Sooheyong Lee

A single-shot transmissive spectrometer for hard x-ray free electron lasers

We report hard x-ray single-shot spectral measurements of the Linac Coherent Light Source. The spectrometer is based on a 10 μm thick cylindrically bent Si single crystal operating in the symmetric Bragg geometry to provide dispersion and high transmission simultaneously. It covers a spectral range >1% using the Si(111) reflection. Using the Si(333) reflection, it reaches a resolving power of better than 42 000 and transmits >83% of the incident flux at 8.3 keV. The high resolution enabled the observation of individual spectral spikes characteristic of a self-amplified spontaneous emission x-ray free electron laser source. Potential applications of the device are discussed.

The soft x-ray instrument for materials studies at the linac coherent light source x-ray free-electron laser.

The soft x-ray materials science instrument is the second operational beamline at the linac coherent light source x-ray free electron laser. The instrument operates with a photon energy range of 480-2000 eV and features a grating monochromator as well as bendable refocusing mirrors. A broad range of experimental stations may be installed to study diverse scientific topics such as: ultrafast chemistry, surface science, highly correlated electron systems, matter under extreme conditions, and laboratory astrophysics. Preliminary commissioning results are presented including the first soft x-ray single-shot energy spectrum from a free electron laser.

Temporal cross-correlation of x-ray free electron and optical lasers using soft x-ray pulse induced transient reflectivity.

The recent development of x-ray free electron lasers providing coherent, femtosecond-long pulses of high brilliance and variable energy opens new areas of scientific research in a variety of disciplines such as physics, chemistry, and biology. Pump-probe experimental techniques which observe the temporal evolution of systems after optical or x-ray pulse excitation are one of the main experimental schemes currently in use for ultrafast studies. The key challenge in these experiments is to reliably achieve temporal and spatial overlap of the x-ray and optical pulses. Here we present measurements of the x-ray pulse induced transient change of optical reflectivity from a variety of materials covering the soft x-ray photon energy range from 500eV to 2000eV and outline the use of this technique to establish and characterize temporal synchronization of the optical-laser and FEL x-ray pulses.

Development of a hard X-ray delay line for X-ray photon correlation spectroscopy and jitter-free pump–probe experiments at X-ray free-electron laser sources

A prototype device capable of splitting an X-ray pulse into two adjustable fractions, delaying one of them with the aim of performing split pulse X-ray photon correlation spectroscopy and pump–probe type studies was designed and manufactured. Time delays up to 2.95 ns have been demonstrated. The achieved contrast values of 56% indicate a feasibility of performing coherence-based experiments with the delay line.

High contrast x-ray speckle from atomic-scale order in liquids and glasses

The availability of ultrafast pulses of coherent hard x rays from the Linac Coherent Light Source opens new opportunities for studies of atomic-scale dynamics in amorphous materials. Here, we show that single ultrafast coherent x-ray pulses can be used to observe the speckle contrast in the high-angle diffraction from liquid Ga and glassy Ni(2)Pd(2)P and B(2)O(3). We determine the thresholds above which the x-ray pulses disturb the atomic arrangements. Furthermore, high contrast speckle is observed in scattering patterns from the glasses integrated over many pulses, demonstrating that the source and optics are sufficiently stable for x-ray photon correlation spectroscopy studies of dynamics over a wide range of time scales.

The X-ray correlation spectroscopy instrument at the Linac Coherent Light Source

A description of the X-ray Correlation Spectroscopy instrument at the Linac Coherent Light Source is presented. Recent highlights illustrate the coherence properties of the source as well as some recent dynamics measurements and future directions.

Interfacial Free Energy Controlling Glass-Forming Ability of Cu-Zr Alloys

Glass is a freezing phase of a deeply supercooled liquid. Despite its simple definition, the origin of glass forming ability (GFA) is still ambiguous, even for binary Cu-Zr alloys. Here, we directly study the stability of the supercooled Cu-Zr liquids where we find that Cu64Zr36 at a supercooled temperature shows deeper undercoolability and longer persistence than other neighbouring compositions with an equivalent driving Gibbs free energy. This observation implies that the GFA of the Cu-Zr alloys is significantly affected by crystal-liquid interfacial free energy. In particular, the crystal-liquid interfacial free energy of Cu64Zr36 in our measurement was higher than that of other neighbouring liquids and, coincidently a molecular dynamics simulation reveals a larger glass-glass interfacial energy value at this composition, which reflects more distinct configuration difference between liquid and crystal phase. The present results demonstrate that the higher crystal-liquid interfacial free energy is a prerequisite of good GFA of the Cu-Zr alloys.

Single shot speckle and coherence analysis of the hard X-ray free electron laser LCLS

The single shot based coherence properties of hard x-ray pulses from the Linac Coherent Light Source (LCLS) were measured by analyzing coherent diffraction patterns from nano-particles and gold nanopowder. The intensity histogram of the small angle x-ray scattering ring from nano-particles reveals the fully transversely coherent nature of the LCLS beam with a number of transverse mode 〈Ms〉 = 1.1. On the other hand, the speckle contrasts measured at a large wavevector yields information about the longitudinal coherence of the LCLS radiation after a silicon (111) monochromator. The quantitative agreement between our data and the simulation confirms a mean coherence time of 2.2 fs and a x-ray pulse duration of 29 fs. Finally the observed reduction of the speckle contrast generated by x-rays with pulse duration longer than 30 fs indicates ultrafast dynamics taking place at an atomic length scale prior to the permanent sample damage.

A single-shot intensity-position monitor for hard x-ray FEL sources

An inline diagnostics device was developed to measure the intrinsic shot-to-shot intensity and position fluctuations of the SASE-based LCLS hard X-ray FEL source. The device is based on the detection of back-scattered X-rays from a partially-transmissive thin target using a quadrant X-ray diode array. This intensity and position monitor was tested for the first time with FEL X-rays on the XPP instrument of the LCLS. Performance analyses showed that the relative precision for intensity measurements approached 0.1% and the position sensitivity was better than 5 μm, limited only by the Poisson statistics of the X-rays collected in a single shot.

Multiple pathways of crystal nucleation in an extremely supersaturated aqueous potassium dihydrogen phosphate (KDP) solution droplet

Significance We successfully achieve unprecedentedly deep levels of supersaturation (S ∼ 4.1) with KH2PO4 (KDP) solutions by using a newly developed device that combines electrostatic levitation with Raman and X-ray scattering. Our study reveals two interesting phenomena. One is an existence of two different solution states, that is, low-concentration KDP solution (LCS) and high-concentration KDP solution (HCS). The other is an emergence of different crystallization paths that depend on the degree of supersaturation: (i) LCS to stable KDP crystal (tetrahedral structure), (ii) LCS to HCS to a metastable KDP crystal (monoclinic structure) to stable KDP crystal. This is a direct in situ observation of multiple pathways of nucleation in aqueous solution. Solution studies have proposed that crystal nucleation can take more complex pathways than previously expected in classical nucleation theory, such as formation of prenucleation clusters or densified amorphous/liquid phases. These findings show that it is possible to separate fluctuations in the different order parameters governing crystal nucleation, that is, density and structure. However, a direct observation of the multipathways from aqueous solutions remains a great challenge because heterogeneous nucleation sites, such as container walls, can prevent these paths. Here, we demonstrate the existence of multiple pathways of nucleation in highly supersaturated aqueous KH2PO4 (KDP) solution using the combination of a containerless device (electrostatic levitation), and in situ micro-Raman and synchrotron X-ray scattering. Specifically, we find that, at an unprecedentedly deep level of supersaturation, a high-concentration KDP solution first transforms into a metastable crystal before reaching stability at room temperature. However, a low-concentration solution, with different local structures, directly transforms into the stable crystal phase. These apparent multiple pathways of crystallization depend on the degree of supersaturation.