Optical properties of small metal clusters

This thesis is devoted to the study of the optical properties of small metal clusters and metal cluster organic compounds. For this purpose a ultra-high vacuum (UHV) experimental apparatus has been built which allows for the deposition of mass selected neutral clusters in a size range up to 16,000 amu which corresponds to Ag150 or Au80. Cluster cations are produced in a gas aggregation cluster source, steered in an ion optics device and co-deposited with electrons and excess rare gas on a cryogenic deposition head. Neutral mono dispersed clusters are analyzed by optical absorption, fluorescence and excitation spectroscopy in the UV-visible energy range. A novel custom-made conical octupole combined to an organic molecule injector has been built, serving as a generator of metal cluster-organic compounds. These are atomically precise model systems which permit to study surface-enhanced Raman scattering (SERS) in order to test the theoretical models and better understand the mechanism of SERS. Optical absorption on size-selected silver clusters of Agn (n= 20 − 120) embedded in neon solid matrix was investigated. The spectra show plasmon-like absorption profiles around a photon energy of 4.0 eV which match in energy molecular like absorption spectra for the smallest Ag clusters but are sensitively higher than what would be expected for the dipolar absorption for bulk silver of about 3.3 eV. The central absorption peak, i.e. the plasmon energy as a function of cluster size is presented. The data fall in a transition between a blueshift of the plasmon resonance with decreasing size to a superimposed red shift for the very smallest sizes. Superimposed, we observe a distinct structure where clusters with atom number of 8,18,34,58,92 show a localized maximum value of the plasmon energy. These numbers correspond perfectly to the fully filled states of 1p, 1d, 1f, 1g and 1h according to the electronic shell model, manifesting a clear sub-shell effect. Optical absorption, fluorescence and excitation spectroscopy on size-selected small gold clusters of Aum (m= 19,20,21) embedded in solid neon were measured. These are the first optical data on ligand free neutral atomically precise Au clusters. Au20, the famous tetraedic structure, shows a rich structured absorption spectrum, superimposed on a continuous background increasing to the UV part of the spectrum. Au21 and Au19 show the same tendency, however without any superimposed structure. Very interestingly, all three Au clusters show strong fluorescence centered at around 1.85 eV, here again Au20 being the prominent species. Comparison to ligand protected Au clusters clearly proves that the fluorescence comes from the Au cluster itself and is only weakly influenced by the ligands. For the first time, size-selected small metal-organic compounds of silver-pyridine AgaPyb (a= 1,3; b= 1,2) which are prototypes for surface enhanced Raman scattering (SERS) have been produced and isolated in a solid Ne matrix. Optical absorption and fluorescence spectra have been measured. Raman scattering cross sections with an enhancement factor of 103 have been found in fair agreement with recent high level time-dependent density functional theory (TDDFT) calculations.

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