Current Issue : January - March Volume : 2018 Issue Number : 1 Articles : 6 Articles
We report on the application of a short working distance von Hamos geometry spectrometer\nto measure the inelastic X-ray scattering (IXS) signals from solids and liquids. In contrast to typical\nIXS instruments where the spectrometer geometry is fixed and the incoming beam energy is scanned,\nthe von Hamos geometry allows measurements to be made using a fixed optical arrangement with\nno moving parts. Thanks to the shot-to-shot capability of the spectrometer setup, we anticipate its\napplication for the IXS technique at X-ray free electron lasers (XFELs). We discuss the capability of\nthe spectrometer setup for IXS studies in terms of efficiency and required total incident photon flux\nfor a given signal-to-noise ratio. The ultimate energy resolution of the spectrometer, which is a key\nparameter for IXS studies, was measured to the level of 150 meV at short crystal radius thanks to the\napplication of segmented crystals for X-ray diffraction. The short working distance is a key parameter\nfor spectrometer efficiency that is necessary to measure weak IXS signals....
NuSTAR is a highly sensitive focusing hard X-ray (HXR) telescope and has observed several small microflares in its\ninitial solar pointings. In this paper, we present the first joint observation of a microflare with NuSTAR and Hinode/\nXRT on 2015 April 29 at âË?¼11:29 UT. This microflare shows the heating of material to several million Kelvin, observed\nin soft X-rays with Hinode/XRT, and was faintly visible in the extreme ultraviolet with SDO/AIA. For three of the four\nNuSTAR observations of this region (pre-flare, decay, and post-flare phases), the spectrum is well fitted by a single\nthermal model of 3.2ââ?¬â??3.5 MK, but the spectrum during the impulsive phase shows additional emission up to 10 MK,\nemission equivalent to the A0.1 GOES class. We recover the differential emission measure (DEM) using SDO/AIA,\nHinode/XRT, and NuSTAR, giving unprecedented coverage in temperature. We find that the pre-flare DEM peaks at\nâË?¼3 MK and falls off sharply by 5 MK; but during the microflareââ?¬â?¢s impulsive phase, the emission above 3 MK is\nbrighter and extends to 10 MK, giving a heating rate of about 2.5 1025 erg sâË?â??1. As the NuSTAR spectrum is purely\nthermal, we determined upper limits on the possible non-thermal bremsstrahlung emission. We find that for the\naccelerated electrons to be the source of heating, a power-law spectrum of 7 with a low-energy cutoff Ec 7 keV\nis required. In summary, this first NuSTAR microflare strongly resembles much more powerful flares....
Molecules often fragment after photoionization in the gas phase. Usually, this process\ncan only be investigated spectroscopically as long as there exists electron correlation between\nthe photofragments. Important parameters, like their kinetic energy after separation, cannot be\ninvestigated. We are reporting on a femtosecond time-resolved Auger electron spectroscopy study\nconcerning the photofragmentation dynamics of thymine. We observe the appearance of clearly\ndistinguishable signatures from thymines neutral photofragment isocyanic acid. Furthermore,\nwe observe a time-dependent shift of its spectrum, which we can attribute to the influence of the\ncharged fragment on the Auger electron. This allows us to map our time-dependent dataset onto\nthe fragmentation coordinate. The time dependence of the shift supports efficient transformation of\nthe excess energy gained from photoionization into kinetic energy of the fragments. Our method is\nbroadly applicable to the investigation of photofragmentation processes....
We report on studies of state-populations during the two-photon absorption process\nusing intense X-ray pulses. The calculations were performed in a time-dependent manner using a\nsimple three-level model expressed by coupled rate equations. We show that the proposed approach\ndescribes well the measured rates of X-rays excited in the one-photon and two-photon absorption\nprocesses, and allows detailed investigation of the state population dynamics during the course\nof the incident X-ray pulse. Finally, we demonstrate that the nonlinear interaction of X-ray pulses\nwith atoms leads to a time-narrowing of state populations. This narrowing-effect is attributed to a\nquadratic incidence X-ray intensity dependence characteristic for nonlinear interactions of photons\nwith matter....
High power attosecond (as) X-ray pulses are in great demand for ultrafast dynamics\nand high resolution microscopy. We numerically demonstrate the generation of a ~230 attosecond,\n1.5 terawatt (TW) pulse at a photon energy of 1 keV, and a 115 attosecond, 1.2 TW pulse at a photon\nenergy of 12.4 keV, using the realistic electron beam parameters such as those of Korean X-ray\nfree electron laser (XFEL) in a tapered undulator configuration. To compensate the energy loss of\nthe electron beam and maximize its radiation power, a tapering is introduced in the downstream\nsection of the undulator. It is found that the tapering helps in not only amplifying a target radiation\npulse but also suppressing the growth of satellite radiation pulses. Tapering allows one to achieve\na terawatt-attosecond pulse only with a 60 m long undulator. Such an attosecond X-ray pulse is\ninherently synchronized to a driving optical laser pulse; hence, it is well suited for the pump-probe\nexperiments for studying the electron dynamics in atoms, molecules, and solids on the attosecond\ntime-scale. For the realization of these experiments, a high level of synchronization up to attosecond\nprecision between optical laser and X-ray pulse is demanded, which can be possible by using an\ninterferometric feedback loop....
Table-top laser-driven hard x-ray sources with kilohertz repetition rates are an attractive\nalternative to large-scale accelerator-based systems and have found widespread\napplications in x-ray studies of ultrafast structural dynamics. Hard x-ray pulses of 100\nfs duration have been generated at the Cu Ka wavelength with a photon flux of up to\n109 photons per pulse into the full solid angle, perfectly synchronized to the sub-\n100-fs optical pulses from the driving laser system. Based on spontaneous x-ray emission,\nsuch sources display a particular noise behavior which impacts the sensitivity of\nx-ray diffraction experiments. We present a detailed analysis of the photon statistics\nand temporal fluctuations of the x-ray flux, together with experimental strategies to optimize\nthe sensitivity of optical pump/x-ray probe experiments. We demonstrate measurements\nclose to the shot-noise limit of the x-ray source....
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