Propriétés optiques de petits agrégats de métaux nobles en matrice de néon

The optical properties of small noble metal clusters have been studied by absorption and fluorescence for sizes between 1 and 10 atoms. The experimental technique consists in depositing size-selected clusters in a neon matrix and analyzing with a spectrometer the transmitted and fluorescing light through the sample in an energy range between 1.8 and 5.5 eV. A new experimental setup has been built. It allows refrigeration of an Aluminium support down to 6 K and condensation of the neon matrix. The design of the sample holder allows a precise measurement of the cluster current during deposition. In comparison to measurements in argon matrices, the optical transmission has been increased by two orders of magnitude on the entire UV-Visible range. Consequently, the signal to noise ratio is greatly improved. Absorption spectroscopy on silver, copper and gold clusters with sizes between 1 and 9 atoms have revealed for each case a large amount of optical transitions with peakwidth as narrow as 50 meV, typical for a molecular-like behavior. Fluorescence measurements by exciting the sample with a tunable laser source have been achieved. For the atomic and dimer gold systems, the experimental results show several new emission lines. The fluorescence spectra obtained for Ag8 and Ag9 in neon confirm the results obtained in argon and demonstrate that the interaction with the environment is relatively weak for larger clusters. A study on the efficient trapping of positively charged clusters inside argon and neon matrices has allowed the measurement of the absorption spectra of the silver trimer and pentamer in their cationic form. TD-DFT calculations have been performed in order to find the lowest energy structures and the excitation spectra for each system. The excellent agreement between theory and experiment allows us to assign with more certainty the measured data to given isomers for all the silver clusters and for about 50% of the copper and gold clusters. A Mulliken population analysis has allowed to find the s, p and d contributions for each calculated spectrum. This analysis shows that we have more or less pure s transitions up to 5 eV for silver clusters while the d electrons participate actively to the transitions even at low energies between 2 and 3 eV in copper and gold. A detailed discussion on the pentamers of silver, copper and gold is presented on the basis of the experimental and theoretical results.


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