Abstract
We present an experimental and theoretical study of the structure of small, neutral gold clusters—Au3, Au4 and Au7—‘tagged’ by krypton atoms. Infrared (IR) spectra of AuN·KrM complexes formed at 100 K are obtained via far-IR multiple photon dissociation in a molecular beam. The theoretical study is based on a statistical (canonical) sampling of the AuN·KrM complexes through ab initio molecular dynamics using density-functional theory in the generalized gradient approximation, explicitly corrected for long-range van-der-Waals (vdW) interactions. The choice of the functional is validated against higher-level first-principle methods. Thereby finite-temperature theoretical vibrational spectra are obtained that are compared with the experimental spectra. This enables us to identify which structures are present in the experimental molecular beam for a given cluster size. For Au2, Au3 and Au4, the predicted vibrational spectra of the Kr-complexed and pristine species differ. For Au7, the presence of Kr influences the vibrational spectra only marginally. This behavior is explained in terms of the formation of a weak chemical bond between Kr and small gold clusters that localizes the Kr atom at a defined adsorption site, whereas for bigger clusters the vdW interactions prevail and the Kr adatom is delocalized and orbits the gold cluster. In all cases, at temperatures as low as T = 100 K, vibrational spectra already display a notable anharmonicity and show, in comparison with harmonic spectra, different position of the peaks, different intensities and broadenings, and even the appearance of new peaks.
Highlights
As a bulk elemental metal, gold is a classic example of inertness [1]
The theoretical study is based on a statistical sampling of the AuN ·KrM complexes through ab initio molecular dynamics using density-functional theory in the generalized gradient approximation, explicitly corrected for long-range van-der-Waals interactions
We thoroughly investigate if this holds, by studying how rare gas (RG) atoms bind to small, neutral gold clusters, and how this binding influences the vibrational spectra at T = 0 K as well as at finite temperatures
Summary
As a bulk elemental metal, gold is a classic example of inertness [1]. at the nanoscale gold exhibits surprising chemical activity [2, 3]. The comparison between experimental far-IR spectra and theoretical spectra calculated at ‘realistic conditions’ (i.e., accurate and validated level of our ab initio electronic structure theory, including van-der-Waals interactions, and finite-temperature statistical sampling in the canonical ensemble), let us identify the (meta)stable structures at all considered sizes, and reveal details on the dynamics of the gold clusters and the cluster-plus-rare-gas complexes. These finite-temperature ab initio molecular dynamics studies provide vibrational spectra that fully include anharmonic effects related to the canonical sampling of the potential energy surface [57].
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