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

Abstract

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 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 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 higherlevel 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 5 Only the structures of neutral Au 7 , Au 19 and Au 20 have been recently derived from farinfrared (IR) multiple photon dissociation (FIR-MPD) spectra of their complexes with krypton atoms and comparison to theoretical predictions [19]. However, this study posed the question of the influence of the Kr messenger on the spectra and the type of the interaction between Kr and the neutral Au clusters. It has been recognized before that even 'inert' rare gas (RG) atoms may influence the IR spectra of metal clusters and their explicit consideration can improve the agreement between experimental and predicted spectra [20,21]. Whereas this was somehow unexpected for neutral clusters, where RG atoms are considered to only physisorb, binding of RG atoms to cationic clusters, especially of late transition metals, is considerably stronger. For instance, it has been previously shown for cationic cobalt clusters [22] how the presence of Ar ligands significantly modifies the vibrational spectrum of the cluster. Similar effects of RG binding, leading even to changes in the energetic ordering of isomers, have been seen, e.g. for cationic vanadium or cerium oxide clusters [23,24].So far, in most cases the experimental FIR spectra of metal clusters are interpreted only by comparison to calculated harmonic spectra (at T = 0 K). However, experiments are performed at finite temperature and even at lowest temperatures anharmonic effects can have a noticeable influence on the vibrational spectra. Nevertheless, a theoretical investigation of Au 7 and Au 7 · Kr motivated by our previous study [19] applying a vibrational configurationinteraction approach did not identify significant anharmonicities for this particular cluster [25] and instead supported the initial conclusion that the Kr binding does not significantly change the vibrational frequencies, 'but has an effect on the IR intensities, which become very similar to those in the experimental spectrum' [19].In this paper, we thoroughly investigate if this indeed holds, by studying how 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. We report F...

Far-IR Spectra of Small Neutral Gold Clusters in the Gas Phase

“…For neutral clusters one might intuitively expect the transition to occur somewhere in between. The only experimentally confirmed structures are those of Au4, Au7 (2D) and Au19, Au20 (3D) [10,11], leaving a large unexplored gap. There have been many theoretical studies addressing this topic.…”

“…By using slow-electron velocity-map imaging (SEVI) [38], transitions to single vibrational levels of Au2 and Au4 have been recently observed [39,40]. We have used photodissociation of rare-gas complexes to obtain the vibrational spectra in the far-IR [10,11]. In combination with quantum chemical calculations, such vibrational spectra can give detailed structural information for metal clusters.…”

“…In the following we report experimental far-IR spectra for neutral gold clusters up to Au8. While the spectra for n = 3, 4, and 7 have been discussed before [10,11], we now include Au5 and Au8. No experimental data is available for Au6 because it is not detected due to its high ionization energy.…”

“…A detailed discussion of the effects of rare-gas ligands and anharmonicity (i.e. through finite temperature effects and fluctionality) has been given elsewhere for n = 3, 4, 7 [11].…”

Effect of Size, Temperature, and Structure on the Vibrational Heat Capacity of Small Neutral Gold Clusters

“…Their ranges for BLYP and BP86 are 26.42-162.22 cm -1 (BLYP) and 29.13-172.35 cm+ (BP86), respectively. Certainly, the most stable structure for Au20 was found through frequency calculations for the (T d ) Au 20 structure by Jun Li, Xi Li, Hua-Jin Zhai, and Lai-Sheng Wang [95,96].…”