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Intense femtosecond X-ray pulses produced at the Linac Coherent Light Source (LCLS) were used for simultaneous X-ray diffraction (XRD) and X-ray emission spectroscopy (XES) of microcrystals of Photosystem II (PS II) at room temperature. This method probes the overall protein structure and the electronic structure of the Mn4CaO5 cluster in the oxygen-evolving complex of PS II. XRD data are presented from both the dark state (S1) and the first illuminated state (S2) of PS II. Our simultaneous XRD/XES study shows that the PS II crystals are intact during our measurements at the LCLS, not only with respect to the structure of PS II, but also with regard to the electronic structure of the highly radiation sensitive Mn4CaO5 cluster, opening new directions for future dynamics studies.

Laser pulses with a power of 10' W and a duration of 10 ' s were focused onto both gas and solid targets. Strong emission of pulsed radiation at terahertz frequencies was observed from the resulting plasmas. The most intense radiation was detected from solid density targets and was correlated with the emission of MeV x rays and electrons. Results indicate that radiative processes in such plasmas are driven by ponderomotively induced space charge fields in excess of 10 V/cm. This work constitutes the first direct observation of a laser-induced wake field.PACS numbers: 52.40.Nk, 42.65.Re, 52.25.Rv, 52.35.Mw Plasmas created by high-intensity laser pulses with subpicosecond duration have received considerable attention as novel sources of radiation. The observed emission includes coherent radiation at high harmonics of the laser frequency [1], incoherent soft x-ray bursts with subpicosecond duration [2], and the generation of hard x rays with photon energies extending beyond 1 MeV [3]. At the low frequency end of the electromagnetic spectrum, strong emission of coherent far-infrared radiation (FIR) at terahertz frequencies was recently predicted [4]. This radiation results from the space charge fields developed in such plasmas. In this Letter we report the first observation of this eff'ect.The generation of strong electric and magnetic fields in laser produced plasmas has been considered before. Electric fields on the order of 10 V/cm have been inferred in experiments involving plasma-wave accelerators [5]. In the context of high-intensity short-pulse laser interaction with plasmas, electric fields of 10 V/cm [6] and magnetic fields of up to 10 G [7,8] were predicted by several groups. Our experiments allow a comparison with this previous work by direct measurement of such fields. We note that the generation of terahertz radiation through the use of femtosecond laser pulses has been considered in a variety of schemes [9]. For example, intense pulses with energies up to 0.8 pJ were produced by illuminating a biased GaAs wafer with short laser pulses [10].In our experiment, the mechanism of FIR generation involves ponderomotive forces present at the focus of an intense laser pulse. These forces generate a large density difference between ionic and electronic charges since the laser pulse length is short enough to inertially confine the ions [6,11]. This charge separation results in a powerful electromagnetic transient [4].To estimate the magnitude of the terahertz emission we employed a hydrodynamic model for the plasma dynamics. We calculated the spatial and temporal dependence of the charge density and acceleration within the focal region and thereby determined the far-field radiation pattern. The electron fiuid can be assumed to be cold, i.e. , the thermal energy is small compared to the ponderomotive energy, U~". U~" is defined in Ref. [12]. The cold Auid approximation is justified since plasmas produced by short pulse lasers tend to have temperatures~10 eV [13], while the ponderomotive energies for our experime...

We discuss the use of second-harmonic generation (SHG) as the nonlinearity in the technique of frequencyresolved optical gating (FROG) for measuring the full intensity and phase evolution of an arbitrary ultrashort pulse. FROG that uses a third-order nonlinearity in the polarization-gate geometry has proved extremely successful, and the algorithm required for extraction of the intensity and the phase from the experimental data is quite robust. However, for pulse intensities less than-1 MW, third-order nonlinearities generate insufficient signal strength, and therefore SHG FROG appears necessary. We discuss the theoretical, algorithmic, and experimental considerations of SHG FROG in detail. SHG FROG has an ambiguity in the direction of time, and its traces are somewhat unintuitive. Also, previously published algorithms are generally ineffective at extracting the intensity and the phase of an arbitrary laser pulse from the SHG FROG trace. We present an improved pulse-retrieval algorithm, based on the method of generalized projections, that is far superior to the previously published algorithms, although it is still not so robust as the polarization-gate algorithm. We discuss experimental sources of error such as pump depletion and group-velocity mismatch. We also present several experimental examples of pulses measured with SHG FROG and show that the derived intensities and phases are in agreement with more conventional diagnostic techniques, and we demonstrate the highdynamic-range capability of SHG FROG. We conclude that, despite the above drawbacks, SHG FROG should be useful in measuring low-energy pulses.