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991.
Perfetti L Georges A Florens S Biermann S Mitrovic S Berger H Tomm Y Höchst H Grioni M 《Physical review letters》2003,90(16):166401
High-resolution angle-resolved photoemission data show that a metal-insulator Mott transition occurs at the surface of the quasi-two-dimensional compound 1T-TaSe2. The transition is driven by the narrowing of the Ta 5d band induced by a temperature-dependent modulation of the atomic positions. A dynamical mean-field theory calculation of the spectral function of the half-filled Hubbard model captures the main qualitative feature of the data, namely, the rapid transfer of spectral weight from the observed quasiparticle peak at the Fermi surface to the Hubbard bands, as the correlation gap opens up. 相似文献
992.
We demonstrate a novel method for improvement of spatial beam quality by use of the angular selectivity of a thick phase grating inserted into various laser cavities. Preliminary experiments with an intentionally degraded beam from a Nd:YVO4 laser have enhanced beam propagation factor M2 by a factor of 4.5. Operation with a broad-area laser diode in an external cavity with a thick grating was achieved. We obtained 100 mW of cw power in a beam with an M2 of 1.3. Moreover, a reduction of the output spectrum by as much as a factor of 3.8 was obtained. 相似文献
993.
The evolution of a spherical gaseous interface accelerated by a plane weak shock wave has been investigated in a square cross section shock tube via a multiple exposure shadowgraph diagnostic. Different gaseous bubbles, i.e., helium, nitrogen, and krypton, were introduced in air at atmospheric pressure in order to study the Richtmyer-Meshkov instability in the spherical geometry for negative, close to zero, and positive initial density jumps across the interface. We show that the bubble distortion is strongly different for the three cases and we present the experimental velocity and volume of the developed vortical structures. We prove that at late times the bubble velocities reach constant values which are in good agreement with previous calculations. Finally, we point out that, in our flow conditions, the gaseous bubble motion and shape are mainly influenced by vorticity and aerodynamic forces. 相似文献
994.
The axial and transverse diffusion coefficients of a band of iodine in a chromatographic column were measured optically as a function of time. It was found that the axial diffusion coefficient remains constant even when the edges of the sample band get close to the wall. By contrast, the radial diffusion coefficient decreases progressively with increasing time when the edges of the sample band leave the core region and begin to diffuse inside the wall region. The local axial and transverse diffusion coefficients of the band decrease from the column center toward the wall. Hence, the increase in local height equivalent to a theoretical plate observed in the region close to the wall must be explained by increasing mass transfer resistances and degree of heterogeneity of the bed. 相似文献
995.
Raphaël JankowskiDelphine Joseph Christian CavéFrançoise Dumas Michèle OurevitchJacqueline Mahuteau Georges MorgantNada Bosnjakovi? Pavlovi? Jean d'Angelo 《Tetrahedron letters》2003,44(22):4187-4190
When conducted in DMSO, the Hünig's base-promoted condensation of 3-quinuclidinone with quinoline-4-carboxaldehyde gave an equimolar mixture of epimeric aldols 8 with an excellent yield. 相似文献
996.
We report molecular dynamics potential of mean force (PMF) simulations on the capture of halide anions X(-) (F(-), Cl(-), Br(-)) by a tetrahedral receptor L(4+) built from four quaternary ammonium sites connected by six (CH(2))(n) chains, leading to the formation of inclusion complexes X(-) subset L(4+). Simulations performed with a reaction field correction of the electrostatics and with PME-Ewald summation gave very similar energy profiles. In aqueous solution, an energy barrier of 12-17 kcal mol(-1) was found for the three anions, mainly due to their dehydration when they enter through the largest triangular face of L(4+). In the inclusion complexes, the anion is anchored near the center of the cavity due to the electrostatic field of the four positively charged ammonium sites, shielded from the surrounding water molecules. It was predicted that L(4+) is selective for Cl(-) over Br(-) which both form stable inclusion complexes, while the F(-) complex should dissociate. The comparison of PMFs in aqueous solution and in the gas phase and the energy component analysis demonstrates the importance of solvent on the nature of these complexes and on the complexation energy profiles. The Cl(-)/Br(-) selectivity obtained from the dissociation pathways in water was in good agreement with the results of free energy perturbation simulations based on the "alchemical route" of a thermodynamic cycle, and consistent with experimental observations. 相似文献
997.
Alary F Heully JL Poteau R Maron L Trinquier G Daudey JP 《Journal of the American Chemical Society》2003,125(36):11051-11061
Using the Effective Group Potentials (EGP) method, optimal geometries, harmonic vibrational frequencies, and relative energies of different sets of metal complexes are calculated. All of the systems under consideration contain the cyclopentadienyl (Cp) ligand. They are as follows: (i). Group V metal Atom complexes showing one Cp ligand, (ii). a tetrameric Al-Cp compound with four Cp ligands, (iii). homometallic lutetium hydrides containing six cyclopentadienyl rings. Various electron correlation treatments have been carried out. All of the results compare very satisfactorily with available experimental data and with all-electron ab initio calculations performed for this work or published in the literature. Furthermore, the performance of the EGP method was tested on a rather large complex for which experimental evidence exists, but no all-electron calculation has been reported so far. 相似文献
998.
We have studied experimentally the spatial photosensitivity distribution of a self-scanned photodiode array. Using this data, we have calculated a reconstruction function which restores exactly the profile of a light distribution sampled by an array infinite extent under the Nyquist conditions. We briefly discuss the noise limitations and the truncation effects introduced by the finite number of diodes of a real array. 相似文献
999.
Franois Cansell Alain Siove Georges Belorgey 《Journal of Polymer Science.Polymer Physics》1990,28(5):647-654
Copolymerization of ethylene (E) with propylene (P) initiated with soluble (polystyrene)100-butadiene3-Li/TiCl4 complexes leads to [St]-b-[E-co-P] diblock copolymers with a random [E-P] block. GPC and 13C-NMR analyses of copolymers with various compositions and block lengths disclose well-defined linear structures. From DSC measurements of such materials, three different endothermal transition regions are observed: two glass transition regions corresponding, respectively, to the Tg of styrenic and olefinic blocks, and one [E-P] block melting region in which propylene units play a primary role. Furthermore, an additional endotherm is observed, depending on the thermal history of the sample. 相似文献
1000.
A semiclassical model for the vibrational relaxation of a diatomic molecule in collision with an atom is presented in the coupled-state framework in the large j limit. Our model realizes an exact quantum resolution of the rotational dynamics, the vibrational dynamics being integrated within a first-order perturbational perturbed-stationary-state approach. The application to CO + 4He confirms an appreciable role of the rotational energy transfers which produce an increase of the rate constant by a factor of ≈ 2.5. 相似文献