Orbital interactions and chemical reactivity of metal particles and metal surfaces

A review is presented with 101 refs. on chem. bonding to metal surfaces and small metal particles demonstrating the power of symmetry concepts to predict changes in chem. bonding. Ab-initio calcns. of chemisorption to small particles, as well as semiempirical extended Hueckel calcns. applied to the study of the reactivity of metal slabs are reviewed. On small metal particles, classical notions of electron promotion and hybridization are found to apply. The surroundings of a metal atom (ligands in complexes, other metal atoms at surfaces), affect bonding and reactivity through the prehybridization they induce. A factor specific for large particles and surfaces is the required localization of electrons on the atoms involved in the metal surface bond. At the surface, the bond energy is found to relate to the grou8p orbital local d. of states at the Fermi level. The use of this concept is extensively discussed and illustrated for chemisorption of CO and dissocn. of NO on metal surfaces. A discussion is given of the current decompn. schemes of bond energies and related concepts (exchange (Pauli)-repulsion, polarization, charge transfer). The role of non-orthogonality of fragment orbitals and of kinetic and potential energy for Pauli repulsion and (orbital) polarization is analyzed. Numerous examples are discussed to demonstrate the impact of those concepts on chem. bonding theor

An annealing study of the R1 EPR centre (the nearest-neighbour di-<100>-split self-interstitial) in diamond

Results are reported of both isochronal and isothermal annealing studies of the R1 EPR centre (known to be a pair of parallel nearest-neighbouring -split self-interstitials) produced by 2 MeV electron irradiation of synthetic type IIa diamonds of very low defect concentration before irradiation. The annealing out of R1 is not associated with any overall change in the concentration of isolated vacancies or interstitials. The isothermal decay corresponds to a first-order process with a rate constant conforming to an Arrhenius relationship with activation energy 0.6(1) eV and an unusually low attempt frequency. That the activation energy is much lower than the migration energy of isolated interstitials (1.68(15) eV) is taken to indicate that it corresponds to a combination of the energy required to excite R1 into an S = 0 state and that required to translate one of the split interstitials over the potential barrier into the next lattice site. This process may be linked to an associated rise in the concentration of the 3H (2.462 eV) optical defect; but the precise relationship between the diamagnetic site formed by the decay of R1 and the 3H optical centre is not revealed by this study

An annealing study of the R1 EPR centre (the nearest-neighbour di-&lt;100&gt;-split self-interstitial) in diamond

Results are reported of both isochronal and isothermal annealing studies of the R1 EPR centre (known to be a pair of parallel nearest-neighbouring &lt;100&gt;-split self-interstitials) produced by 2 MeV electron irradiation of synthetic type IIa diamonds of very low defect concentration before irradiation. The annealing out of R1 is not associated with any overall change in the concentration of isolated vacancies or interstitials. The isothermal decay corresponds to a first-order process with a rate constant conforming to an Arrhenius relationship with activation energy 0.6(1) eV and an unusually low attempt frequency. That the activation energy is much lower than the migration energy of isolated interstitials (1.68(15) eV) is taken to indicate that it corresponds to a combination of the energy required to excite R1 into an S = 0 state and that required to translate one of the split interstitials over the potential barrier into the next lattice site. This process may be linked to an associated rise in the concentration of the 3H (2.462 eV) optical defect; but the precise relationship between the diamagnetic site formed by the decay of R1 and the 3H optical centre is not revealed by this study

An annealing study of the R1 EPR centre (the nearest-neighbour di-&lt;100&gt;-split self-interstitial) in diamond

Results are reported of both isochronal and isothermal annealing studies of the R1 EPR centre (known to be a pair of parallel nearest-neighbouring &lt;100&gt;-split self-interstitials) produced by 2 MeV electron irradiation of synthetic type IIa diamonds of very low defect concentration before irradiation. The annealing out of R1 is not associated with any overall change in the concentration of isolated vacancies or interstitials. The isothermal decay corresponds to a first-order process with a rate constant conforming to an Arrhenius relationship with activation energy 0.6(1) eV and an unusually low attempt frequency. That the activation energy is much lower than the migration energy of isolated interstitials (1.68(15) eV) is taken to indicate that it corresponds to a combination of the energy required to excite R1 into an S = 0 state and that required to translate one of the split interstitials over the potential barrier into the next lattice site. This process may be linked to an associated rise in the concentration of the 3H (2.462 eV) optical defect; but the precise relationship between the diamagnetic site formed by the decay of R1 and the 3H optical centre is not revealed by this study

Magnetic resonance studies of irradiation damage defects in diamond: The effect of light on the neutral and negatively charged vacancy and the Di-&lt;100&gt;-split interstitial

New measurements on the R1 intrinsic radiation damage centre in diamond, recently identified as a di-[100]-split interstitial (DSI), indicate that (a) it can be spin polarised by radiation of energy greater than 1.7 eV, and (b) it is destroyed by annealing at about 600K with an associated transfer of an electron to a neutral vacancy, producing a temporary increase in the density of negatively charged vacancies

Electron paramagnetic resonance (EPR) and optical absorption studies of defects created in diamond by electron irradiation damage at 100 and 350 K

We present a study, using electron paramagnetic resonance (EPR) and optical absorption spectroscopies, of high purity synthetic type IIa diamonds, which have been irradiated with 2 MeV electrons in a specially developed dewar; allowing irradiation at a measured sample temperature down to 100 K, at doses of 2 x 10(17) --&gt; 4 x 10(18)e(-) cm(-2). The production rate of vacancies (1.53(10) cm(-1)) was the same for irradiation at 100 as at 350 K; as was the production rate of the EPR centre R1, known to be two nearest-neighbour (0 0 1)-split interstitials (0.014(4) cm(-1)). However, the production rate of the (0 0 1)-split self-interstitial (R2 EPR centre) is 1.1(1) cm(-1) at 100 K and only 0.10(5) cm(-1) at 350 K. That R1 is created at 100 K indicates that the self-interstitial is mobile under these conditions of irradiation. Production rates have also been measured for the R3 and R14 EPR centres and a new centre, labelled O3. (C) 1999 Elsevier Science B.V. All rights reserved