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Quantum chemical studies of formate on Cu(100) and Cu(110)
A179 Surface Science 183 (1987) 449-468 North-Holland, Amsterdam QUANTUM CHEMICAL AND Cu(ll0) Jos6 A. R O D R I G U E Z
STUDIES
449 OF FORMATE
a n d C h a r l e s T. C A M P B E L L
ON Cu(100) *
Chemistry Department, Indiana University, Bloomington, IN 47405, USA Received 9 October 1986; accepted for publication 10 December 1986
The chemisorption of HCOO on Cu(100) and Cu(ll0) has been studied with semi-empirical quantum-mechanical calculations (INDO/S) of Cu clusters." The results indicate that the HCOO chemisorption bond is formed on both surfaces by net ~r-donation of electrons from formate to the metal, and net o-backdonation of electrons from the metal to HCOO. The total bond-order index between formate and the clusters is almost the same on both surfaces. Upon HCOO adsorption, it is found that the C - O bond-order index decreases and that the C - H bond is not affected (relative to the free formate radical). Peak assignments for the ultraviolet photoemission spectrum of HCOO on Cu(ll0) and of HCOO on Cu(100) are made based on these results. The possible effects of electronic changes in the Cu surface upon the formate species are analyzed in light of the molecular-orbital description of the chemisorption bond. Possible pathways for the dissociation of adsorbed formate are discussed.
Surface Science 183 (1987) 469-483 North-Holland, Amsterdam MNDO
ANALYSIS
P. B A D Z I A G
469
OF THE OXIDISED
and W.S. VERWOERD
DIAMOND
(100) S U R F A C E
*
Department of Physics, University of South Africa, PO Box 392, Pretoria, 0001 South Africa Received 28 August 1986; accepted for publication 10 December 1986
An analysis of the geometry and the electronic structure of oxidised diamond (100) surface based on the semiempirical quantum chemistry MNDO program is presented. Results show that the most stable configuration of the oxidised surface is one which removes the (2 × 1) reconstruction. There can, however, be a substantial energetic barrier preventing the oxidised surface from this configuration. With this in mind, possible ways of oxidisation and two other structures of oxidised surfaces are examined. The structures realise ratios of adsorbed oxygen atoms to surface carbon atoms 1 : 1 and 1 : 2, respectively, and both preserve a (2 × 1) reconstruction structure. The first one is likely to be a result of molecular oxidisation while the second one can be a result of atomic oxidisation of the clean surface. Energies of atomic desorption for these structures are 4.2 and 5.3 eV, respectively. The same quantity for the stable configuration is estimated to be 6.2 eV. Analysis of.the electronic structure of the surface done with the help of program POPLOC by Verwoerd [Chem. Phys. 44 (1979) 151] shows that in the stable configuration all the dangling bonds are saturated by oxygen while in the other two configurations carbon atoms form symmetric dimers with a stretched bond (1.67 ~,).
STUDIES
449 OF FORMATE
a n d C h a r l e s T. C A M P B E L L
ON Cu(100) *
Chemistry Department, Indiana University, Bloomington, IN 47405, USA Received 9 October 1986; accepted for publication 10 December 1986
The chemisorption of HCOO on Cu(100) and Cu(ll0) has been studied with semi-empirical quantum-mechanical calculations (INDO/S) of Cu clusters." The results indicate that the HCOO chemisorption bond is formed on both surfaces by net ~r-donation of electrons from formate to the metal, and net o-backdonation of electrons from the metal to HCOO. The total bond-order index between formate and the clusters is almost the same on both surfaces. Upon HCOO adsorption, it is found that the C - O bond-order index decreases and that the C - H bond is not affected (relative to the free formate radical). Peak assignments for the ultraviolet photoemission spectrum of HCOO on Cu(ll0) and of HCOO on Cu(100) are made based on these results. The possible effects of electronic changes in the Cu surface upon the formate species are analyzed in light of the molecular-orbital description of the chemisorption bond. Possible pathways for the dissociation of adsorbed formate are discussed.
Surface Science 183 (1987) 469-483 North-Holland, Amsterdam MNDO
ANALYSIS
P. B A D Z I A G
469
OF THE OXIDISED
and W.S. VERWOERD
DIAMOND
(100) S U R F A C E
*
Department of Physics, University of South Africa, PO Box 392, Pretoria, 0001 South Africa Received 28 August 1986; accepted for publication 10 December 1986
An analysis of the geometry and the electronic structure of oxidised diamond (100) surface based on the semiempirical quantum chemistry MNDO program is presented. Results show that the most stable configuration of the oxidised surface is one which removes the (2 × 1) reconstruction. There can, however, be a substantial energetic barrier preventing the oxidised surface from this configuration. With this in mind, possible ways of oxidisation and two other structures of oxidised surfaces are examined. The structures realise ratios of adsorbed oxygen atoms to surface carbon atoms 1 : 1 and 1 : 2, respectively, and both preserve a (2 × 1) reconstruction structure. The first one is likely to be a result of molecular oxidisation while the second one can be a result of atomic oxidisation of the clean surface. Energies of atomic desorption for these structures are 4.2 and 5.3 eV, respectively. The same quantity for the stable configuration is estimated to be 6.2 eV. Analysis of.the electronic structure of the surface done with the help of program POPLOC by Verwoerd [Chem. Phys. 44 (1979) 151] shows that in the stable configuration all the dangling bonds are saturated by oxygen while in the other two configurations carbon atoms form symmetric dimers with a stretched bond (1.67 ~,).