Alkali clusters, electron affinities

As mentioned in Section 3.4, clusters of metal atoms of varying sizes can be prepared. The presence of alkali atom clusters in the vapour phase is well documented. Such clusters have a much lower ionization energy than that of an isolated atom and also have a high electron affinity. The probability of electron transfer is therefore considerably greater in a metal cluster. It is indeed known in the case of caesium that as the density of caesium increases (from isolated atoms in a low-density gas to a liquid), larger clusters form and charge-transfer becomes increasingly favoured as the density... 

E. Eliav, M.J. Vilkas, Y. Ishikawa, U. Kaldor, Extrapolated intermediate Hamiltonian coupled-cluster approach Theory and pilot application to electron affinities of alkali atoms, J. Chem. Phys. 122 (22) (2005) 224113. 

Table A. Approximate Electron Affinities for Alkali Metal Clusters in eV...

The transition from the atom to the cluster to the bulk metal can best be understood in the alkali metals. For example, the ionization potential (IP) (and also the electron affinity (EA)) of sodium clusters Na must approach the metallic sodium work function in the limit N - . We previously displayed this (1) by showing these values from the beautiful experiments by Schumacher et al. (36, 37) (also described in this volume 38)) plotted versus N". The electron affinity values also shown are from (39), (40) and (34) for N = 1,2 and 3, respectively. A better plot still is versus the radius R of the N-mer, equivalent to a plot versus as shown in Figure 1. The slopes of the lines labelled "metal sphere" are slightly uncertain those shown are 4/3 times the slope of Wood ( j ) and assume a simple cubic lattice relation of R and N. It is clear that reasonably accurate interpolation between the bulk work function and the IP and EA values for small clusters is now possible. There are, of course, important quantum and statistical effects for small N, e.g. the trimer has an anomalously low IP and high EA, which can be readily understood in terms of molecular orbital theory (, ). The positive trimer ions may in fact be "ionization sinks" in alkali vapor discharges a possible explanation for the "violet bands" seen in sodium vapor (20) is the radiative recombination of Na. Csj may be the hypothetical negative ion corresponding to EA == 1.2 eV... 

It was mentioned in Sect. 4 that electronic-shell effects appear in the mass abundance [10,43], ionization potentials [88], and electron affinities [89] of noble metal clusters that are very similar to those observed for alkalis. These can be readily interpreted within the spherical jellium model if we treat the noble metal atoms as monovalent, that is, each atom contributes its external s-el tron only. Even more, odd-even effects are also observed for small N in the properties mentioned above, and have been explained by Penzar and Ekardt [32] within the context of the spheroidally deformed jellium model. 

From the odd-even effects observed in the ionization potential and electron affinity of small clusters (see Sect. 2.4) we can also expect odd-even oscillations in the reactivity. For instance, the ionization potential I(Np) of a small alkali cluster with an even number of atoms (Np) is larger than I(Np -i- 1) and I(Np — 1). At the same time the electron affinity A(Np) is smaller than A(Np -I- 1) and A(Np — 1). Consequently I(Np) —A(Np) will be larger than I(Np-I- 1) — A(Np -I- 1) or I(Np — 1) — A(Np — 1). In summary, the spin pairing effect induces odd-even oscillations in the reactivity. 

The ground-state properties of nonstoichiometrie X Y clusters (X = Na, Li, K and Y = Cl, F) with single and multiple excess electrons have been extensively studied experimentally [41-43] and theoretically [44-46] since they are good candidates for possible metal-insulator transitions and metallic-ionic segregation in finite systems. Hydrogenation of lithium clusters has been also investigated [47, 48]. It is of interest to establish similarities and differences among properties of alkali halide and alkali hydride clusters, since both bulk materials have a common structure but the electron affinities of F and H atoms are very different (3.4 versus 0.75 eV). The question can be raised to what extent these differences are reflected in properties of small finite systems. 

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