Structural Evolution of Gold Clusters Au_<i>n</i>^– (<i>n</i> = 21–25) Revisited

We performed a combined theoretical and experimental photoelectron spectroscopy study of the structural evolution of gold anion clusters Au_<i>n</i>^– in the size range <i>n</i> = 21–25, a special size range for gold anion clusters where extensive structural changes from the pyramidal structure at Au₂₀^– toward the core–shell structure at Au₂₆^– were expected to occur. Density functional theory calculations with inclusion of spin–orbit effects were employed to produce the simulated spectra for the selected low-energy isomers obtained from basin-hopping global minimum search. The comparison of these simulated spectra with reasonably well-resolved experimental photoelectron spectra resulted in the identification of the low-lying structures of the gold clusters. The <i>fused-planar</i> and <i>hollow-tubular</i> structures are found dominant in this special size range. The highly stable tetrahedral Au₂₀ unit (viewed as the fragment of face-centered cubic (FCC) bulk gold) was found intact only in the minor isomer at <i>n</i> = 21, whereas hollow-tubular structures were found prevalent in the <i>n</i> = 22–25 range. At <i>n</i> = 25, the dominant structure is a hollow-tubular one with two of gold pyramids fused together, but not a core–shell one as previously believed