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Figure 2.6 Reaction between two Na₈ clusters in vacuum. (a) Time evolution of the potential energy relative to its value in the initial configuration (solid curve, scale on the left) and the center‐of‐mass distance of the two clusters (dotted curve, scale on the right). The two snapshots indicate the initial configuration (left) and the configuration at 2.6 ps (right). (b) Time evolution of the Kohn‐Sham eigen values. The dotted curves indicate empty states.

      Source: Hakkinen and Manninen [24]. © American Physical Society.

While Na₈ was found to see its geometry destroyed when interacting with another Na₈ cluster or when supported on a Na (100) surface, the result for Au₂₀ is different. Note that according to the jellium model, Au₂₀ is also a closed shell cluster. Although it has a pyramidal geometry (Figure 2.8), its molecular orbitals show the same pattern as in the jellium model [25]. As shown in Figure 2.9, Au₂₀ maintains its pyramidal structure when deposited on a carbon substrate [26]. However, it is not clear if Au₂₀ would continue to maintain its gas phase geometry when interacting with each other or when supported on an Au substrate? No studies have yet been done to make any conclusion. However, based on extensive studies of Al₁₃, another free electron meal cluster [27–36], it is unlikely that Au₂₀ would maintain its virgin geometry in the above situation.

With 39 valence electrons and an electronic configuration of 1S² 1P⁶ 1D¹⁰ 2S² 1F¹⁴ 2P⁵, Al₁₃ is known to mimic the chemistry of a halogen atom. Indeed, its electron affinity of 3.57 eV is almost identical to that of the Cl atom. It was theoretically predicted [30] and experimentally verified [31] that KAl₁₃ is an ionically bonded cluster where an electron is transferred from K to Al₁₃. Evidence that Al₁₃ behaves like a halogen also came from an experiment of Bergeron et al. who showed that Al₁₃I₂ ⁻ can be viewed as Al₁₃ ⁻.2I, making it look like a triiodide (I₃ ⁻) ion [32]. Similarly, Al₁₄I₃ ⁻ can be viewed as Al₁₄ ²⁺.3I⁻ with Al₁₄ behaving like an alkaline earth element. From the above results, the authors concluded that Al₁₃ and Al₁₄ exhibit a new form of superatom chemistry in which superatoms behave like atoms when they react with other atoms/molecules. However, a different conclusion was reached by Han and Jung who examined whether Al _n clusters exhibit multiple atomic characteristics depending upon n by studying halogenated Al _n (n = 11–15) complexes and plotted the charge (Q) distribution in MX and MX₂ systems (M = Al₁₁–Al₁₅, X = F, Cl, Br, I) vs electronegativity, η of X [33, 34]. The results are presented in Figure 2.10. Noting that the charge transfer Q(M) is nearly independent of n in Al _n clusters in both the systems, the authors concluded that “there is no evidence of an alkaline earth superatom in the Al₁₄ clusters” and that “there are no theoretical grounds to regard Al₁₃I₂ ⁻ as Al₁₃ ⁻.2I.”

Figure 2.7 The initial (left) and the final (right) configuration of the collapse of Na₈ on Na (100). Note the epitaxial arrangement of the adatoms at the end of the run (at 2.8 ps). Both side and top views of the two configurations are shown.

      Source: Hakkinen and Manninen [24]. © American Physical Society.

Figure 2.8 (a) Structure and super atomic‐molecule models of Au₂₀ (TAu4). (b) Schematic representation for the superatom−atom D³S−s bonding of Au₂₀ (TAu4).

      Source: Cheng et al. [25]. © Royal Society of Chemistry.

Figure 2.9 Direct atomic imaging and dynamical fluctuations of the tetrahedral Au20 cluster soft‐landed on amorphous carbon substrate.

      Source: Adapted with permission from Wang et al. [26]. © Royal Society of Chemistry.

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