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3867. ir study of molecules adsorbed on metal surfaces by surface electromagnetic wave spectroscopy. (USA)
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Editor's note
The/abe/immediately following the title of each item denotes country or origin of pub//cation, and that at the end of each abstract indicates country of origin of work (where known).
I. General v a c u u m science and engineering 11. GASES A N D SOLIDS 11 3866. Chemisorbed phases of O2 on TiO2 and SrTiO3. (USA) We have investigated the chemisorption of O2 on TiO2 and SrTiO3 surfaces by using ultraviolet photoemission difference spectroscopy. Both nearly perfect surfaces and surfaces having a high density of defects associated with filled Ti(3d) surface states were studied. On all surfaces low O2 exposures produced an initial adsorbed phase, with a sticking coefficient of 0.2-1, that yields a difference spectrum consisting of two peaks separated by 2.5 eV. The location of these peaks corresponds closely to the valance-band photoemission structure of the substrate, suggesting that the chemisorbed phase consists of O 2- ions. For surfaces having initially filled Ti(3d) surface states, formation of this phase is accompanied by charge transfer from the substrate. With increasing exposure, a second adsorbed phase, with a sticking coefficient of 10- 3 or less, is formed on some surfaces. This phase exhibits a sharp peak about 2 eV below the bottom of the substrate valence band and a broad peak overlapping the valance band. V E Henrich et al, J P'ac Sci Technol, 15 (2), 1978, 534--537. I1 3867. ir study of molecules adsorbed on metal surfaces by surface electromagnetic wave spectroscopy. (USA) The infrared spectra of monolayer coverages of two different molecules (KReO4 and SF6) physisorbed on metal surfaces have been measured in the 10 ~m region by means of a new surface electromagnetic wave (SEW) coupling technique in conjunction with a CO2 laser. Coverages as low as two monolayers can be readily monitored in agreement with predictions based on an optical sum rule. The new coupling configuration is based on the principle of inhornogeneous wave coupling and allows the matching of both the propagation wave vector and the spatial profile of the SEW. Our method is both more efficient and more convenient for SEW spectroscopy than the conventional prism technique. Y J Chabal and A J Sievers, J Vac Sci Technol, 15 (2), 1978, 638-641. II 3868. Oxidation of tin: an ESCA study. (USA) Tin exposed to dry oxygen (104-106 L) at 100°C has been studied by ESCA. The ESCA shift for the tin 3ds/z line was 1.65 eV and the separation between this line and the oxygen Is line was 43.90 eV. The oxide was identified by comparing its ESCA spectra with those of SnO and SnO2 samples carefully prepared in vacuum. Clean SnO samples were prepared in vacuum by decomposing SnO2 crystallites at high temperature and depositing the vapor product on a gold substrate. The resulting films were identified to be red SnO by ion backscattering and by M6ssbauer experiments. Clean SnO2 samples were prepared by diamond-filing bulk SnO2 crystals in vacuum. Both of these oxides were found to have the same separation between the oxygen Is and the tin oxide 3ds/2 lines and, hence, the same chemical shift. For this reason, the oxidized tin was identified by comparing its valence band and O/Sn ratio, rather than its chemical shift, with those of the SnO and SnO2 samples. The oxidized tin was found to consist of both SnO and SnO2. C L Lau and G K Wertheim, J Vac Sci Technol, 15 (2), 1978, 622-624. 11 3869. LEED structure analysis of a p(2 × 2) chemisorbed layer of CO on Ti(0001). (USA) Exposure of a clean Ti(0001) surface to CO gas gives rise to the formation of first a p(2 x 2) and then a 1 × 1 LEED pattern. An
intensity analysis of the p(2 x 2) phase has been carried out using six integral-order and four fractional-order spectra. Several models involving the CO molecule with axis normal to the surface have been tested with negative results. Fair agreement between observed and calculated intensities has been obtained for models involving C and O atoms (both 1/4 monolayer) in the same plane parallel to the surface and either both in the 'fcc' or both the 'hcp' threefold symmetrical hollows. The former model is better than the latter, but for neither is the agreement wholly satisfactory. Experience with the N/Ti system suggests that the 'correct' model may involve formation of an underlayer. This model, however, cannot be tested with our computer programs at the present time. H D Shih et al, J Vac Sci Technol, 15 (2), 1978, 596-599. 11 3870. Oxygen uptake on epitaxial PbTe(lU) surfaces. (USA) The uptake of oxygen on A.r÷-cleaned (111) surfaces of PbTe epitaxial films was studied by means of UPS and XPS. Two distinct stages of uptake process were observed and they were separated by a phaselike change at an exposure of l0 s L. The first stage, characteristic of an adsorption process, has a high oxygen uptake rate. In this stage, weak bonding between oxygen and the substrate is suggested by the observation of the relatively narrow linewidth of the Pb 5dsl2 core level. The most important feature, however, is the lowering of the Fermi level with oxygen coverage, which confirms the band bending model developed by previous studies, based on electrical measurements. The second stage, characteristic of an oxidation process, has a lower uptake rate. Strong bonds between oxygen and individual Pb and Te ions appear to prevail in this stage, as is evident from the broadening of the Pb 5ds/2 level and the appearance of the Te oxide species in the XPS spectra. The Fermi level stops lowering and in fact rises slightly at high oxygen coverages. T S Sun et al, J Vac Sci Technol, 15 (2), 1978, 585-589. 11 3871. Chlorine adsorption on silver surfaces. (USA) The properties and structure of adsorbed chlorine layers on clean silver surfaces have been studied. On the A g ( l l l ) surface, isobars over the temperature range from ~300 to ~:)00 K have been obtained using Auger electron spectroscopy. A model dependent, initial heat of adsorption of ,--,50 kcai/mol was obtained. Structural observations using LEED showed that an epitaxial layer of AgCI(I 11) was formed. Electron impact desorption measurements give a desorption cross-section for chlorine. On the AgO 10) surface a set of isotherms was obtained and the model independent isosteric initial heat of adsorption was also calculated to be ~ 5 0 kcal/mol. Yung-Yi Tu and J M Blakely, J Vac Sci Technol, 15 (2), 1978, 563-567. II 3872. Kinetics of thermal desorption and thermal conversion of adsorbates: AES studies. (USA) A surface analysis technique, called 'thermal conversion spectroscopy' (TCS), has been developed which employs Auger electron spectroscopy (AES) to measure adsorbate coverage as a continuous function of temperature. The technique is expected to be particularly useful in studies of the kinetics of surface processes which involve a conversion from one adsorbate state to another without causing any corresponding change in the gas phase. To evaluate the technique it has been applied to the desorption of CO from a W substrate and a direct comparison has been made with conventional thermal desorption spectrometry (TDS) done with a mass spectrometer. The Auger detection method is found to give the lower signal-to-noise ratio but the resolution is relatively high since complications arising from sample thermal gradients and overall vacuum system response (pumping speed, wall effects, etc.) are absent. By monitoring the carbon Auger spectrum, data analogous to TDS curves have been 185
Editor's note
The/abe/immediately following the title of each item denotes country or origin of pub//cation, and that at the end of each abstract indicates country of origin of work (where known).
I. General v a c u u m science and engineering 11. GASES A N D SOLIDS 11 3866. Chemisorbed phases of O2 on TiO2 and SrTiO3. (USA) We have investigated the chemisorption of O2 on TiO2 and SrTiO3 surfaces by using ultraviolet photoemission difference spectroscopy. Both nearly perfect surfaces and surfaces having a high density of defects associated with filled Ti(3d) surface states were studied. On all surfaces low O2 exposures produced an initial adsorbed phase, with a sticking coefficient of 0.2-1, that yields a difference spectrum consisting of two peaks separated by 2.5 eV. The location of these peaks corresponds closely to the valance-band photoemission structure of the substrate, suggesting that the chemisorbed phase consists of O 2- ions. For surfaces having initially filled Ti(3d) surface states, formation of this phase is accompanied by charge transfer from the substrate. With increasing exposure, a second adsorbed phase, with a sticking coefficient of 10- 3 or less, is formed on some surfaces. This phase exhibits a sharp peak about 2 eV below the bottom of the substrate valence band and a broad peak overlapping the valance band. V E Henrich et al, J P'ac Sci Technol, 15 (2), 1978, 534--537. I1 3867. ir study of molecules adsorbed on metal surfaces by surface electromagnetic wave spectroscopy. (USA) The infrared spectra of monolayer coverages of two different molecules (KReO4 and SF6) physisorbed on metal surfaces have been measured in the 10 ~m region by means of a new surface electromagnetic wave (SEW) coupling technique in conjunction with a CO2 laser. Coverages as low as two monolayers can be readily monitored in agreement with predictions based on an optical sum rule. The new coupling configuration is based on the principle of inhornogeneous wave coupling and allows the matching of both the propagation wave vector and the spatial profile of the SEW. Our method is both more efficient and more convenient for SEW spectroscopy than the conventional prism technique. Y J Chabal and A J Sievers, J Vac Sci Technol, 15 (2), 1978, 638-641. II 3868. Oxidation of tin: an ESCA study. (USA) Tin exposed to dry oxygen (104-106 L) at 100°C has been studied by ESCA. The ESCA shift for the tin 3ds/z line was 1.65 eV and the separation between this line and the oxygen Is line was 43.90 eV. The oxide was identified by comparing its ESCA spectra with those of SnO and SnO2 samples carefully prepared in vacuum. Clean SnO samples were prepared in vacuum by decomposing SnO2 crystallites at high temperature and depositing the vapor product on a gold substrate. The resulting films were identified to be red SnO by ion backscattering and by M6ssbauer experiments. Clean SnO2 samples were prepared by diamond-filing bulk SnO2 crystals in vacuum. Both of these oxides were found to have the same separation between the oxygen Is and the tin oxide 3ds/2 lines and, hence, the same chemical shift. For this reason, the oxidized tin was identified by comparing its valence band and O/Sn ratio, rather than its chemical shift, with those of the SnO and SnO2 samples. The oxidized tin was found to consist of both SnO and SnO2. C L Lau and G K Wertheim, J Vac Sci Technol, 15 (2), 1978, 622-624. 11 3869. LEED structure analysis of a p(2 × 2) chemisorbed layer of CO on Ti(0001). (USA) Exposure of a clean Ti(0001) surface to CO gas gives rise to the formation of first a p(2 x 2) and then a 1 × 1 LEED pattern. An
intensity analysis of the p(2 x 2) phase has been carried out using six integral-order and four fractional-order spectra. Several models involving the CO molecule with axis normal to the surface have been tested with negative results. Fair agreement between observed and calculated intensities has been obtained for models involving C and O atoms (both 1/4 monolayer) in the same plane parallel to the surface and either both in the 'fcc' or both the 'hcp' threefold symmetrical hollows. The former model is better than the latter, but for neither is the agreement wholly satisfactory. Experience with the N/Ti system suggests that the 'correct' model may involve formation of an underlayer. This model, however, cannot be tested with our computer programs at the present time. H D Shih et al, J Vac Sci Technol, 15 (2), 1978, 596-599. 11 3870. Oxygen uptake on epitaxial PbTe(lU) surfaces. (USA) The uptake of oxygen on A.r÷-cleaned (111) surfaces of PbTe epitaxial films was studied by means of UPS and XPS. Two distinct stages of uptake process were observed and they were separated by a phaselike change at an exposure of l0 s L. The first stage, characteristic of an adsorption process, has a high oxygen uptake rate. In this stage, weak bonding between oxygen and the substrate is suggested by the observation of the relatively narrow linewidth of the Pb 5dsl2 core level. The most important feature, however, is the lowering of the Fermi level with oxygen coverage, which confirms the band bending model developed by previous studies, based on electrical measurements. The second stage, characteristic of an oxidation process, has a lower uptake rate. Strong bonds between oxygen and individual Pb and Te ions appear to prevail in this stage, as is evident from the broadening of the Pb 5ds/2 level and the appearance of the Te oxide species in the XPS spectra. The Fermi level stops lowering and in fact rises slightly at high oxygen coverages. T S Sun et al, J Vac Sci Technol, 15 (2), 1978, 585-589. 11 3871. Chlorine adsorption on silver surfaces. (USA) The properties and structure of adsorbed chlorine layers on clean silver surfaces have been studied. On the A g ( l l l ) surface, isobars over the temperature range from ~300 to ~:)00 K have been obtained using Auger electron spectroscopy. A model dependent, initial heat of adsorption of ,--,50 kcai/mol was obtained. Structural observations using LEED showed that an epitaxial layer of AgCI(I 11) was formed. Electron impact desorption measurements give a desorption cross-section for chlorine. On the AgO 10) surface a set of isotherms was obtained and the model independent isosteric initial heat of adsorption was also calculated to be ~ 5 0 kcal/mol. Yung-Yi Tu and J M Blakely, J Vac Sci Technol, 15 (2), 1978, 563-567. II 3872. Kinetics of thermal desorption and thermal conversion of adsorbates: AES studies. (USA) A surface analysis technique, called 'thermal conversion spectroscopy' (TCS), has been developed which employs Auger electron spectroscopy (AES) to measure adsorbate coverage as a continuous function of temperature. The technique is expected to be particularly useful in studies of the kinetics of surface processes which involve a conversion from one adsorbate state to another without causing any corresponding change in the gas phase. To evaluate the technique it has been applied to the desorption of CO from a W substrate and a direct comparison has been made with conventional thermal desorption spectrometry (TDS) done with a mass spectrometer. The Auger detection method is found to give the lower signal-to-noise ratio but the resolution is relatively high since complications arising from sample thermal gradients and overall vacuum system response (pumping speed, wall effects, etc.) are absent. By monitoring the carbon Auger spectrum, data analogous to TDS curves have been 185