Ionization potential, clusters atoms

Figure Cl. 1.3 shows a plot of tire chemical reactivity of small Fe, Co and Ni clusters witli FI2 as a function of size (full curves) [53]. The reactivity changes by several orders of magnitudes simply by changing tire cluster size by one atom. Botli geometrical and electronic arguments have been put fortli to explain such reactivity changes. It is found tliat tire reactivity correlates witli tire difference between tire ionization potential (IP) and tire electron affinity...

The ionization potential (7.9 eV) falls right outside the bracket of experimental IP s reported for carbon clusters with 40 to 100 atoms (6.42 eV IP 7.87 eV, Ref. 11). Inclusion of correlation effects will lower the calculated ASCF IP by 0.25 to 0.50 eV, so that the corrected IP will be at the upper end of the experimental IP>bracket. Due to the diffuseness of the n orbital from which an electron is removed, the correlation error in the ASCF value will be smaller than in cases where an electron is removed from a well localized bond. In these cases a correction of 1 eV is usually applied. 

In our third example (52), dissociative chemisorption of Li2, B2, C2, 02, N2, F2, CO, NO and ethylene on (100)W and Ni surfaces was examined. The metal surfaces are represented by means of nine-atom clusters, arranged as in Fig. 35. Experimental geometry was used for the adsorbates. The standard EHT method was used, i.e. with charge-independent atomic ionization potentials. Charge transfer between adsorbate and surface was explored... 

Energy levels of heavy and super-heavy (Z>100) elements are calculated by the relativistic coupled cluster method. The method starts from the four-component solutions of the Dirac-Fock or Dirac-Fock-Breit equations, and correlates them by the coupled-cluster approach. Simultaneous inclusion of relativistic terms in the Hamiltonian (to order o , where a is the fine-structure constant) and correlation effects (all products smd powers of single and double virtual excitations) is achieved. The Fock-space coupled-cluster method yields directly transition energies (ionization potentials, excitation energies, electron affinities). Results are in good agreement (usually better than 0.1 eV) with known experimental values. Properties of superheavy atoms which are not known experimentally can be predicted. Examples include the nature of the ground states of elements 104 md 111. Molecular applications are also presented. 

In recent publications [120, 121, 122,123] it has been shown that both the ionization potentials and the optical properties of bare and uncharged mercury clusters in a molecular beam experiment demonstrate a gradual size dependent evolution of metallic properties, starting at about 13 atoms and already bulklike at about 70 atoms. It has been predicted theoretically [124] that plasmons should begin to develop for such mercury clusters at about Hgi5. We should keep this in mind in the discussion of the electronic properties of AU55. 

It was also observed, in 1973, that the fast reduction of Cu ions by solvated electrons in liquid ammonia did not yield the metal and that, instead, molecular hydrogen was evolved [11]. These results were explained by assigning to the quasi-atomic state of the nascent metal, specific thermodynamical properties distinct from those of the bulk metal, which is stable under the same conditions. This concept implied that, as soon as formed, atoms and small clusters of a metal, even a noble metal, may exhibit much stronger reducing properties than the bulk metal, and may be spontaneously corroded by the solvent with simultaneous hydrogen evolution. It also implied that for a given metal the thermodynamics depended on the particle nuclearity (number of atoms reduced per particle), and it therefore provided a rationalized interpretation of other previous data [7,9,10]. Furthermore, experiments on the photoionization of silver atoms in solution demonstrated that their ionization potential was much lower than that of the bulk metal [12]. Moreover, it was shown that the redox potential of isolated silver atoms in water must... 

Fig. 6 compares the nuclearity effect on the redox potentials [19,31,63] of hydrated Ag+ clusters E°(Ag /Ag )aq together with the effect on ionization potentials IPg (Ag ) of bare silver clusters in the gas phase [67,68]. The asymptotic value of the redox potential is reached at the nuclearity around n = 500 (diameter == 2 nm), which thus represents, for the system, the transition between the mesoscopic and the macroscopic phase of the bulk metal. The density of values available so far is not sufficient to prove the existence of odd-even oscillations as for IPg. However, it is obvious from this figure that the variation of E° and IPg do exhibit opposite trends vs. n, for the solution (Table 5) and the gas phase, respectively. The difference between ionization potentials of bare and solvated clusters decreases with increasing n as which corresponds fairly well to the solvation free energy of the cation deduced from the Born solvation model [45] (for the single atom, the difference of 5 eV represents the solvation energy of the silver cation) [31]. 

An S or Au atom adsorbed at a positive kink site would promote electron capture. The presence of S reduces the ionization potential of odd-sized Ag clusters, but has the reverse effect on even-sized clusters. 

Ionization Potentials. Ionization potentials have been measured for small alkali metal clusters183-185 as a function of cluster size. The ionization potential decreases with cluster size (approximately 5eV at M2 down to 3.5 eV at M14) but even for 14 atom clusters the value is far above the 2.3 eV found for the work function of bulk metal. 

Lithium clusters have been a popular model for the calculation of metal properties because of their low atomic number. Lasarov and Markov (49) used a Hiickel procedure to determine the properties of a 48-atom Li crystal. They found a transition to metal properties with the binding energy per atom approaching 1.8 eV at 30 atoms. The ionization potential approached the bulk value since some electrons occupy antibonding molecular orbitals, as observed for Ag clusters. The calculated properties of the largest cluster were not those of a bulk metal. 

The electronic properties of Ag cluster adsorbed on a C model have been examined by Baetzold (60). In this CNDO calculation, properties of interest included bond energies, ionization potential, and charge transfer with the substrate. A 10-atom C model consisting of fused hexagons was employed as the substrate. The 2 s and 2p orbitals of C are included in this calculation as well as the 4d, 5s, and 5p orbitals of Ag. 

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