Not so loosely bound rare gas atoms: finite-temperature vibrational fingerprints of neutral gold-cluster complexes

We present an experimental and theoretical study of the structure of small, neutral gold clusters-Au-3, Au-4 and Au-7-'tagged' by krypton atoms. Infrared (IR) spectra of Au-N . Kr-M complexes formed at 100K are obtained via far-IR multiple photon dissociation in a molecular beam. The theoretical study is based on a statistical (canonical) sampling of the Au-N . Kr-M 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 Au-2, Au-3 and Au-4, the predicted vibrational spectra of the Kr-complexed and pristine species differ. For Au-7, 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.