Oxidation of chromium carbide

The oxidation of chromium carbide has been studied gravimetrically. Products of reaction have been examined by gas sorption analysis and X-ray diffraction. Changes in phase composition, crystallinity and crystallite size are correlated with the reaction conditions.Chromium carbide, Cr₃C₂, differs from most of the transitional metal carbides in that it forms stable films of metal oxide (Cr₂O₃) around the remaining carbide particles, inhibiting further oxidation. Thus chromium carbide additive inhibits oxidation of interstitial zirconium carbide, ZrC, by forming some chromic oxide which stabilises the zirconia (ZrO₂) layer around the remaining carbide crystallites.     

Evaluation of the texture of polycrystalline aggregate from ultrasonic measurements

The propagation of ultrasonic waves in a polycrystalline aggregate is considered for a bulk sample with orthorhombic symmetry made of cubic crystals. Use is made of the Jaynes principle of maximum Shannon entropy to show how information about the crystallite orientation distribution function can be derived from ultrasonic velocity measurements. It is shown that such information may be derived from ultrasonic velocity measurements both immediately, and after calculating the effective Young modulus in the respective direction. The results obtained from the ultrasonic velocity measurements are compared with those obtained from neutron diffraction measurements.     

Auger-spectroscopy studies of the electronic structure of amorphous carbon films

Information on the density of valence states (DOVS) of aC films has been determined by Auger spectroscopy. The films were prepared by magnetron sputtering of graphite and plasma decomposition of hydrocarbon gas. The relative contents of graphite-like and diamond-like components in aC have been determined by comparison of the area under the π-band of aC with that of graphite. The effect of annealing on the aC structure has benn studied.     

Studies of alkali adsorption on Rh(111) using optical second-harmonic generation

Dramatic changes were observed in the optical second-harmonic generation from the Rh(111) crystal surface as the surface was covered with alkali atoms. For low alkali coverage, these changes are explained qualitatively in terms of optical transitions between alkali atomic-derived electronic states and, for high coverage, in terms of the plasmon resonance of the alkali layer. The results establish the surface specificity of second-harmonic generation to the topmost 1 or 2 atomic layers of alkali metals.     

Energy and angular distributions for scattering of K+ from W(110) at normal incidence

Using a K⁺ ion beam and a K neutral beam at normal incidence on a W(110) surface, we have measured the angle and energy distributions of scattered K⁺ ions at different azimuthal angles. The primary energy is in the range 12–100 eV. The measured distributions show a complex peak structure. The peaks can be assigned to different scattering mechanisms, giving rise to various rainbow phenomena.     

Coadsorption of oxygen and cesium on Ru(001)

The coadsorption of oxygen and Cs on Ru(001) has been studied by means of thermal desorption, Auger and electron loss spectroscopy and work function measurements. The initial sticking coefficients for oxygen adsorption and oxygen saturation coverages increase with increasing Cs coverage, θ_Cs. Irrespective of the initial θ_Cs, the Cs desorption energy always increases under the influence of the coadsorbed oxygen, the effect becoming stronger with increasing oxygen coverage. At θ_O>0.5and θ_Cs>0.14 the work function, electron loss changes and thermal desorption data give evidence of strong CsO interactions and the formation of a CsO “surface compound”.     

Inelastic electron tunneling spectroscopy of a model catalyst: CO on Rh/Al2O3

Using tunneling spectroscopy we have studied the preparation and behavior of dispersed rhodium model catalysts supported on alumina. Samples were prepared by vacuum evaporation from Rh metal or Rh₂O₃ sources onto an oxidized Al film and CO was adsorbed in-situ. The tunnel junctions were formed by adding a Pb top electrode and the vibrational spectra of the adsorbed species were measured. We observed qualitatively different spectra when the preparation procedure was varied. Special care was taken to monitor and control background gases. We obtained different results from Rh of Rh₂O₃ sources and the presence of oxygen or water affects the vibrational spectra of the adsorbed CO. We also study the effect of the Rh thickness on the spectral intensity. Other experiments were measurement of the superconducting tunneling spectra of the Pb and a TEM study of Rh particle size. Previously reported data from tunneling and IR measurements are compared with the present work. Based on these results, we conclude that there are two species present, either a linear Rh-CO or doubly (geminal) adsorbed Rh(CO)₂ depending upon the degree of dispersion and oxidation of the Rh. The evidence also indicates that in both instances a dispersed form of Rh, rather than relatively large Rh metal particles, is responsible for the observed spectra.     

The effect of bending vibrations on the dipole moment of a linear polyatomic molecule: A theoretical treatment of the vibrational changes and transition moments

The vibrational effect on the dipole moment of a linear molecule is theoretically considered from the aspects of the dipole moment changes with the excitation of bending vibrations and the transition moments for the overtone, combination, and difference bands associated with bending modes. Such dipole moment changes and transition moments consist of two components, one depending on the first dipole moment derivatives with respect to bond lengths and the other depending on the second dipole moment derivatives with respect to bond angles. We show that the first component normally contributes little, and propose an approximation in which only the second component is retained. This approximation is practically important because the second component can be calculated without the anharmonic force constants. We derive formulas for the dipole moment changes and transition moments to facilitate a simultaneous analysis of different isotopic species. We introduce the concept of the equivalent mode, by which we may readily understand the correlation between the dipole moment change for a bending mode and the transition moment for a vibrational band.