Coherent evolution of parahydrogen induced polarisation using laser pump, NMR probe spectroscopy : Theoretical framework and experimental observation

We recently reported a pump-probe method that uses a single laser pulse to introduce parahydrogen (p-H2) into a metal dihydride complex and then follows the time-evolution of the p-H2-derived nuclear spin states by NMR. We present here a theoretical framework to describe the oscillatory behaviour of the resultant hyperpolarised NMR signals using a product operator formalism. We consider the cases where the p-H2-derived protons form part of an AX, AXY, AXYZ or AA′XX′ spin system in the product molecule. We use this framework to predict the patterns for 2D pump-probe NMR spectra, where the indirect dimension represents the evolution during the pump-probe delay and the positions of the cross-peaks depend on the difference in chemical shift of the p-H2-derived protons and the difference in their couplings to other nuclei. The evolution of the NMR signals of the p-H2-derived protons, as well as the transfer of hyperpolarisation to other NMR-active nuclei in the product, is described. The theoretical framework is tested experimentally for a set of ruthenium dihydride complexes representing the different spin systems. Theoretical predictions and experimental results agree to within experimental error for all features of the hyperpolarised 1H and 31P pump-probe NMR spectra. Thus we establish the laser pump, NMR probe approach as a robust way to directly observe and quantitatively analyse the coherent evolution of p-H2-derived spin order over micro-to-millisecond timescales

Influence of the stress field due to pressurized nanometric He bubbles on the mobility of an edge dislocation in iron

Voids and He bubbles are strong obstacles to dislocation, which induce hardening and loss of ductility. In Fe, molecular dynamics simulation is used to investigate the basic mechanisms of the interaction between a moving edge dislocation and a void or He bubble, as a function of its He content, temperature, interatomic potentials and interaction geometry. Different interatomic potentials for Fe-Fe and Fe-He interactions are used. It appears that temperature eases the dislocation release, due to the increased mobility of the screw segments appearing on the dislocation line upon bowing from the void or He bubble. The mobility includes the cross-slipping of these segments, which leads to the formation of a jog. It appears that the He bubble induces an inhomogeneous stress field in its surroundings, which strongly influences the dislocation passage depending on the geometry of the interaction

Aluminum partitioning during phase separation in Fe-20%Cr-6%Al

Phase separation in a commercial Fe–20 wt.% Cr–6%Al oxide dispersion-strengthened PM 2000 steel has been characterized with a local-electrode atom probe after isothermal aging at 708 Kand 748 Kfor times up to 3,600 h. A progressing decrease in the Al content of the Cr-rich a0 phase was observed with time at both aging temperatures. The Al partitioning trend was consistent with theoretical calculations. However, the experimentally observed Al partitioning factor was significantly lower than the predicted equilibrium value. A *10 nm diameter, roughly spherical, Al- and Ti-enriched b0 Fe(AlTi) phase was also observedSpanish Ministerio de Educación y Ciencia in the form of a Coordinate Project in the Energy Area of Plan Nacional 2006 (ENE2006-15170-C02). Research at the Oak Ridge National Laboratory SHaRE User Facility was sponsored by Basic Energy Sciences, U.S. Department of Energy. PM 2000™ is a trademark of Plansee SEPeer reviewe