Vibrational distortions of the Au 7 + hexagonal cluster
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Vibrational distortions of the Au7+ hexagonal cluster J. R. Soto1, B. Molina1, and J. J. Castro2 1 Facultad de Ciencias, Universidad Nacional Autónoma de México, Apdo. Postal 70- 646, 04510 México, D.F. 2 Departamento de Física, CINVESTAV del IPN, Apdo. Postal 14-740, 07000 México D.F. México
ABSTRACT The study of the 2D-3D structural transition in Au7+ nanocluster as a function of the number of gold atoms has been a long standing problem due to contradictory results between experiments, that show a 2D structure, and some theoretical results predicting 3D. We present a theoretical analysis, based on the pseudo Jahn-Teller effect that explains the origin of the 2D-3D structural transition controversy. It is shown that the usually assumed 2D non-degenerate ground state cluster structure with D6h symmetry is unstable due to a vibronic coupling between the ground state and one excited state, producing a puckering effect ending in a 3D stable structure with D3d symmetry. This structure presents the same surface area than the 2D, being therefore compatible with ion mobility experimental results. We discuss the effect of symmetry breaking on the Raman, IR and UV-vis spectra, which might indicate some possible sensor capabilities for this subnanometric cluster. The study is based on scalar relativistic and time-dependent DFT calculations in the Zero Order Regular Approximation (ZORA). . INTRODUCTION The fact that small clusters and nanostructures show a completely different physical and chemical behavior than in their bulk phase has opened an intense field of research for more than two decades. In particular, the study of supported gold clusters has been of special interest due to their unique electronic, optical and catalytic properties when compared to other metallic clusters. The study of nanoclusters on the gas phase provides a natural framework to understanding the dependence of those properties on the size and structure stability. Recently, new experimental techniques have been developed to study the geometrical structure of both gaseous and supported gold nanoclusters in different charged states. For anionic structures in gaseous phase, the techniques used are mainly based on photoelectron spectroscopy [1], electron diffraction [2], and ion mobility spectroscopy [3]. For cationic clusters, only the ion mobility technique has proved to be successful. All those experimental techniques are not able to determine in a unique way the geometrical structure, and it is therefore necessary to be complemented with theoretical predictions, being the density functional theory (DFT) the most widely accepted technique. From a theoretical point of view, small gold clusters are characterized by a rather flat adiabatic potential energy surface (APES), with several minima differing in energy by no more than 200 meV [4], presenting therefore the difficulty in finding the global minima. This fact, together with the dependence of some of the results on the functional and basis used in DFT calculations [4, 5], have made the determination
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