Ionization potential, metal clusters

Ionization potential, metal clusters, 36 144 Ionizing radiation see also Radiation... 

Work function of metals Ionization potential of clusters 1()2... 

For larger clusters much spectroscopic detail is lost, but measurements of the photoionization thresholds provide information concerning cluster ionization potentials. Sodium clusters show a relatively smooth decrease in ionization potential from 5.1 eV for the atom to 3.5 eV for the 14-atom cluster. This is still significantly above the 1.6-eV work function of bulk sodium. For the smaller clusters the odd sizes have a lower ionization potential than the neighboring even-sized clusters because of effects due to open versus closed shell configurations. Recently measurements have been made of the ionization potential of iron clusters up to 25 atoms. In this experiment the ionization potentials were bracketed by the use of various ionizing lasers. There is a decrease in the ionization potential from 7.870 eV for an iron atom to the 4.4-eV work function of the bulk metal, but the trend is by no means linear. Thus, the ionization potential of Fea is about 5.9 eV, while those of Fes and Fe4 are above 6.42 eV. Clusters in the range of 9-12 atoms have ionization potentials below... 

The spherical shell model can only account for tire major shell closings. For open shell clusters, ellipsoidal distortions occur [47], leading to subshell closings which account for the fine stmctures in figure C1.1.2(a ). The electron shell model is one of tire most successful models emerging from cluster physics. The electron shell effects are observed in many physical properties of tire simple metal clusters, including tlieir ionization potentials, electron affinities, polarizabilities and collective excitations [34]. 

Yang S and Knickelbein M B 1990 Photoionization studies of transition metal clusters ionization potentials for Fe... 

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]. 

Figure4.17 Ionization potentials (IP) and electron affinities (EA) of Group 11 clusters M up ton = 23 (in eV). The bulk metal work-functions for the (1 00) plane are also shown on the left hand side in open symbols. Experimental values from Refs. [370-374].

Cheeseman, M.A. and Eyler, J.R. (1992) Ionization potentials and reactivity of coinage metal clusters. The Journal of Physical Chemistry, 96, 1082-1087. 

Ionization potential of metal clusters is one of the factors affected by cluster size [33]. This study represents the most extensive effort so far to determine the size dependence of IP. The measurements on these clusters showed a decreasing IP with size with apparent oscillatory trend. Even-size particles had a relatively larger IP compared to their odd-size counterparts. The data show oscillatory behavior for small Na clusters with a loss of this oscillation for the larger Na clusters. The IP decreases with cluster size, but even at Nai4 the value 3.5 eV is far from... 

The recent interest in the exploration of electrocatalytic phenomena from first principles can be traced to the early cluster calculations of Anderson [1990] and Anderson and Debnath [1983]. These studies considered the interaction of adsorbates with model metal clusters and related the potential to the electronegativity determined as the average of the ionization potential and electron affinity—quantities that are easily obtained from molecular orbital calculations. In some iterations of this model, changes in reaction chemistry induced by the electrochemical environment... 

Photoionization ti me-of-fli ght mass spectrometry is almost exclusively the method used in chemical reaction studies. The mass spectrometers, detectors and electronics are almost identical. A major distinction is the choice of ionizing frequency and intensity. For many stable molecules multi photon ionization allowed for almost unit detection efficiency with controllable fragmentation(20). For cluster systems this has been more difficult because high laser intensities generally cause extensive dissociation of neutrals and ions(21). This has forced the use of single photon ionization. This works very well for low i oni zati on potential metals ( < 7.87 eV) if the intensity is kept fairly low. In fact for most systems the ionizing laser must be attenuated. A few very small... 

Lewis Bases. A variety of other ligands have been studied, but with only a few of the transition metals. There is still a lot of room for scoping work in this direction. Other reactant systems reported are ammoni a(2e), methanol (3h), and hydrogen sulfide(3b) with iron, and benzene with tungsten (Tf) and plati num(3a). In a qualitative sense all of these reactions appear to occur at, or near gas kinetic rates without distinct size selectivity. The ammonia chemisorbs on each collision with no size selective behavior. These complexes have lower ionization potential indicative of the donor type ligands. Saturation studies have indicated a variety of absorption sites on a single size cluster(51). 

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... 

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]. 

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. 

---