Infoscience

Thesis

Study of the Optical UV-Visible Electron Induced Luminescence of Small Size Selected Metal Clusters in Rare Gas Solids

The optical properties of small size selected metal clusters have been studied by fluorescence spectroscopy. Cun (n = 1-5, 8), Agn (n = 1-5, 8, 9, 20), Aun (n = 1-5, 8) and Nin (n = 1-3) clusters embedded in condensed neon are photoionized by VUV photons in the solid, and the fluorescence originating from the reneutralization of the transient cluster ions is recorded. The VUV photons result from the radiative decay of excitons which are generated by the impact of electrons of variable energy on the solid. A new experimental setup has been developed, which permits the excitation of the rare gas solid by electrons with an energy between 0 and 200 eV. A field emission electron source based on carbon nanotubes provides an intense electron beam without any light generation. A custom built cryopumped gas purification system allows the production of rare gas solids with a purity in the order of up to 10-5. The characteristics and a physical descriptive model of electron induced luminescence is presented. The measurement of electron currents through the solid are thoroughly analyzed. Based on localized exciton production and radiative deexcitation of the host rare gas atoms near the surface of the solid, the excitation can be spread over distances as large as 20-30 µm by high energy photons which excite the species contained in the solid. Quenching and amplification effects on the luminescence intensities are observed depending on the material and the size of the dopants contained in the rare gas solids. These effects are believed to originate from the local enhanced electric field induced by the metal clusters which act as antennas. The analysis of the luminescence spectra reveals rich spectroscopic signatures from the UV to the near IR. A certain amount of fluorescent transitions could be identified from conventional excitation fluorescence spectroscopy. This proves that this new method is sensitive and reliable. The so far reported new transitions await interpretation on the basis of sophisticated quantum calculations.

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