Transition metal properties ionization potentials

Besides these many cluster studies, it is currently not knovm at what approximate cluster size the metallic state is reached, or when the transition occurs to solid-statelike properties. As an example. Figure 4.17 shows the dependence of the ionization potential and electron affinity on the cluster size for the Group 11 metals. We see a typical odd-even oscillation for the open/closed shell cases. Note that the work-function for Au is still 2 eV below the ionization potential of AU24. Another interesting fact is that the Au ionization potentials are about 2 eV higher than the corresponding CUn and Ag values up to the bulk, which has been shown to be a relativistic effect [334]. A similar situation is found for the Group 11 cluster electron affinities [334]. 

The electron density centered at M is the only central contributor at the nuclear position M, as in this case the nucleus coincides with the field point P, which is excluded from the integrals. For transition metal atoms, the central contributions are the largest contributors to the properties at the nuclear position, which can be compared directly with results from other experimental methods. The electric field gradient at the nucleus, for instance, can be measured very accurately for certain nuclei with nuclear quadrupole resonance and/or Mdssbauer spectroscopic methods, while the electrostatic potential at the nucleus is related to the inner-shell ionization energies of atoms, which are accessible by photoelectron and X-ray spectroscopic methods. 

It is commonly accepted that chemisorption of CO on transition metals takes place in a way that is quite similar to bond formation in metal carbonyls (4). First experimental evidence for this assumption was obtained from a comparison of the C—O stretching frequencies (5) and was later confirmed by data on the bond strength (6) as well as by valence and core level ionization potentials obtained by photoelectron spectroscopy (7). Recent investigations have in fact shown that polynuclear carbonyl compounds with more than about 3-4 metal atoms exhibit electronic properties that are practically identical to those of corresponding CO chemisorption systems (8, 9), thus supporting the idea that the bond is relatively strongly localized to a small number of metal atoms forming the chemisorption site. 

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. 

In view of the fact that the probability of the molecular electron shell restructuring is due solely to the atom nearest to the radioactive one, in Fig. 3 we show the dependence of the probability (of remaining in the ground state) on the ionization potential /f for the shell of the atom nearest to He which is most distorted by the transition T - He+. As follows from Fig. 3, for metal hydrides there is a linear dependence on the potential For nonmetals (C, O, or F) the probability is independent of Apparently, the obtained dependence on /, is an illustration of the degree of ionicity of the chemical bond. As a whole, the problem of the influence of properties of an immediate neighbor of the radioactive atom on the /1-decay-induced excitations of the molecule requires further study. 

A systematic study of the first ionization potentials of Zn clusters as a function of cluster size with up to 43 atoms reveals that at least 20 transition metals are needed to approach surface (bulk) electronic structure properties. In addition, for the first time LSDF theory predicts a stable Zn dimer with a realistic bond length and binding energy. 

The problem may not be as severe for the/-shell metals particularly in the 4f series. The / electrons (in most cases n — 3 electrons in the Fn column for 4 < < 17) are so well localized that they may be treated as core electrons. The corresponding values were calculated from the ionization potentials of the atoms (see Problem 16-2,c), and are listed in the Solid State Table. Then the bonding properties of the /-shell metals can be treated exactly as the simple metals (or as beginning transition series if the effects of d states are sufficiently large to make that necessary) and effects of the /-shell electrons (such as the magnetic properties discussed in Section 20-F) can be treated separately. As an example, the equilibrium spacing of the rare earths is discussed in Problem 20-2, in which any d-state effects are ignored. This is a rather crude approximation, since with three non-/ electrons there is always some occupation of /-like bands. 

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