Determination of K to L shell total vacancy transfer probabilities using a weak gamma source: An alternative method

The K-L total vacancy transfer probabilities (η_KL) of selected elements in the atomic range 42≤Z≤82 have been determined using a weak gamma source. The targets are excited by 123.6 keV gamma photons from a weak ⁵⁷Co source and K X-ray photons are measured by an ORTEC type GMX-10P HPGe detector coupled to 8 K multichannel analyzer. By measuring the K X-ray intensity ratio and K shell fluorescence yield, the K to L total vacancy transfer probabilities have been determined. Measured values have been compared with theoretical and other experimental values.     

On the evolution of the ground state in the system U x La 1 - x S: Polarized neutron diffraction and X-ray magnetic circular dichroism study

In the U_xLa_1-xS system there is an abrupt loss of the long-range ferromagnetic ordering found in pure US at a critical concentration x _c ∼ 0.57, which is far above the percolation limit. As the magnetic ground state in such a system can be strongly affected by small variations of the 5f localization, we have investigated a set of samples with different x by polarized neutron diffraction and X-ray magnetic circular dichroism (XMCD). The neutron results are consistent with early measurements performed on pure US. Even at the lowest U content (x = 0.15, below x _c ) the shape of the induced form factor (f ( Q )) is comparable with that found for x = 1 and is well reproduced by either a U⁴⁺ or a U³⁺ state. The ratio between the orbital and the effective spin moments in the XMCD measurements confirms this result, but the evolution of the shape at the M₅ edge suggests an abrupt change in the distribution of the electrons (holes) in the 5 f density of states around x _c . Received 31 January 2001     

A study on anisotropy of L2 to L3 Coster-Kronig transition (f23) for Th and U elements

The Coster-Kronig transition, f₂₃, was determined using differential fluorescence cross sections of L_l X-ray for Th and U. The targets were irradiated an Am-241 radioisotope at the different incident angle. The L_l X-rays were counted with a Si (Li) detector at the different scattering angle varying from 60° to 90° at 10° intervals. For each angle, the Coster-Kronig transition probability, f₂₃, was found. An obtained Coster-Kronig transition probability value was fitted versus emission angle. According to present results we can say that the Coster-Kronig transition probability, f₂₃, shows isotropic distribution.     

Surface chemistry and catalysis of oxide model catalysts from single crystals to nanocrystals

Fundamental understandings of surface chemistry and catalysis of solid catalysts are of great importance for the developments of efficient catalysts and corresponding catalytic processes, but have been remaining as a challenge due to the complex nature of heterogeneous catalysis. Model catalysts approach based on catalytic materials with uniform and well-defined surface structures is an effective strategy. Single crystals-based model catalysts have been successfully used for surface chemistry studies of solid catalysts, but encounter the so-called “materials gap” and “pressure gap” when applied for catalysis studies of solid catalysts. Recently catalytic nanocrystals with uniform and well-defined surface structures have emerged as a novel type of model catalysts whose surface chemistry and catalysis can be studied under the same operational reaction condition as working powder catalysts, and they are recognized as a novel type of model catalysts that can bridge the “materials gap” and “pressure gap” between single crystals-based model catalysts and powder catalysts. Herein we review recent progress of surface chemistry and catalysis of important oxide catalysts including CeO₂, TiO₂ and Cu₂O acquired by model catalysts from single crystals to nanocrystals with an aim at summarizing the commonalities and discussing the differences among model catalysts with complexities at different levels. Firstly, the complex nature of surface chemistry and catalysis of solid catalysts is briefly introduced. In the following sections, the model catalysts approach is described and surface chemistry and catalysis of CeO₂, TiO₂ and Cu₂O single crystal and nanocrystal model catalysts are reviewed. Finally, concluding remarks and future prospects are given on a comprehensive approach of model catalysts from single crystals to nanocrystals for the investigations of surface chemistry and catalysis of powder catalysts approaching the working conditions as closely as possible.