bonding. The NH2-derived orbitals appe ar at - 19.6, - 9.6, - 4.1 and - 2.0 eV. Above about 370 K, further de composition occurs and at 490 K oxygen in the form of GeO(x) and some atomic N are visible in photoemission. If the surface is exposed first to NO at 100-130 K, partial decomposition occurs and NO and O are obs erved. Coexposure to NH3 leads first to NH3-NO-complex formation. Furt her NH3 adsorbs molecularly and is polarized by the O atoms. Coadsorpt ion of NH3 to O or OH + H on the surface yields polarized NH3 with pea k shifts corresponding to the strength of the interaction. On Ge(111), coadsorbed NO dissociates partly and forms NO and O. A similar NH3-NO complex is formed. Because of the overlap with O(ad)-derived features , the formation of OH + NH2 upon annealing cannot unambiguously be ded uced from the spectra.