Be measuring the degree of linear polarization we identify the orbital and spin contributions to the xray magnetic scattering in holmium. When the incident x-ray energy is tuned through the L\\\ absorption edge, we observe a fiftyfold resonant enhancement of the magnetic signal, and resonant integer harmonics. The line shapes of the two linear components scattered parallel and perpendicular to the diffraction plane are distinct in energy with a 6-eV splitting.

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Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms. DOI: 10.1103/PhysRevLett.94.114801 PACS numbers: 41.60.Cr, 41.75.Ht, 42.65.Re Ultrafast x-ray pulses are providing our first view of subpicosecond atomic motion. New sources based on high harmonic generation [1,2] and laser-produced plasmas [3] as well as femtosecond laser-sliced synchrotron emission [4] have been demonstrated. These sources produce x-ray pulses with durations of less than a few hundred femtoseconds, the time scale of vibrations in solids and molecules and the making and breaking of chemical bonds. While these sources provide the time resolution necessary to study these dynamics, their relatively low brightness limits their application and often hinders attempted experiments.A new generation of linear-accelerator-based x-ray free electron lasers (XFELs) will be more than 20 orders of magnitude brighter than laser-plasma-based sources and have the potential to produce x-ray pulses below one femtosecond in duration [5]. With x rays from an XFEL, researchers can expect to image chemistry in real time on the atomic scale. While these new XFELs will be far brighter than any other ultrafast x-ray source, their physical size and complexity introduce new challenges which, if left unaddressed, will restrict their application. A major obstacle will be the inability to precisely synchronize the time-dependent process being studied with the x-ray pulse generated by a large accelerator-based source.Subpicosecond time-dependent phenomena are typically studied with pump-probe techniques in which the dynamics are initiated by an ultrafast laser or laser-driven source and then probed after a time delay. If these experiments can be self-synchronized, with the pump and probe having a common laser source, then precise time delays can be produced using different optical path lengths. The time resolution is then limited by the overlap of the pump and probe pulses that can be as short as a fraction of a PRL 94, 114801 (2005) P H Y S I C A L

Results are presented for the first time-resolved x-ray absorption measurements with a time resolution of 300 microseconds on a dynamically evolving chemical system. By synchronizing a neodymium: yttrium-aluminum-garnet pulsed laser with the bursts of x-rays emitted from the Cornell High Energy Synchrotron Source, it was possible to monitor at room temperature the recombination of carbon monoxide with myoglobin after laser photolysis. Changes in the pre-edge structure and in the position of the iron edge of this protein were detected as a function of time.

To use the unique element-specific nature of polarized x-ray techniques to study a wide variety of problems related to magnetic materials, we have developed a dual-branch sector that simultaneously provides both hard and soft x-ray capabilities. This facility, which is located in sector 4, is equipped with two different insertion devices providing photons in both the intermediate (0.5–3 keV) and hard x-ray regions (3–100 keV). This facility is designed to allow the simultaneous branching of two undulator beams generated in the same straight section of the ring.

Calculations are presented for the femtosecond time‐evolution of intensities of beams diffracted by perfect Bragg crystals illuminated with radiation expected from X‐ray free‐electron lasers (XFELs) operating through the self‐amplified spontaneous emission (SASE) process. After examining the case of transient diffraction of an electromagnetic delta‐function impulse through flat, single‐ and double‐crystal monochromators, the propagation of a 280 fs‐duration SASE XFEL pulse of 8 keV photons through the same optics is discussed. The alteration of the sub‐femtosecond spiky microbunched temporal structure of the XFEL pulse after it passes through the system is shown for both low‐order (broad bandwidth) and high‐order (narrow bandwidth) crystal reflections. Finally, the shot‐to‐shot statistical fluctuations of the integrated diffracted intensity is simulated. Implications of these results for XFEL applications are addressed.

We have used the pulsed time structure of the Cornell High-Energy Synchrotron Source (CHESS) to carry out a nanosecond resolution time-resolved x-ray study of silicon during pulsed-laser irradiation. Time-resolved temperature distributions and interfacial overheating and undercooling were measured on 〈111〉 and 〈100〉 silicon during 25 ns UV laser pulses through the analysis of thermal expansion induced strain. The temperature gradients were found to be > 107 K/cm at the liquid-solid interface and the temperature distributions have been shown to be in agreement with numerical heat flow calculations for these laser conditions. The combined overheating and undercooling (during ∼ 10 m/s melting and ∼ 6 m/s regrowth) was measured to be 110 ± 30 K on 〈111〉 oriented silicon and 50 ± 25 K on 〈100〉 silicon. These values have been interpreted in terms of velocity coefficients of overheating and undercooling.

Parametric down conversion of X-ray photons in diamond crystals was detected in two experiments, both using the phase-matching scheme ®rst employed in the X-ray regime by Eisenberger & McCall [Phys. Rev. Lett. (1971), 26, 684±688]. The conversion events were detected by a combination of timecorrelation spectroscopy and energy discrimination, using Si drift-chamber detectors. The timecorrelation spectra give a direct comparison of the conversion rate over the accidental coincidence rate. Mechanisms for possible detection of false events and ways to cross check against them are discussed in detail.