Dynamical changes in the structure factor of liquid water, S(Q,t), are measured using time-resolved x-ray diffraction techniques with 100ps resolution. On short time scales following femtosecond optical excitation, we observe temperature-induced changes associated with rearrangements of the hydrogen-bonded structure at constant volume, before the system has had time to expand. We invert this data to extract transient changes in the pair correlation function associated with isochoric heating effects, and interpret these in terms of a decrease in the local tetrahedral ordering.

We report on the first demonstration of femtosecond x-ray absorption spectroscopy, made uniquely possible by the use of broadly tunable bending-magnet radiation from "laser-sliced" electron bunches within a synchrotron storage ring. We measure the femtosecond electronic rearrangements that occur during the photoinduced insulator-metal phase transition in VO2. Symmetry- and element-specific x-ray absorption from V2p and O1s core levels (near 500 eV) separately measures the filling dynamics of differently hybridized V3d-O2p electronic bands near the Fermi level.

Laminar gratings can be used to perform x-ray absorption measurements dispersively in energy, thereby making an efficient use of the available x-ray intensity. To examine the performance of a laminar grating in diffracting short wavelength x rays, efficiency measurements of a 600l∕mm grating were performed over the photon energy range from 3 to 8 keV. A peak efficiency of 5.8% was observed without surface normal rotation (SNR); with a SNR of 60° a maximum efficiency of 14.1% was measured. The measured grating efficiencies are in good qualitative agreement with values calculated by a code based on the Neviere theory. By considering both the peak efficiency and the diffracted bandwidth, a gain of 97 is obtained for the 600l∕mm grating with a SNR of 60° in comparison with a germanium (111) crystal. Sufficient energy resolution for extended x-ray absorption fine structure experiments can be achieved by a grating at short wavelengths.

Even the most basic properties of liquid carbon have long been debated due to the challenge of studying the material at the required high temperature and pressure. Liquid carbon is volatile and thus inherently transient in an unconstrained environment. In this paper we use a new technique of picosecond time-resolved x-ray absorption spectroscopy to study the bonding of liquid carbon at densities near that of the solid. As the density of the liquid increases, we see a change from predominantly sp-bonded atomic sites to a mixture of sp, sp(2), and sp(3) sites and compare these observations with molecular dynamics simulations.

The Linac-based Ultrafast X-ray source (LUX) is a proposed recirculating linear accelerator for the purpose of producing intense, tunable, high repetition rate ultrafast x-ray pulses. An angle-time or position-time correlation is induced in the electron bunches by a dipole-mode RF cavity. Undulators and wigglers are sources of synchrotron radiation. Asymmetrically-cut crystals are used as optical elements of an x-ray pulse compression scheme. X-ray pulse durations of 50-100 fs are obtained over a range of photon energies from 2 to 12 keV. An undulator beamline consists of a collimating mirror, two asymmetric crystals and Kirkpatrick-Baez mirrors and provides compressed, monochromatic and focused x-rays for time-resolved experiments.

The performance of CsI photocathodes has been characterized for use with grazing incidence soft x-rays. The total electron yield and pulsed quantum efficiency from a CsI photocathode has been measured in a reflection geometry as a function of photon energy (100 eV to 1 keV), angle of incidence and the electric field between the anode and photocathode. The total electron yield and pulsed quantum efficiency increase as the x-ray penetration depth approaches the secondary electron escape depth. Unit quantum efficiency in a grazing incidence geometry is demonstrated. A weak electric field dependence is observed for the total yield measurements; whilst no significant dependence is found for the pulsed quantum efficiency. Theoretical predictions agree accurately with experiment.

We directly measure the photoinduced insulator-to-metal transition in VO2 using time-resolved near-edge x-ray absorption. Picosecond pulses of synchrotron radiation are used to detect the redshift in the vanadium L3edge at 516 eV, which is associated with the transient collapse of the low-temperature band gap. We identify a two-component temporal response, corresponding to an ultrafast transformation over a 50 nm surface layer, followed by 40 m/s thermal growth of the metallic phase into the bulk.

Femtosecond synchrotron pulses of <200 fs duration are generated at the Advanced Light Source beamline 5.3.1 via laser manipulation of the stored electron beam. We demonstrate a peak laser acceleration of >13 MeV relative to the nominal 1.9 GeV beam energy. Femtosecond pulses are effectively isolated from the long-pulse background using the transverse dispersion of the storage ring in combination with an x-ray imaging optic and a pair of slits to achieve a signal/background ratio of similar to1.

ALS Beamline 6.0 is designed for x‐ray absorption spectroscopy with femtosecond x‐ray pulses generated by the bunch ‘slicing’ technique. The fs x‐ray pulses are isolated by imaging the source and translating slits a small vertical distance from the optical axis. Soft and hard x‐ray branch lines will cover a wide photon energy range from 120 eV to 10 keV. A soft x‐ray spectrograph will collect absorption spectra dispersively.

We present a setup which allows the measurement of time-resolved x-ray absorption spectra with picosecond temporal resolution on liquid samples at the Advanced Light Source at Lawrence Berkeley National Laboratories. The temporal resolution is limited by the pulse width of the synchrotron source. We characterize the different sources of noise that limit the experiment and present a single-pulse detection scheme.

We have performed experiments where DKDP has been irradiated by short (100 fs), laser pulses. Subsequently X-ray pulses with a duration of 100 ps were used as a probe. Time-resolved X-ray diffraction enables monitoring of the transitions between the paraelectric and ferroelectric phases. By recording the intensity of a peak only present in the paraelectric phase, we observe indications of a phase-transition following laser-irradiation of DKDP in the ferroelectric phase. We have estimated the laser heating effect, by measuring the strain (peak shifts) in the diffraction patterns. Furthermore, the orientation of the ferroelectric domains has been observed. In spite of the fact that the temperature did not rise above the Curie temperature, following interaction with this radiation, the polarization of ferroelectric domains was modified. This indicates a mechanism where short pulses impulsively excite phonons, which enable either reversal of entire domains, the shift of domain walls and/or the broadening of the domain wall widths.

Core-level photoemission spectroscopy provides a local probe of expansion dynamics and associated transient chemical properties as a highly pressurized, metallic fluid expands into vacuum following impulsive heating of a semiconductor by an intense, ultrashort laser pulse. Transient photoemission peak shifts reveal that metal-insulator transitions occur rapidly following laser heating. These experiments probe constituents species and solidification kinetics occurring in the early moments of material ejection and provide insight into how particles arise in the current laser ablation regime.

Accurate determination of the transient electronic structures, which drive photochemical reactions, is crucial in chemistry and biology. We report the detection of transient chemical changes on the picosecond time scale by x-ray-absorption near-edge structure of photoexcited aqueous [Ru(bpy)(3)](2+). Upon ultrashort laser pulse excitation a charge transfer excited state having a 300 ns lifetime is formed. We detect the change of oxidation state of the central Ru atom at its L3 and L2 edges, at a temporal resolution of 100 ps with the zero of time unambiguously determined.

Time-resolved x-ray spectroscopy at the Si L edges is used to probe the electronic structure of an amorphous Si foil as it melts following absorption of an ultrafast laser pulse. Picosecond temporal resolution allows observation of the transient liquid phase before vaporization and before the liquid breaks up into droplets. The melting causes changes in the spectrum that match predictions of molecular dynamics and ab initio x-ray absorption codes.

We report, to our knowledge, the first use of pump-probe photoemission spectroscopy to study core-level dynamics during a laser-driven phase transition. Synchrotron x-rays probe the kinetics of cluster formation during the initial moments of femtosecond laser ablation.

Time-resolved x-ray diffraction with picosecond temporal resolution is used to probe the product state of a coherent control experiment in which a single acoustic mode in a bulk semiconductor is driven to large amplitude or canceled out. It is demonstrated that by shaping ultrafast acoustic pulses one can coherently control the x-ray diffraction efficiency of a crystal on the time scale of a vibrational period, with application to coherent switching of x-ray beams.

Time-resolved x-ray absorption spectroscopy is used to study the electronic structure of laser heated foils of silicon and carbon on time scales early enough to observe the liquid phase before break-up into droplets.

We demonstrate a novel scheme for generating ultrashort pulses of synchrotron radiation. Our approach is to create femtosecond time-structure on a long electron bunch by using a femtosecond laser pulse to modulate the energy of an ultrashort slice of the bunch Femtosecond synchrotron pulses are directly measured from a bend-magnet beamline at the Advanced Light Source (ALS). The measured pulse duration is determined by the storage ring dispersion integrated from the wiggler to the radiation bend-magnet. A new beamline, dedicated to ultrafast X-ray spectroscopy is currently under construction at the ALS and will provide X-ray pulses of <100 fs duration. The femtosecond time structure is invariant over the entire spectral range of bend-magnet emission from the near infrared to the X-ray regime, making this a powerful tool for femtosecond X-ray spectroscopy