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Clusters and surface processes: Electron states in medium-nuclearity transition-metal cluster carbonyls
A426 Surface Science 189/190 (1987) 583-589 North-Holland, Amsterdam
583
CLUSTERS AND SURFACE PROCESSES: ELECTRON STATES IN MEDIUM-NUCLEARITY
TRANSITION-METAL
CLUSTER CARBONYLS D.W.
BULLETT
Sehool of Physics, Unieersi(v of Bath, Bath BA2 7A Y, UK Received 26 March 1987; accepted for publication 8 May 1987 Studies of carbonyl clusters provide a probe of the evolution of electronic structure from the small molecule towards the chemisorbed overlayer on a macroscopic metal surface. Apart from forming useful signposts towards the surface science of the bulk metal, such clusters also have potential catalytic applications in their own right. Modern computational methods permit for the first time reliable non-empirical calculations of electronic structure for clusters containing up to ten or more transition-metal atoms. Calculations for the family of osmium carbonyls show that these molecules adopt geometries which induce substantial energy gaps Eg between the highest filled and lowest unoccupied molecular levels. Eg tends to decrease as the size of the cluster increases or as its symmetry decreases, and beyond a critical size of ten Os atoms the clusters tend to switch to a high-spin configuration. The results provide a unifying interpretation of known optical, magnetic and photoemission properties within this family of compounds.
590
Surface Science 189/190 (1987) 590-604 North-Holland, A m s t e r d a m
FINE S T R U C T U R E IN E L E C T R O N E N E R G Y - L O S S A N D AUGER SPECTRA Applications to the local geometry determination of surfaces and interfaces J. D E R R I E N ,
E. C H A ] N E T ,
M, DE CRESCENZI
* and C. NOGUERA
**
Centre National de la Recherche Scientifique, Laboratoire d'Etudes des Propridt~s Electroniques des So[ides associ~ a l'Universitd Scientifique, Technologique et M~dieale de Grenoble, B.P. 166, 38042 Grenoble Cedex, France Received 24 March 1987; accepted for publication 4 May 1987 We review recent and main contributions to the extended fine structures observed in reflection mode electron energy-loss spectra. Those structures allow the local g e o m e t ~ determination of surfaces and interfaces like the well-known surface extended X-ray absorption fine structure technique (SEXAFS) usually performed with synchrotron facilities. The physical principles of the surface extended electron energy-loss fine structure (SEELFS) technique, its capabilities and limitations will be discussed. Several examples will be given and particular attention will be paid to the local order of clean surface, adsorbate monolayer on clean surface, initial stages of metal-semiconductor interface formation and metallic cluster growth on solid substrate. Furthermore, we obtain evidence of extended fine structures observed above core valence-valence Auger transitions in Auger spectra of several materials. We tentatively suggest their underlying physical origins and analyse these oscillating structures following the same standard EXAFS formalism. The lattice parameters deduced from these extended fine Auger structures (EXFAS) are in good agreement with those deduced from synchrotron radiation EXAFS data, conferring therefore to the conventional Auger technique new capabilities as a surface sensitive local order probe.
583
CLUSTERS AND SURFACE PROCESSES: ELECTRON STATES IN MEDIUM-NUCLEARITY
TRANSITION-METAL
CLUSTER CARBONYLS D.W.
BULLETT
Sehool of Physics, Unieersi(v of Bath, Bath BA2 7A Y, UK Received 26 March 1987; accepted for publication 8 May 1987 Studies of carbonyl clusters provide a probe of the evolution of electronic structure from the small molecule towards the chemisorbed overlayer on a macroscopic metal surface. Apart from forming useful signposts towards the surface science of the bulk metal, such clusters also have potential catalytic applications in their own right. Modern computational methods permit for the first time reliable non-empirical calculations of electronic structure for clusters containing up to ten or more transition-metal atoms. Calculations for the family of osmium carbonyls show that these molecules adopt geometries which induce substantial energy gaps Eg between the highest filled and lowest unoccupied molecular levels. Eg tends to decrease as the size of the cluster increases or as its symmetry decreases, and beyond a critical size of ten Os atoms the clusters tend to switch to a high-spin configuration. The results provide a unifying interpretation of known optical, magnetic and photoemission properties within this family of compounds.
590
Surface Science 189/190 (1987) 590-604 North-Holland, A m s t e r d a m
FINE S T R U C T U R E IN E L E C T R O N E N E R G Y - L O S S A N D AUGER SPECTRA Applications to the local geometry determination of surfaces and interfaces J. D E R R I E N ,
E. C H A ] N E T ,
M, DE CRESCENZI
* and C. NOGUERA
**
Centre National de la Recherche Scientifique, Laboratoire d'Etudes des Propridt~s Electroniques des So[ides associ~ a l'Universitd Scientifique, Technologique et M~dieale de Grenoble, B.P. 166, 38042 Grenoble Cedex, France Received 24 March 1987; accepted for publication 4 May 1987 We review recent and main contributions to the extended fine structures observed in reflection mode electron energy-loss spectra. Those structures allow the local g e o m e t ~ determination of surfaces and interfaces like the well-known surface extended X-ray absorption fine structure technique (SEXAFS) usually performed with synchrotron facilities. The physical principles of the surface extended electron energy-loss fine structure (SEELFS) technique, its capabilities and limitations will be discussed. Several examples will be given and particular attention will be paid to the local order of clean surface, adsorbate monolayer on clean surface, initial stages of metal-semiconductor interface formation and metallic cluster growth on solid substrate. Furthermore, we obtain evidence of extended fine structures observed above core valence-valence Auger transitions in Auger spectra of several materials. We tentatively suggest their underlying physical origins and analyse these oscillating structures following the same standard EXAFS formalism. The lattice parameters deduced from these extended fine Auger structures (EXFAS) are in good agreement with those deduced from synchrotron radiation EXAFS data, conferring therefore to the conventional Auger technique new capabilities as a surface sensitive local order probe.