Ultrafast Laser-Driven Microfocus X-Ray Plasma Source Increases Time Resolution in Diffraction Experiments
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searchers reported. At temperatures of 400°C and lower, the spectra show the formation of cationic gold, which is active for the oxidation of hydrogen in the water gasshift reaction (CO2+ H2 = CO+H2O). The investigators concluded that the optimal process for obtaining the selective catalyst must result in the formation of metallic gold in Fe2O3 particles and they established a two-stage process: 3 h at 400°C and 2 h at 550°C. The researchers demonstrated a heterogeneous catalyst that remained stable for more than 80 h of continuous use. This development, said the research team, has the potential benefit of eliminating the need for a multistage reactor currently required to eliminate CO from the reformer fuel. SIARI SOSA
Ultrafast Laser-Driven Microfocus X-Ray Plasma Source Increases Time Resolution in Diffraction Experiments Ultrafast laser technology involves the use of a femtosecond laser, with the potential of producing laser pulses with enormous peak powers and power densities. High peak-power ultrafast laser pulses have been used at a number of facilities to generate ultrafast x-ray pulses from laserinduced plasmas. Such technology facilitates the study of very fast chemical reactions and their intermediate and transition
states. When an ultrafast laser with a small spot size is applied in a diffraction experiment, the temporal resolution of the response depends on the ability to control the jitter between the x-ray pulse and the laser pulse. This is the idea behind a broadband microfocus x-ray source designed as a result of the collaboration between a research group from the Max Born Institute led by T. Elsaesser and another from the Friedrich Schiller University led by E. Förster as described in the July 1 issue of Optics Letters (p. 1737). The experimental setup for the microfocus x-ray source included a 1 kHz Ti:sapphire laser source with 45 fs duration laser pulses incident upon a 20-μmthick copper foil, which was spooled such that it could be moved with precision so that each laser pulse was incident upon a fresh surface. When the laser intensity exceeded 1012 W/cm2, a high-temperature plasma developed at the target. At intensities higher than 1016 W/cm2, hot electrons were created that penetrated the target, generating incoherent x-rays. Similar levels of Cu Kα flux were detected by the investigators in both transmitted and reflected directions, increasing with laser intensity to a maximum of 6.8 × 10 10 photons/s, remaining stable in a 10 h span. The transmission geometry has two main advantages over the standard reflection geome-
try, according to N. Zhavoronkov of Max Born Institute, “The main advantages of this new setup are the possibility to determine the initial pulse time point with very good accuracy, and a significantly diminished temporal jitter, because of the novel transmission geometry introduced.” An image of the x-ray emitting area captured with a CCD and measured using a knife-edge technique gave a source diameter after deconvolution of 10 μm ± 2 μm full width at half maximum (FWHM).
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