Abraham Szöke

Femtosecond diffractive imaging with a soft-X-ray free-electron laser

Theory predicts1,2,3,4 that, with an ultrashort and extremely bright coherent X-ray pulse, a single diffraction pattern may be recorded from a large macromolecule, a virus or a cell before the sample explodes and turns into a plasma. Here we report the first experimental demonstration of this principle using the FLASH soft-X-ray free-electron laser. An intense 25 fs, 4×1013 W cm−2 pulse, containing 1012 photons at 32 nm wavelength, produced a coherent diffraction pattern from a nanostructured non-periodic object, before destroying it at 60,000 K. A novel X-ray camera assured single-photon detection sensitivity by filtering out parasitic scattering and plasma radiation. The reconstructed image, obtained directly from the coherent pattern by phase retrieval through oversampling5,6,7,8,9, shows no measurable damage, and is reconstructed at the diffraction-limited resolution. A three-dimensional data set may be assembled from such images when copies of a reproducible sample are exposed to the beam one by one10.

Raman pulse compression of excimer lasers for application to laser fusion

Application of efficient ultraviolet excimer lasers such as the 248 nm KrF laser to laser fusion requires that long laser pulses be efficiently converted to short pulses at high intensity. The backward Raman amplifier is shown to be a promising candidate for this application. Gain, saturation, and limits to amplifier performance are described. It is shown that pump beams of poor spatial quality may be converted to output beams of high spatial quality. Several common gaseous vibrational Raman scatterers are discussed, and it is shown that a simple KrF-pumped backward Raman amplifier using methane at atmospheric pressure will have a saturation fluence near 1 J/cm2and can produce an output five times as intense as the pump in a ten times shorter pulse with an efficiency of about 50 percent. Design tradeoffs and possible techniques for further improving the performance of such amplifiers are discussed.

Massively parallel X-ray holography

Stefano Marchesini, 2 Sébastien Boutet, 4 Anne E. Sakdinawat, Michael J. Bogan, Sas̆a Bajt, Anton Barty, Henry N. Chapman, 6 Matthias Frank, Stefan P. Hau-Riege, Abraham Szöke, Congwu Cui, Malcolm R. Howells, David A. Shapiro, John C. H. Spence, Joshua W. Shaevitz, Johanna Y. Lee, Janos Hajdu, 4 and Marvin M. Seibert Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550, USA. Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron rd. Berkeley, CA 94720, USA∗ Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, 2575 Sand Hill Road, Menlo Park, California 94025, USA. Laboratory of Molecular Biophysics, Department of Cell and Molecular Biology, Uppsala University, Husargatan 3, Box 596, SE-75124 Uppsala, Sweden. Center for X-ray Optics, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. 6 Centre for Free-Electron Laser Science U. Hamburg, DESY, Notkestraße 85, Hamburg, Germany. Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504, USA Department of Physics and Lewis-Sigler Institute, 150 Carl Icahn Laboratory, Princeton, New Jersey 08544, USA. Department of Plant and Microbial Biology, University of California, Berkeley, 648 Stanley Hall 3220, Berkeley, California 94720, USA. (Dated: February 9, 2008)

Prepulse energy suppression for high-energy ultrashort pulses using self-induced plasma shuttering

We demonstrate the technique of self-induced plasma shuttering as a means of suppressing prepulse energy that accompanies high-energy, ultrashort laser pulses. This technique makes possible the generation of clean, high-intensity subpicosecond laser pulses even in the presence of high levels of amplified spontaneous emission from the laser system. Low prepulse energy is important in applications such as the generation of solid-density subpico-second plasmas.

Coherent X-ray diffractive imaging: applications and limitations.

The inversion of a diffraction pattern offers aberration-free diffraction-limited 3D images without the resolution and depth-of-field limitations of lens-based tomographic systems, the only limitation being radiation damage. We review our experimental results, discuss the fundamental limits of this technique and future plans.

Auger-electron cascades in diamond and amorphous carbon

We have analyzed cascades of secondary electrons in diamond and amorphous carbon generated by the thermalization of a single Auger electron. The elastic electron mean free path was calculated as a function of impact energy in the muffin-tin potential approximation. The inelastic scattering cross section and the energy loss of electrons ~expressed in terms of the differential inverse mean free path! were estimated from two ‘‘optical’’ models that utilize the measured dielectric constants of the materials. Using these data, a Monte Carlo model describing the time evolution of the cascade was constructed. The results show that at most around 20‐ 40 secondary cascade electrons are released by a single Auger electron in a macroscopic sample of diamond or amorphous carbon. Consideration of the real band structure of diamond reduces this number further. The release of the cascade electrons happens within the first 100 fs after the emission of the primary Auger electron. The results have implications to planned experiments with femtosecond x-ray sources.

Vibration-vibration energy transfer in the v 3 mode of CO 2

The vibration-vibration energy transfer of CO 2 gas initially excited to the first asymmetric stretch level (0001) has been observed. Collisional pumping to the (0111) combination level is measured by monitoring the fluorescence due to the (0111) → (0110) band. The rate constant for the process: CO 2 (0111) + CO 2 (0000) → CO 2 (0001) + CO 2 (0110) is found to be (5.3 \pm 1) \times 10^{6} s-1torr-1.

Space-time evolution of electron cascades in diamond

The impact of a primary electron initiates a cascade of secondary electrons in solids, and these cascades play a significant role in the dynamics of ionization. Here we describe model calculations to follow the spatiotemporal evolution of secondary electron cascades in diamond. The band structure of the insulator has been explicitly incorporated into the calculations as it affects ionizations from the valence band. A Monte Carlo model was constructed to describe the path of electrons following the impact of a single electron of energy E;250 eV. This energy is similar to the energy of an Auger electron from carbon. Two limiting cases were considered: the case in which electrons transmit energy to the lattice, and the case where no such energy transfer is permitted. The results show the evolution of the secondary electron cascades in terms of the number of electrons liberated, the spatial distribution of these electrons, and the energy distribution among the electrons as a function of time. The predicted ionization rates ( ;5 ‐13 electrons in 100 fs! lie within the limits given by experiments and phenomenological models. Calculation of the local electron density and the corresponding Debye length shows that the latter is systematically larger than the radius of the electron cloud, and it increases exponentially with the radial size of the cascade. This means that the long-range Coulomb field is not shielded within this cloud, and the electron gas generated does not represent a plasma in a single impact cascade triggered by an electron of E;250 eV energy. This is important as it justifies the independent-electron approximation used in the model. At 1 fs, the ~average! spatial distribution of secondary electrons is anisotropic with the electron cloud elongated in the direction of the primary impact. The maximal radius of the cascade is about 50 A at this time. At 10 fs the cascade has a maximal radius of ;70 A, and is already dominated by low-energy electrons (.50%, E,10 eV). These electrons do not contribute to ionization but exchange energies with the lattice. As the system cools, energy is distributed more equally, and the spatial distribution of the electron cloud becomes isotropic. At 90 fs, the maximal radius is about 150 A. An analysis of the ionization fraction shows that the ionization level needed to create an Auger electron plasma in diamond will be reached with a dose of ;2310 5 impact x-ray photons per A 2 if these photons arrive before the cascade electrons recombine. The Monte Carlo model described here could be adopted for the investigation of radiation damage in other insulators and has implications for planned experiments with intense femtosecond x-ray sources.

Holographic Methods in X-ray Crystallography. V. Multiple IsomorphousReplacement, Multiple Anomalous Dispersion and Non-crystallographic Symmetry

The holographic method for the recovery of the electron density of macromolecules is based on the expansion of the electron density into Gaussian basis functions. The technique makes consistent use of real- and reciprocal-space information to produce electron-density maps. It enforces positivity of the recovered electron density and makes effective use of previously known information about the electron density, such as knowledge of a solvent region or knowledge of a partial structure. In this paper, we summarize the theory underlying the holographic method, and describe how we extend the range of information that can be used by the method to include information from multiple-isomorphous-replacement (MIR) data, multiple-anomalous-dispersion (MAD) data and knowledge of non-crystallographic symmetry. The convergence properties and the limiting accuracy of the method are discussed. Its power for synthetic problems is demonstrated and the method is applied to experimentally measured MIR data from kinesin, a motor protein domain that has recently been solved. Appendix A gives a detailed description of the algorithms and the equations used in EDEN, the computer program that implements the holographic method.

Shake-up and shake-off excitations with associated electron losses in X-ray studies of proteins

Photoionization of an atom by X‐rays usually removes an inner shell electron from the atom, leaving behind a perturbed “hollow ion” whose relaxation may take different routes. In light elements, emission of an Auger electron is common. However, the energy and the total number of electrons released from the atom may be modulated by shake‐up and shake‐off effects. When the inner shell electron leaves, the outer shell electrons may find themselves in a state that is not an eigen‐state of the atom in its surroundings. The resulting collective excitation is called shake‐up. If this process also involves the release of low energy electrons from the outer shell, then the process is called shake‐off. It is not clear how significant shake‐up and shake‐off contributions are to the overall ionization of biological materials like proteins. In particular, the interaction between the outgoing electron and the remaining system depends on the chemical environment of the atom, which can be studied by quantum chemical methods. Here we present calculations on model compounds to represent the most common chemical environments in proteins. The results show that the shake‐up and shake‐off processes affect ∼20% of all emissions from nitrogen, 30% from carbon, 40% from oxygen, and 23% from sulfur. Triple and higher ionizations are rare for carbon, nitrogen, and oxygen, but are frequent for sulfur. The findings are relevant to the design of biological experiments at emerging X‐ray free‐electron lasers.