Advantages of a synchrotron bending magnet as the sample illuminator for a wide-field X-ray microscope.

In this paper the choice between bending magnets and insertion devices as sample illuminators for a hard X-ray full-field microscope is investigated. An optimized bending-magnet beamline design is presented. Its imaging speed is very competitive with the performance of similar microscopes installed currently at insertion-device beamlines. The fact that imaging X-ray microscopes can accept a large phase space makes them very well suited to the output characteristics of bending magnets which are often a plentiful and paid-for resource. There exist opportunities at all synchrotron light sources to take advantage of this finding to build bending-magnet beamlines that are dedicated to transmission X-ray microscope facilities. It is expected that demand for such facilities will increase as three-dimensional tomography becomes routine and advanced techniques such as mosaic tomography and XANES tomography (taking three-dimensional tomograms at different energies to highlight elemental and chemical differences) become more widespread.

Full-field transmission x-ray microscopy for bio-imaging.

A full-field hard-x-ray microscope at SSRL has successfully imaged samples of biological and environmental origin at 40 nm resolution. Phase contrast imaging of trabeculae from a female mouse tibia, loaded in vivo to study the effects of weight-bearing on bone structure, revealed a complex network of osteocytes and canaliculi. Imaging of cordgrass roots exposed to mercury revealed nanoparticles with strong absorption contrast. 3D tomography of yeast cells grown in selenium rich media showed internal structure.

X-ray diffuse scattering measurements of nucleation dynamics at femtosecond resolution.

Femtosecond time-resolved small and wide angle x-ray diffuse scattering techniques are applied to investigate the ultrafast nucleation processes that occur during the ablation process in semiconducting materials. Following intense optical excitation, a transient liquid state of high compressibility characterized by large-amplitude density fluctuations is observed and the buildup of these fluctuations is measured in real time. Small-angle scattering measurements reveal snapshots of the spontaneous nucleation of nanoscale voids within a metastable liquid and support theoretical predictions of the ablation process.

Carrier-density-dependent lattice stability in InSb.

The ultrafast decay of the x-ray diffraction intensity following laser excitation of an InSb crystal has been utilized to observe carrier dependent changes in the potential energy surface. For the first time, an abrupt carrier dependent onset for potential energy surface softening and the appearance of accelerated atomic disordering for a very high average carrier density have been observed. Inertial dynamics dominate the early stages of crystal disordering for a wide range of carrier densities between the onset of crystal softening and the appearance of accelerated atomic disordering.

Ultrafast bond softening in bismuth: mapping a solid's interatomic potential with X-rays.

Intense femtosecond laser excitation can produce transient states of matter that would otherwise be inaccessible to laboratory investigation. At high excitation densities, the interatomic forces that bind solids and determine many of their properties can be substantially altered. Here, we present the detailed mapping of the carrier density-dependent interatomic potential of bismuth approaching a solid-solid phase transition. Our experiments combine stroboscopic techniques that use a high-brightness linear electron accelerator-based x-ray source with pulse-by-pulse timing reconstruction for femtosecond resolution, allowing quantitative characterization of the interatomic potential energy surface of the highly excited solid.

X-ray microprobe of orbital alignment in strong-field ionized atoms.

We have developed a synchrotron-based, time-resolved x-ray microprobe to investigate optical strong-field processes at intermediate intensities (10(14) - 10(15) W/cm2). This quantum-state specific probe has enabled the direct observation of orbital alignment in the residual ion produced by strong-field ionization of krypton atoms via resonant, polarized x-ray absorption. We found strong alignment to persist for a period long compared to the spin-orbit coupling time scale (6.2 fs). The observed degree of alignment can be explained by models that incorporate spin-orbit coupling. The methodology is applicable to a wide range of problems.

Observation of structural anisotropy and the onset of liquidlike motion during the nonthermal melting of InSb.

The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements observe the delayed onset of diffusive atomic motion, signaling the appearance of liquidlike dynamics. They also demonstrate that the root-mean-squared displacement in the [111] direction increases faster than in the [110] direction after the first 500 fs. This structural anisotropy indicates that the initially generated fluid differs significantly from the equilibrium liquid.

Clocking femtosecond X rays.

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.

Atomic-scale visualization of inertial dynamics.

The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.

Multiphoton double ionization via field-independent resonant excitation.

The double ionization of xenon in the multiphoton regime has been studied at two wavelengths (0.77 and 0.79 microm) using an electron-ion coincidence technique and an intensity binned ion ratio method. Sharp resonant structures in the electron energy distribution correlated with the doubly charged ion, as well as a wavelength dependence of the Xe(2+)/Xe(+) ratio provides new insights. A mechanism involving the shelving of population in Rydberg states followed by excitation of a core electron is proposed.

Observation of a transition in the dynamics of strong-field double ionization.

The double ionization of argon and xenon in an intense laser field has been studied in detail using an electron-ion coincidence technique. The observed double ionization electron spectra in xenon show resonancelike structures here resolved for the first time. In argon, the featureless spectra are consistent with rescattering. This represents a clear transition in the dynamics of strong-field double ionization, analogous to the well-known transition between the tunneling and multiphoton regimes in single ionization.