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Abstract:
The coadsorption and reaction of NH3 and NO on the (001) and (111) surfaces of Ge has been studied by valence-band and core-level photoelectron 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 Ev, then also on the up-atoms. Coadsorbed NO binds molecularly to the up-atoms and causes displacement of NH3 from these sites. A strongly polarized NH3-NO complex, stabilized by hydrogen bonding and the antiparallel orientation of the molecular 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 splitting off the N atom from NO. Coadsorbed OH and NH2 are formed, probably again stabilized by hydrogen bonding. The NH2-derived orbitals appear at −19.6, −9.6, −4.1 and −2.0 eV. Above about 370 K, further decomposition occurs and at 490 K oxygen in the form of GeOx 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 observed. Coexposure to NH3 leads first to NH3-NO-complex formation. Further NH3 adsorbs molecularly and is polarized by the O atoms. Coadsorption of NH3 to O or OH + H on the surface yields polarized NH3 with peak 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 Oad-derived features, the formation of OH + NH2 upon annealing cannot unambiguously be deduced from the spectra.
