Armin Shayeghi

The Nature of Bonding between Argon and Mixed Gold–Silver Trimers

The controversial nature of chemical bonding between noble gases and noble metals is addressed. Experimental evidence of exceptionally strong Au-Ar bonds in Ar complexes of mixed Au-Ag trimers is presented. IR spectra reveal an enormous influence of the attached Ar atoms on vibrational modes, particularly in Au-rich trimers, where Ar atoms are heavily involved owing to a relativistically enhanced covalency. In Ag-rich trimers, vibrational transitions of the metal framework predominate, indicating a pure electrostatic character of the Ag-Ar bonds. The experimental findings are analyzed by means of DFT calculations, which show how the relativistic differences between Au and Ag are manifested in stronger Au-Ar binding energies. Because of the ability to vary composition and charge distribution, the trimers serve as ideal model systems to study the chemical nature of the bonding of noble gases to closed-shell systems containing gold.

Optical and electronic properties of mixed Ag-Au tetramer cations

We present experimental and theoretical studies of the optical response of mixed Ag(n)Au(+)(4-n) (n=1-3) clusters in the photon energy range ℏω = 1.9-3.5 eV. Absorption spectra are recorded by a newly built longitudinal molecular beam depletion spectroscopy apparatus providing lower limits to absolute photodissociation cross sections. The experimental data are compared to optical response calculations in the framework of long-range corrected time-dependent density functional theory with initial cluster geometries obtained by the unbiased Birmingham Cluster Genetic Algorithm coupled with density functional theory. Experiments and excited state calculations shed light on the structural and electronic properties of the mixed Ag-Au tetramer cations.

Charge-induced dipole vs. relativistically enhanced covalent interactions in Ar-tagged Au-Ag tetramers and pentamers

Vibrational spectra of Au(n)Ag(m)(+)⋅Ar(k) (n + m = 4, 5; k = 1-4) clusters are determined by far-infrared resonant multiple photon dissociation spectroscopy in the range ν̃=100-250 cm(-1). The experimental spectra are assigned using density functional theory for geometries obtained by the Birmingham cluster genetic algorithm. Putative global minimum candidates of the Ar complexes are generated by adding Ar atoms to the Au(n)Ag(m)(+) low energy isomers and subsequent local optimization. Differential Ar binding energies indicate exceptionally strong Au-Ar bonds in Au-rich clusters, leading to fundamental changes to the IR spectra. The stronger Ar binding is attributed to a relativistically enhanced covalent character of the Au-Ar bond, while in Au-rich species charge-induced dipole interactions overcompensate the relativistic affinity to Au. Moreover, not only the absolute composition but also the topologies are essential in the description of Ar binding to a certain cluster.

Communication: Global minimum search of Ag 10+ with molecular beam optical spectroscopy

The present study is focused on the optical properties of the Ag₁₀⁺ cluster in the photon energy range ℏω = 1.9-4.4 eV. Absorption spectra are recorded by longitudinal molecular beam depletion spectroscopy and compared to optical response calculations using time-dependent density functional theory. Several cluster isomers obtained by the new pool-based parallel implementation of the Birmingham Cluster Genetic Algorithm, coupled with density functional theory, are used in excited state calculations. The experimental observations, together with additional simulations of ion mobilities for the several geometries found within this work using different models, clearly identify the ground state isomer of Ag₁₀⁺ to be composed of two orthogonal interpenetrating pentagonal bipyramids, having overall D(2d) symmetry.

Influence of spin-orbit effects on structures and dielectric properties of neutral lead clusters

Combining molecular beam electric deflection experiments and global optimization techniques has proven to be a powerful tool for resolving equilibrium structures of neutral metal and semiconductor clusters. Herein, we present electric molecular beam deflection experiments on PbN (N = 7-18) clusters. Promising structures are generated using the unbiased Birmingham Cluster Genetic Algorithm approach based on density functional theory. The structures are further relaxed within the framework of two-component density functional theory taking scalar relativistic and spin orbit effects into account. Quantum chemical results are used to model electric molecular beam deflection profiles based on molecular dynamics calculations. Comparison of measured and simulated beam profiles allows the assignment of equilibrium structures for the most cluster sizes in the examined range for the first time. Neutral lead clusters adopt mainly spherical geometries and resemble the structures of lead cluster cations apart from Pb10. Their growth pattern deviates strongly from the one observed for tin and germanium clusters.

Charge and Compositional Effects on the 2D–3D Transition in Octameric AgAu Clusters

Abstract The unbiased density functional-based Birmingham Cluster Genetic Algorithm is employed to locate the global minima of all neutral and mono-ionic silver-gold octamer clusters. Structural, energetic and electronic trends are determined across the series, in order to clarify the role of composition and charge on the position of the 2D–3D transition in ultrasmall coinage metal systems. Our calculations indicate a preference for three dimensional structures at high silver concentrations, which varies significantly with charge. The minimum in composition dependent mixing energies is independent of the charge, however, with a preference for the maximally mixed clusters, Ag4Au4 ν for all charge states ν. The sensitivity of isomeric preference to ν is found to be greater for electron-rich and electron-deficient clusters, implying a complexity of unambiguous determination of cluster motifs in related experiments. Vertical ionization potentials and detachment energies are calculated to probe electronic behaviour, providing numerical predictions for future spectroscopic studies.