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Lateral interactions and azimuthal orientation of pure and coadsorbed benzene layers on Ni(111)
A591 Surface Science 258 (1991) 1-15 North-Holland
On the formation of mixed ordered structures in the coadsorption system benzene + NO on N i ( l l l ) P. Zebisch, W. Huber and H.-P. Steinriick Physik-Department E20, Technische Universith't Miinchen, D-8046 Garching, Germany Received 4 April 1991; accepted for publication 6 June 1991 The adsorption, desorption and ordering behavior of coadsorbed benzene + NO layers on Ni(111) have been studied by TPD and LEED. With increasing NO exposure dissociation of benzene upon heating is more and more suppressed and molecular benzene desorption is observed in a variety of peaks between 370 and 440 K which strongly depend on the initial benzene and NO coverages. Under certain conditions dissociation of benzene can be completely suppressed. The desorption behavior of NO is essentially unchanged as compared to NO desorption from the bare Ni(lll) surface. Depending on initial benzene and NO 'coverage four ordered benzene + NO structures have been observed: (2~/3 x 2Vr3-)R30o, (3 × 3), c(27~ × 4)rect and ( 2 ~ x 4,,/3-)R30 o. In all cases the coadsorbed layers were disordered after adsorption at 120 K and ordering was only observed after annealing to 250 K. The observed structures can be correlated to different molecular benzene desorption peaks. The formation of the various ordered benzene + NO structures and their correlation to the TPD spectra is discussed.
Surface Science 258 (1991) 16-22 North-Holland
Lateral interactions and azimuthal orientation of pure and coadsorbed benzene layers on N i ( l l l ) W. Huber, P. Zebisch, T Bornemann and H.-P. Steinriick Physik-Department E20, Technische Unicersitiit Miinchen, D-8046 Garchmg, Germany Received 14 January 1991; accepted for publication 10 June 1991 The adsorption of benzene and the coadsorption of benzene with electronegative coadsorbates on Ni(lll) have been studied by angle-resolved UPS using linearly polarized synchrotron radiation. The electronic structure of benzene coadsorbed with NO, CO and O in the ordered (2v/3- x 2Vff)R30° layers and for the coadsorbed (3 x 3) benzene + NO layer is similar to that of pure benzene layers. Uniform shifts of the various benzene levels to lower binding energy (0.2-0.3 eV) in the coadsorbed layers are attributed to electrostatic initial state effects. In all pure and coadsorbed layers studied, the benzene molecules are oriented with their molecular plane parallel to the surface with the molecular symmetry reduced to C3v. In the nonsaturated benzene layer and in the coadsorbed layers with (2v~ x 2v~)R30 o structure the molecules are azimuthally oriented with the hydrogens pointing along the [211] directions of the substrate. In the saturated pure (V~ × ~-)R19.1 ° layer and the (3 × 3) benzene + NO layer the molecules are azimuthally reorientated due to lateral interactions. The proposed azimuthal reorientations are in agreement with expectations from steric arguments. For the saturated pure (v0- × v0)R19.1 ° layer the 2alg levels exhibits a dispersion of 0.45 eV, also indicative of lateral interactions.
Surface Science 258 (1991) 23-34 North-Holland
Adsorption and reaction on oxide surfaces: NO, NO2 on C r 2 0 3 ( l l l ) / C r ( l 1 0 ) C. Xu, M. Hassel, H. Kuhlenbeck and H.-J. Freund Lehrstuhl fiir Physikalische Chemie I, Ruhr-Unicersitiit Bochum, Unicersitiitsstrasse 150, D-4630 Bochum 1, Germany Received 18 March 1991: accepted for publication 30 April 1991 We report results of electron spectroscopic measurements, i.e., LEED, EELS, ARUPS, XPS, and NEXAFS on NO, and NO 2 adsorbed on a thin Cr20.~ film with (111) orientation grown on top of a Cr(ll0) single-crystal surface via an oxidation procedure. It is shown that the Cr203(111) surface is likely to consist of Cr-terminated and O-terminated terraces and that the Cr-atoms loca[ed within the oxide surface are in oxidation states different from the bulk. Our results indicate that these sites are involved in the dissociation of NO 2 at rather low temperature to yield adsorbed NO and adsorbed oxygen.
On the formation of mixed ordered structures in the coadsorption system benzene + NO on N i ( l l l ) P. Zebisch, W. Huber and H.-P. Steinriick Physik-Department E20, Technische Universith't Miinchen, D-8046 Garching, Germany Received 4 April 1991; accepted for publication 6 June 1991 The adsorption, desorption and ordering behavior of coadsorbed benzene + NO layers on Ni(111) have been studied by TPD and LEED. With increasing NO exposure dissociation of benzene upon heating is more and more suppressed and molecular benzene desorption is observed in a variety of peaks between 370 and 440 K which strongly depend on the initial benzene and NO coverages. Under certain conditions dissociation of benzene can be completely suppressed. The desorption behavior of NO is essentially unchanged as compared to NO desorption from the bare Ni(lll) surface. Depending on initial benzene and NO 'coverage four ordered benzene + NO structures have been observed: (2~/3 x 2Vr3-)R30o, (3 × 3), c(27~ × 4)rect and ( 2 ~ x 4,,/3-)R30 o. In all cases the coadsorbed layers were disordered after adsorption at 120 K and ordering was only observed after annealing to 250 K. The observed structures can be correlated to different molecular benzene desorption peaks. The formation of the various ordered benzene + NO structures and their correlation to the TPD spectra is discussed.
Surface Science 258 (1991) 16-22 North-Holland
Lateral interactions and azimuthal orientation of pure and coadsorbed benzene layers on N i ( l l l ) W. Huber, P. Zebisch, T Bornemann and H.-P. Steinriick Physik-Department E20, Technische Unicersitiit Miinchen, D-8046 Garchmg, Germany Received 14 January 1991; accepted for publication 10 June 1991 The adsorption of benzene and the coadsorption of benzene with electronegative coadsorbates on Ni(lll) have been studied by angle-resolved UPS using linearly polarized synchrotron radiation. The electronic structure of benzene coadsorbed with NO, CO and O in the ordered (2v/3- x 2Vff)R30° layers and for the coadsorbed (3 x 3) benzene + NO layer is similar to that of pure benzene layers. Uniform shifts of the various benzene levels to lower binding energy (0.2-0.3 eV) in the coadsorbed layers are attributed to electrostatic initial state effects. In all pure and coadsorbed layers studied, the benzene molecules are oriented with their molecular plane parallel to the surface with the molecular symmetry reduced to C3v. In the nonsaturated benzene layer and in the coadsorbed layers with (2v~ x 2v~)R30 o structure the molecules are azimuthally oriented with the hydrogens pointing along the [211] directions of the substrate. In the saturated pure (V~ × ~-)R19.1 ° layer and the (3 × 3) benzene + NO layer the molecules are azimuthally reorientated due to lateral interactions. The proposed azimuthal reorientations are in agreement with expectations from steric arguments. For the saturated pure (v0- × v0)R19.1 ° layer the 2alg levels exhibits a dispersion of 0.45 eV, also indicative of lateral interactions.
Surface Science 258 (1991) 23-34 North-Holland
Adsorption and reaction on oxide surfaces: NO, NO2 on C r 2 0 3 ( l l l ) / C r ( l 1 0 ) C. Xu, M. Hassel, H. Kuhlenbeck and H.-J. Freund Lehrstuhl fiir Physikalische Chemie I, Ruhr-Unicersitiit Bochum, Unicersitiitsstrasse 150, D-4630 Bochum 1, Germany Received 18 March 1991: accepted for publication 30 April 1991 We report results of electron spectroscopic measurements, i.e., LEED, EELS, ARUPS, XPS, and NEXAFS on NO, and NO 2 adsorbed on a thin Cr20.~ film with (111) orientation grown on top of a Cr(ll0) single-crystal surface via an oxidation procedure. It is shown that the Cr203(111) surface is likely to consist of Cr-terminated and O-terminated terraces and that the Cr-atoms loca[ed within the oxide surface are in oxidation states different from the bulk. Our results indicate that these sites are involved in the dissociation of NO 2 at rather low temperature to yield adsorbed NO and adsorbed oxygen.