Structure of the active site of sulfite dehydrogenase from Starkeya novella

In this paper, we report the results of molybdenum K-edge X-ray absorption studies performed on the oxidized and reduced active sites of the sulfite dehydrogenase from Starkeya novella. Our results provide the first direct structural information on the active site of the oxidized form of this enzyme and confirm the conclusions derived from protein crystallography that the molybdenum coordination is analogous to that of the sulfite oxidases. The molybdenum atom of the oxidized enzyme is bound by two Mo=O ligands at 1.73 A and three thiolate Mo-S ligands at 2.42 A, whereas the reduced enzyme has one oxo at 1.74 A, one long oxygen at 2.19 A (characteristic of Mo-OH2), and three Mo-S ligands at 2.40 A.     

Probing the transition in bulk Ce under pressure: a direct investigation by resonant inelastic X-ray scattering

We report on the most complete investigation to date of the -electron properties at the transition in elemental Ce by resonant inelastic x-ray scattering (RIXS). The Ce 2p3d-RIXS spectra were measured directly in the bulk material as a function of pressure through the transition. The spectra were simulated within the Anderson impurity model. The occupation number n(f) and f double occupancy were derived from the calculations in both gamma and alpha phases in the ground state. We find that the electronic structure changes result mainly from band formation of 4f electrons which concurs with reduced electron correlation and increased Kondo screening at high pressure.     

Magneto-optical interactions in single-molecule magnets: Low-temperature photon-induced demagnetization

We show that the irradiation of SMM molecules at optical wavelengths can drive an increase or a decrease of the magnetic moment of a SMM, even though the energy of the photons does not correspond to a precise electronic or spin transition, the light pulse triggering a phonon-assisted spin transition. The process is sensitive to the power of the incident light. This result most probably explains why it has been so far impossible to observe the opening of the hysteresis loop on thin films of SMM with the XMCD technique. The consequences of these observations are manifold: they bring a means of controlling molecular magnets, open prospects in the field of quantum computing, and may enable the realization of coherent microwave sources through stimulated superradiance.     

Loss of single-molecule-magnet behavior of a Mn12-based compound assembled in a monolayer

Magnetic circular dichroism magnetization measurements on a monolayer of single molecule magnets have been performed and the results are presented. At the measurement temperature (2 K) the hysteresis cycle shows no opening, contrary to what is seen in the bulk with SQUID magnetometry. This result could surprisingly imply that XMCD is unsuited for such measurements given the fact that it excites the core electrons at the origin of the magnetism. This result could also mean that the blocking temperature is greatly decreased or that these molecules loose their hallmark when deposited as adsorbed films, precluding therefore their much sought-after use in magnetic memories.     

4f ground state properties of cerium in AuCu3 intermetallic compounds studied by x-ray absorption spectroscopy

L_III absorption edges of cerium in ternary AuCu₃ compounds Ce(Pd_1?xM_x)₃ (M = Ag and Rh) and (Ce_1?xY_x)Pd₃ are presented. We find that Ce in these compounds is mixed valent with n_f ? 0.7. The valence V of Ce(Pd_1?xRh_x)₃ becomes concentration independent of V = 3.29 ± 0.02 for x ? 0.18. In Ce_1?xY_xPd₃ we observe the valence to increase up to V = 3.32 ± 0.02 at the highest concentration of Y. We show, that a critical volume, available for the Ce atom, is associated with the concentration independent valence around 3.3. For both systems, linewidths W₁, W₀ and Δ = E(4f¹) ? E(4f⁰), as derived from L_III double-bump structure, are given. Possible falsifications due to “final states effects” on the valence are discussed.     

Heisenberg picture operators in the stochastic wave function approach to open quantum systems

A fast simulation algorithm for the calculation of multitime correlation functions of open quantum systems is presented. It is demonstrated that any stochastic process which “unravels” the quantum Master equation can be used for the calculation of matrix elements of reduced Heisenberg picture operators, and thus for the calculation of multitime correlation functions, by extending the stochastic process to a doubled Hilbert space. The numerical performance of the stochastic simulation algorithm is investigated by means of a standard example. Received: 30 May 1997 / Revised: 4 November 1997 / Accepted: 7 November 1997     

Correlation between XPS, Raman and TEM measurements and the gas sensitivity of Pt and Pd doped SnO2 based gas sensors

Nanocrystalline thick-film SnO₂ sensors with different dopants were fabricated by an optimized screen printing process and subsequent annealing. Powders were used as starting materials which were prepared by a wet chemical process from SnCl₄. Microanalysis was performed of both, the precursors and the final sensor materials with their different annealing conditions. Gas sensing tests with CO, CH₄ and NO₂ in air with controlled humidity were correlated with results from X-ray photoemission spectroscopy (XPS), Raman spectroscopy and transmission electron microscopy (TEM). As an interesting result, the distribution of the transition metal dopants Pd and Pt (as deduced from TEM and XPS data) rules out the existence of metallic clusters or even atoms in the metallic state at the surface. This finding does not allow to explain the sensor effects on SnO₂ based materials as usually done by means of spill-over effects or Fermi energy control.