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Ru(0001): a Monte Carlo study
A138 Dissociative scattering on a Ag(lll) crystal surface of neutral and positively charged diatomics (H~, H I , I-le~, He J ) at tow normal incidence energies (E~ < 1 eV) has been studied by measuring relative velocity distributions of the neutral fragments. The dissociation fragments were recorded by means of a two-particle detector with a condition of coincidence for arrival of both fragments, On taking into account independent straggling of the fragments after dissociation, the spectra are interpreted as resulting from direct dissociation. A comparison with theory is presented. The experimental data support Auger neutralization in the case of helium. For hydrogen we interpret the results in terms of resonant neutralization.
Surface Science 280 (1993) 170-178 North-Holland
Exoelectron emission during the oxidation of Na films A. B6ttcher, R. Grobecker, T. Greber, A. Morgante * and G. Ertl Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-IO00 Berlin 33, Germany Received 22 July 1992; accepted for publication 16 September 1992 Oxidation of Na films is accompanied by a low yield of electron emission which is, however, confined to the later stages ot reaction in which transformation of peroxide (Na20 2) into superoxide (NaP 2) species at the surface takes place, By probing the electronic properties of the outermost layer by means of metastable deexcitation spectroscopy (MDS) and by recording the energy distribution of the emitted exoelectrons the mechanism of this process was found to be analogous to that established previously tbl the oxidation of Cs films and as proposed earlier theoretically. It involves decay of a hole state derived from the affinity level of the impinging 0 2 molecule in front of the surface via an Auger transition which, on the other hand, may efficiently be quenched by resonance ionisation from metallic electrons near the Fermi level, Experiments with Na submonolayers adsorbed on a Ru(i)001) substrate reveal that for coverages < 0.6 ML the latter effect dominates so strongly that exoelectron emission i~ t~o hmger observed.
Surface Science 280 (1993) 179-184 North-Holland
Surface chemistry and molecular reactions on KNbO 3 single crystal surfaces K. Szot 1, j. Keppels, W. Speier, K. Besocke 2 M. Teske 2 and W. Eberhardt IFF, Forschungszentrum Jiilich, Postfach 1913, D-517O Jiilich, Germany Received 23 June 1992; accepted for publication 18 August 1992 We have exposed the surface of KNbO 3 single crystals, cleaned in UHV by scraping, to a variety of simple molecules {H 2, O2, N2, CO, CO 2, H20, NH 3, NO 2, C2H 4, and (CH3)2CO). From photoemission and work function measurements we derive that out of these molecules only CO 2 and acetone ((CH3)2CO) react with the surface. Initially CO 2 reacts with the regions of the (I{X}} surface terminated by fragments of the KO(100)bulk plane, forming a surface carbonate. At high exposures, after the KO facets are saturated, CO2 dissociates at the NbO 2 terminated surface regions into O and CO. The oxygen atoms react with the Nb atom~ restoring the full bulk octahedral oxygen coordination of these atoms, while the CO fragments are desorbing.
Surface Science 280 (1993) 185-196 North-Holland
Phase diagram and phase transitions in the system a Monte Carlo study M. Sandhoff a, H. Pfniir b and H.-U. Everts a lnstitut fiir Theoretische Physik, Universitiit Hannover, Appelstrasse 2, D-3000 Hannover 1, Germany h lnstitut fiir Festk6rperphysik; Universitiit Hanno~er, Appelstrasse 2, D-3000 Hannover l, Germany Received 6 July 1992; accepted for publication 22 September 1992
A139 We study a lattice gas model with pair and three-body interactions up to the third neighbour shell on a triangular lattice by Monte Carlo methods to simulate the adsorbate system hydrogen/ruthenium(0001). This system has an asymmetric O-T phase diagram with respect to coverage 0 = 1/2. To get qualitative agreement we need only first and second neighbour pair interactions and one three-body term, while the quantitative agreement requires an additional three-body term. We investigate the nature of the order-disorder phase transitions by calculating critical exponents to compare our model with the experiment. Two continuous phase transitions and one weak first-order transition were found. The numerical results are accurate within 10% to the experimental data.
Surface Science 280 (1993) 197-207 North-Holland
Structural study, by surface differential diffraction of neutrons, of the surface of a nickel powder covered with deuterium J.P. Beaufils lnstitut Luue-LangeL~in, B.P. 156, 38042 Grenoble Cedex, France Received 15 May 1992; accepted for publication 10 September 1992 The analysis of the data of a neutron surface differential diffraction experiment, using H positions determined in the literature whenever available, supports the following model: Grains of the studied nickel powder are limited by (20 _+ 10)% of (111 ) faces and 80% of (110) faces. The (110) faces expose (111) and (111) facets because of anisotropic roughening. A new interpretation of the streaks observed in L E E D patterns and a discussion of the coverage provides additional arguments in favour of the roughening. On the facets of the rough faces, the D atoms are displaced from the position they would have on a genuine (111) face. They are no longer on a C3 axis. The Ni atoms of the rough faces suffer an average displacement of 0.015 _+ 0.015 nm in the [110] outward direction. The shortest N i - D distance on the rough face is equal to 0.19 + 0.015 rim.
Surface Science 280 (1993) 208-216 North-Holland
The production of heavy multiply charged ions in secondary ion mass spectrometry P.A.W. van der Heide, J.B. Metson, D.L. Tui Department of Chemistry, and Research Centre for Surface and Materials Science, Unit~ersity of Auckland, Pri~,ate Bag, Auckland, New Zealand
and W.M. Lau Surface Science Western, Unil~'ersity of Western Ontario, London, Ontario, Canada, N6A 3K7 Received 21 May 1992; accepted for publication 14 September 1992 In this study it was found that the multiply charged ion emissions (M 2+ and M 3+) of AI, Au, Ag, Cd, Co, Cu, Hg, In, Mo. Sc, Te. Ta, and V, arising from O b o m b a r d m e n t of metal surfaces, exhibited a great variety of energy distributions. This is attributed to the competing surface and gas phase ionization mechanisms. Investigations of the gas phase collision process were carried out by studying the primary ion current density effects on the intensities of AI "+, Ag ~+, Cd n+, and I n " - . (n = 1, 2, 3), the isotope fractionation exhibited by energy deficient Cu 2+ ions, and the image displacement of multiply charged ions with respect to intrinsically formed singly charged ions. These revealed that both doubly and triply charged ions from the heavier elements arc formed predominantly from gas phase collisions between the secondary neutral population and the secondary electron, or p r i m a ~ ion population, during single primary ion events. A kinematics study however, discounted the possibility of primary ion interaction. whether surface scattered or not.
Surface Science 280 (1993) 170-178 North-Holland
Exoelectron emission during the oxidation of Na films A. B6ttcher, R. Grobecker, T. Greber, A. Morgante * and G. Ertl Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-IO00 Berlin 33, Germany Received 22 July 1992; accepted for publication 16 September 1992 Oxidation of Na films is accompanied by a low yield of electron emission which is, however, confined to the later stages ot reaction in which transformation of peroxide (Na20 2) into superoxide (NaP 2) species at the surface takes place, By probing the electronic properties of the outermost layer by means of metastable deexcitation spectroscopy (MDS) and by recording the energy distribution of the emitted exoelectrons the mechanism of this process was found to be analogous to that established previously tbl the oxidation of Cs films and as proposed earlier theoretically. It involves decay of a hole state derived from the affinity level of the impinging 0 2 molecule in front of the surface via an Auger transition which, on the other hand, may efficiently be quenched by resonance ionisation from metallic electrons near the Fermi level, Experiments with Na submonolayers adsorbed on a Ru(i)001) substrate reveal that for coverages < 0.6 ML the latter effect dominates so strongly that exoelectron emission i~ t~o hmger observed.
Surface Science 280 (1993) 179-184 North-Holland
Surface chemistry and molecular reactions on KNbO 3 single crystal surfaces K. Szot 1, j. Keppels, W. Speier, K. Besocke 2 M. Teske 2 and W. Eberhardt IFF, Forschungszentrum Jiilich, Postfach 1913, D-517O Jiilich, Germany Received 23 June 1992; accepted for publication 18 August 1992 We have exposed the surface of KNbO 3 single crystals, cleaned in UHV by scraping, to a variety of simple molecules {H 2, O2, N2, CO, CO 2, H20, NH 3, NO 2, C2H 4, and (CH3)2CO). From photoemission and work function measurements we derive that out of these molecules only CO 2 and acetone ((CH3)2CO) react with the surface. Initially CO 2 reacts with the regions of the (I{X}} surface terminated by fragments of the KO(100)bulk plane, forming a surface carbonate. At high exposures, after the KO facets are saturated, CO2 dissociates at the NbO 2 terminated surface regions into O and CO. The oxygen atoms react with the Nb atom~ restoring the full bulk octahedral oxygen coordination of these atoms, while the CO fragments are desorbing.
Surface Science 280 (1993) 185-196 North-Holland
Phase diagram and phase transitions in the system a Monte Carlo study M. Sandhoff a, H. Pfniir b and H.-U. Everts a lnstitut fiir Theoretische Physik, Universitiit Hannover, Appelstrasse 2, D-3000 Hannover 1, Germany h lnstitut fiir Festk6rperphysik; Universitiit Hanno~er, Appelstrasse 2, D-3000 Hannover l, Germany Received 6 July 1992; accepted for publication 22 September 1992
A139 We study a lattice gas model with pair and three-body interactions up to the third neighbour shell on a triangular lattice by Monte Carlo methods to simulate the adsorbate system hydrogen/ruthenium(0001). This system has an asymmetric O-T phase diagram with respect to coverage 0 = 1/2. To get qualitative agreement we need only first and second neighbour pair interactions and one three-body term, while the quantitative agreement requires an additional three-body term. We investigate the nature of the order-disorder phase transitions by calculating critical exponents to compare our model with the experiment. Two continuous phase transitions and one weak first-order transition were found. The numerical results are accurate within 10% to the experimental data.
Surface Science 280 (1993) 197-207 North-Holland
Structural study, by surface differential diffraction of neutrons, of the surface of a nickel powder covered with deuterium J.P. Beaufils lnstitut Luue-LangeL~in, B.P. 156, 38042 Grenoble Cedex, France Received 15 May 1992; accepted for publication 10 September 1992 The analysis of the data of a neutron surface differential diffraction experiment, using H positions determined in the literature whenever available, supports the following model: Grains of the studied nickel powder are limited by (20 _+ 10)% of (111 ) faces and 80% of (110) faces. The (110) faces expose (111) and (111) facets because of anisotropic roughening. A new interpretation of the streaks observed in L E E D patterns and a discussion of the coverage provides additional arguments in favour of the roughening. On the facets of the rough faces, the D atoms are displaced from the position they would have on a genuine (111) face. They are no longer on a C3 axis. The Ni atoms of the rough faces suffer an average displacement of 0.015 _+ 0.015 nm in the [110] outward direction. The shortest N i - D distance on the rough face is equal to 0.19 + 0.015 rim.
Surface Science 280 (1993) 208-216 North-Holland
The production of heavy multiply charged ions in secondary ion mass spectrometry P.A.W. van der Heide, J.B. Metson, D.L. Tui Department of Chemistry, and Research Centre for Surface and Materials Science, Unit~ersity of Auckland, Pri~,ate Bag, Auckland, New Zealand
and W.M. Lau Surface Science Western, Unil~'ersity of Western Ontario, London, Ontario, Canada, N6A 3K7 Received 21 May 1992; accepted for publication 14 September 1992 In this study it was found that the multiply charged ion emissions (M 2+ and M 3+) of AI, Au, Ag, Cd, Co, Cu, Hg, In, Mo. Sc, Te. Ta, and V, arising from O b o m b a r d m e n t of metal surfaces, exhibited a great variety of energy distributions. This is attributed to the competing surface and gas phase ionization mechanisms. Investigations of the gas phase collision process were carried out by studying the primary ion current density effects on the intensities of AI "+, Ag ~+, Cd n+, and I n " - . (n = 1, 2, 3), the isotope fractionation exhibited by energy deficient Cu 2+ ions, and the image displacement of multiply charged ions with respect to intrinsically formed singly charged ions. These revealed that both doubly and triply charged ions from the heavier elements arc formed predominantly from gas phase collisions between the secondary neutral population and the secondary electron, or p r i m a ~ ion population, during single primary ion events. A kinematics study however, discounted the possibility of primary ion interaction. whether surface scattered or not.