Spectroscopic characterization of Al2O3-Ni selective absorbers for solar collectors

Cataloged from PDF version of article.Optical spectroscopy of electrochemically prepared Ni-pigmented aluminum oxide selective absorbers have been determined in the 200—20000 nm range. It was found that samples anodized under the same conditions and pigmented using nickel acetate resulted in better thermal emittance values when compared with nickel sulfate although both have comparable solar absorbance values. Electron spectroscopic investigation revealed that only a small fraction of Ni is present on the surface with an oxidation state of *#2. The O/Al ratio determined by XPS is larger than 1.5. This information together with the measured Al 2p Auger parameter indicated that the surfaces contain additional OH groups which was also confirmed by the presence of a broad hyrogen-bonded band in the region 3000—3400 cm~1 observed in the reflection—absorption IR spectra of these samples. ( 1998 Elsevier Science B.V. All rights reserved

Electron self-energy in A3C60 (A=K, Rb): Effects of t1u plasmon in GW approximation

The electron self-energy of the t1u states in A3C60 (A=K, Rb) is calculated using the so-called GW approximation. The calculation is performed within a model which considers the t1u charge carrier plasmon at 0.5 eV and takes into account scattering of the electrons within the t1u band. A moderate reduction (35 %) of the t1u band width is obtained.Comment: 4 pages, revtex, 1 figure more information at http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene

Diffraction of complex molecules by structures made of light

We demonstrate that structures made of light can be used to coherently control the motion of complex molecules. In particular, we show diffraction of the fullerenes C60 and C70 at a thin grating based on a standing light wave. We prove experimentally that the principles of this effect, well known from atom optics, can be successfully extended to massive and large molecules which are internally in a thermodynamic mixed state and which do not exhibit narrow optical resonances. Our results will be important for the observation of quantum interference with even larger and more complex objects.Comment: 4 pages, 3 figure

Renormalization Group Approach to the Coulomb Pseudopotential for C_{60}

A numerical renormalization group technique recently developed by one of us is used to analyse the Coulomb pseudopotential (${\mu^*}$) in ${{\rm C}_{60}}$ for a variety of bare potentials. We find a large reduction in ${\mu^*}$ due to intraball screening alone, leading to an interesting non-monotonic dependence of ${\mu^*}$ on the bare interaction strength. We find that ${\mu^*}$ is positive for physically reasonable bare parameters, but small enough to make the electron-phonon coupling a viable mechanism for superconductivity in alkali-doped fullerides. We end with some open problems.Comment: 12 pages, latex, 7 figures available from [email protected]

Optical absorption spectra of A6C60 and A6C70: Reduction of effective Coulomb interactions in Frenkel excitons

We theoretically investigate optical absorption spectra of \soc^{6-} and \rug^{6-}, and discuss relations with the optical properties of alkali metal doped fullerides A_6\soc and A_6\rug. This is a valid approach for systems where Frenkel exciton effects are dominant. We use a tight binding model with long ranged Coulomb interactions and bond disorder. Optical spectra are obtained by the Hartree-Fock approximation and the configuration interaction method. We find that the Coulomb interaction parameters, which are relevant to the optical spectra of A_6\soc (A_6\rug) in order to explain the excitation energies and relative oscillator strengths of absorption peaks, are almost the half of those of the neutral \soc (\rug). The reduction of the effective Coulomb interactions is concluded for the heavily doped case of \soc and \rug. This finding is closely related with the experimental fact that dielectric constants of fullerides which are maximumly doped with alkali metals become about twice as large as those of the neutral systems.Comment: Note: A full preprint with figures should be requested to the author. It will be sent by air-mail.; E-mail: [email protected]