Observation of earlier two-to-three dimensional structural transition in gold cluster anions by isoelectronic substitution: MAu(n)(-) (n=8-11; M=Ag,Cu).

The effects of isoelectronic substitution on the electronic and structural properties of gold clusters are investigated in the critical size range of the two-dimensional (2D)-three-dimensional (3D) structural transition (MAu(n)(-), n=8-11; M=Ag,Cu) using photoelectron spectroscopy and density functional calculations. Photoelectron spectra of MAu(n)(-) are found to be similar to those of the bare gold clusters Au(n+1)(-), indicating that substitution of a Au atom by a Ag or Cu atom does not significantly alter the geometric and electronic structures of the clusters. The only exception occurs at n=10, where very different spectra are observed for MAu(10)(-) from Au(11)(-), suggesting a major structural change in the doped clusters. Our calculations confirm that MAu(8)(-) possesses the same structure as Au(9)(-) with Ag or Cu simply replacing one Au atom in its C(2v) planar global minimum structure. Two close-lying substitution isomers are observed, one involves the replacement of a center Au atom and another one involves an edge site. For Au(10)(-) we identify three coexisting low-lying planar isomers along with the D(3h) global minimum. The coexistence of so many low-lying isomers for the small-sized gold cluster Au(10)(-) is quite unprecedented. Similar planar structures and isomeric forms are observed for the doped MAu(9)(-) clusters. Although the global minimum of Au(11)(-) is planar, our calculations suggest that only simulated spectra of 3D structures agree with the observed spectra for MAu(10)(-). For MAu(11)(-), only a 3D isomer is observed, in contrast to Au(12)(-) which is the critical size for the 2D-3D structural transition with both the 2D and 3D isomers coexisting. The current work shows that structural perturbations due to even isoelectronic substitution of a single Au atom shift the 2D to 3D structural transition of gold clusters to a smaller size.