Titanium carbide (TiC) and titanium nitride (TiN) possess remarkable physical properties, such as extremely high hardness and melting point, that promote their use as antiwear materials under harsh tribological conditions. These physical properties must arise from chemical bonding phenomena that result from the inclusion of the non-metal atom within the metallic matrix, and these bonding phenomena should be apparent in measurements of the valenceband electronic structures of TiC and TiN. This paper explores the surface electronic structure and bonding in TiC(lOO) and TiN(l 10) with core and valence level photoelectron spectroscopies (PES's) using X-rays (1486.6 eV) and synchrotron radiation in the range 28-180 eV. Intensity changes in the valence-band features are followed as a function of incident photon energy; these changes are then compared to theoretical atomic photoionization cross sections to determine the atomic origins of these features. Resonant PES at the Ti 3p absorption edge is used to determine titanium 3d contributions to the valence band and to show differences in the electronic structures in TiC and TiN. A new resonance phenomenon near the Ti 3s edge in TiC was observed, and its possible assignment is discussed. The electronic structure and bonding in these materials is well described by molecular orbital theory, where the Ti and non-metal ions in their formal oxidation states (e.g., Ti4+ and C4-in TiC) undergo covalent bonding interactions. Overall, the PES results indicate greater covalent mixing for TiC as compared to TiN, consistent with the differences in the electronegativities of the atoms. Specifically, stronger covalent interactions between the C 2s, 2p and the Ti 3d, 4s, 4p levels must occur to explain the spectroscopic differences between TiC and TiN. In addition, there is no evidence for an occupied TiC valence level having predominantly Ti character (unlike TiN), precluding the existence of direct Ti-Ti bonding in TiC. Any such orbital overlap is significantly affected by the carbon atoms in the lattice.

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Application of highresolution electron energyloss spectroscopy to the study of adsorbates on metal oxide surfaces: Methylacetylene and allene on ZnO (0001) The interface produced by vapor deposition ofMn on the MoS 2 (0001) surface has been studied in situ by high-resolution photoelectron spectroscopy using synchrotron radiation. The evolution of the Mo 3d, Mn 3p, and S 2p core levels and of the valence-band spectra during growth of thin films (10--58 A) is consistent with partial conversion of the Mn overlayer to MnS via the overall reaction 2Mn + MoS 2 --+ 2MnS + Mo. The persistence of the substrate components of the Mo 3d and S 2p spectra for thicknesses> 35 A are consistent with the Volmer-Weber growth mode. Annealing a 58-A film to 770 K resulted in an overlayer film consisting mostly ofMnS coexisting with some metallic Mn. Analysis of the Mo 3d core levels indicates the production of a MoS 2 (000 1 ) surfact; with S vacancy defects. Annealing to temperatures between 850 and 1040 K drove the reaction to completion (as shown by the valence band and Mn 3p core level spectra). Annealing of the sample to 1130 K resulted in uncovering the MoS 2 (000 1 ) surface due to breakup of the reacted layer. In addition, low-energy electron diffraction indicated the formation of (0001 )-2 X 2 regions on the surface. This surface structure is interpreted in terms of an ordered, MoS 2 _ x sulfur vacancy defect structure rather than a Mn-Mo-S compound.

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A polydimethylsiloxane polymer was irradiated with 1470-and 1236-Á radiation, and the quantum yields of the gas products were determined. Analysis of the data indicates that the breaking of the Si-CH3 bond is the most probable reaction occurring at both wavelengths. For surface irradiations, C2He, CH4, and Ha are primary products, while for immersion irradiations, only CH4 and Ha are produced. The differences in the two types of experiments are ascribed to the competition between bimolecular CH3 combination and radical abstraction reactions; the surface irradiation favors the former mechanism. The quantum yields for 1470 were 0.033, 0.020, and 0 009 for CaHe, CH4, and Ha, respectively, with surface irradiation. Quantum yields for the immersion irradiation were 0.087 and 0.021 for CH4 and Ha. The precision of these numbers is ±0.002 for all values. For 1236-Á surface irradiations, secondary photolytic reactions were complicating factors and quantum yields were not obtained. In the immersion irradiations, 1236-A radiation gave quantum yields of 0.087 ± 0.016 and 0.049 ± 0.005 for CH4 and H2, respectively. A omixed methyl-phenyl polymer £(CH3)(Ph)2Si-0-Si(CH3)2-0-Si(CH3) (Ph)2]i was also irradiated at 1470 A, and quantum yields of 1.0 ± 0.5 X 10-3 and $4X 10-3 were found for CH4 and H2, respectively, in both surface and immersion irradiations. No C2H6 was found for this polymer. Also solutions of the phenyl-containing polymer in the polydimethylsiloxane were irradiated. It was found that the gas yields were reduced by both radical addition to the phenyl groups as well as intermolecular energy transfer between the two polymers; however, there is no evidence for the presence of long-range transfer involving energy transfer along the chain. Analysis of the data indicates that the rate constant for H atom addition to a phenyl group is ~10 times larger than the corresponding rate constant for CH3 radicals.