Photoelectron Imaging Spectroscopy and Density Functional Theory Calculations of Al-Doped Silver Cluster Anions:...

We report a joint study of photoelectron imaging spectroscopy and density functional theory (DFT) calculation for aluminum-doped silver cluster anions, Ag_nAl^- (n = 4-17). To explore their electronic and geometric structures, we measured size-dependent photoelectron spectra at a photon energy of 3.87 eV and compared them with simulated spectra for DFT-optimized structures. It is found that the Al atom is exposed on the cluster surface at 4 ≤ n ≤ 15, whereas it is fully encapsulated at n = 16 and 17; a highly symmetric framework similar to the Frank-Kasper-type tetrahedron is obtained as the most stable structure for n = 16. For n = 15, DFT calculations suggest the coexistence of endohedral forms; however, no clear experimental signatures supporting this prediction were obtained. Also found is that the vertical detachment energies of Ag_nAl^- with n = odd are lower than those for the neighboring sizes. Since the total number of the valence electrons of Ag_nAl^- is n + 4, this observation indicates that the outermost orbitals of the odd-numbered clusters are singly occupied. The present DFT calculation confirms that their spin multiplicities are indeed doublet, whereas those of the even-numbered clusters are singlet. Photoelectron angular distribution from the outermost orbital for n = 15 exhibits a large positive anisotropy with respect to the laser polarization, which manifests a 1S²1P⁶1D¹⁰2S¹ electronic configuration as observed in valence isoelectronic systems of Ag₁₈^- and Ag₁₅Sc^-. While all sizes exhibit electronic shell structures similar to those observed in undoped silver clusters, the energy ordering of the discrete levels in the endohedrally doped clusters, Al@Ag_n^- (n = 16 and 17), is not as predicted by the spherical jellium model; 2S-like orbitals are located in the manifold of 1D-like orbitals.