The Ultimate X-ray Machine

Chemical effects on the dynamics of organic molecules irradiated with high intensity x rays

The interaction of a high intensity x-ray pulse with matter causes ionization of the constituent atoms through various atomic processes, and the system eventually goes through a complex structural dynamics. Understanding this whole process is important from the perspective of structure determination of molecules using single particle imaging. XMDYN, which is a classical molecular dynamics-Monte Carlo based hybrid approach, has been successful in simulating the dynamical evolution of various systems under intense irradiation over the past years. The present study aims for extending the XMDYN toolkit to treat chemical bonds using the reactive force field. In order to study its impact, a highly intense x-ray pulse was made to interact with the simplest amino acid, glycine. Different model variants were used to highlight the consequences of charge rearrangement and chemical bonds on the time evolution. The charge-rearrangement-enhanced x-ray ionization of molecules effect is also discussed to address the capability of a classical MD based approach, i.e., XMDYN, to capture such a molecular phenomenon.

Probing C60 Fullerenes from within Using Free Electron Lasers

Fullerenes, such as C60, are ideal systems to investigate energy redistribution following substantial excitation. Ultra-short and ultra-intense free electron lasers (FELs) have allowed molecular research in a new photon energy regime. FELs have allowed the study of the response of fullerenes to X-rays, which includes femtosecond multi-photon processes, as well as time-resolved ionization and fragmentation dynamics. This perspective: (1) provides a general introduction relevant to C60 research using photon sources, (2) reports on two specific X-ray FEL-based photoionization investigations of C60, at two different FEL fluences, one static and one time-resolved, and (3) offers a brief analysis and recommendations for future research.

Towards the theoretical limitations of X-ray nanocrystallography at high intensity: the validity of the effective-form-factor description

X-ray free-electron lasers (XFELs) broaden horizons in X-ray crystallography. Facilitated by the unprecedented high intensity and ultrashort duration of the XFEL pulses, they enable us to investigate the structure and dynamics of macromolecules with nano-sized crystals. A limitation is the extent of radiation damage in the nanocrystal target. A large degree of ionization initiated by the incident high-intensity XFEL pulse alters the scattering properties of the atoms leading to perturbed measured patterns. In this article, the effective-form-factor approximation applied to capture this phenomenon is discussed. Additionally, the importance of temporal configurational fluctuations at high intensities, shaping these quantities besides the average electron loss, is shown. An analysis regarding the applicability of the approach to targets consisting of several atomic species is made, both theoretically and via realistic radiation-damage simulations. It is concluded that, up to intensities relevant for XFEL-based nanocrystallography, the effective-form-factor description is sufficiently accurate. This work justifies treating measured scattering patterns using conventional structure-reconstruction algorithms.

The LAMP instrument at the Linac Coherent Light Source free-electron laser

The Laser Applications in Materials Processing (LAMP) instrument is a new end-station for soft X-ray imaging, high-field physics, and ultrafast X-ray science experiments that is available to users at the Linac Coherent Light Source (LCLS) free-electron laser. While the instrument resides in the Atomic, Molecular and Optical science hutch, its components can be used at any LCLS beamline. The end-station has a modular design that provides high flexibility in order to meet user-defined experimental requirements and specifications. The ultra-high-vacuum environment supports different sample delivery systems, including pulsed and continuous atomic, molecular, and cluster jets; liquid and aerosols jets; and effusive metal vapor beams. It also houses movable, large-format, high-speed pnCCD X-ray detectors for detecting scattered and fluorescent photons. Multiple charged-particle spectrometer options are compatible with the LAMP chamber, including a double-sided spectrometer for simultaneous and even coincident measurements of electrons, ions, and photons produced by the interaction of the high-intensity X-ray beam with the various samples. Here we describe the design and capabilities of the spectrometers along with some general aspects of the LAMP chamber and show some results from the initial instrument commissioning.

A perspective for investigating photo-induced molecular dynamics from within with femtosecond free electron lasers

Photo-induced molecular dynamics can now be investigated using free electron lasers (FELs) whose attributes are unprecedented brightness, few femtosecond pulses duration and in the near future few hundreds of attosecond pulse duration.

Calculation of x-ray scattering patterns from nanocrystals at high x-ray intensity

We present a generalized method to describe the x-ray scattering intensity of the Bragg spots in a diffraction pattern from nanocrystals exposed to intense x-ray pulses. Our method involves the subdivision of a crystal into smaller units. In order to calculate the dynamics within every unit, we employ a Monte-Carlo-molecular dynamics-ab-initio hybrid framework using real space periodic boundary conditions. By combining all the units, we simulate the diffraction pattern of a crystal larger than the transverse x-ray beam profile, a situation commonly encountered in femtosecond nanocrystallography experiments with focused x-ray free-electron laser radiation. Radiation damage is not spatially uniform and depends on the fluence associated with each specific region inside the crystal. To investigate the effects of uniform and non-uniform fluence distribution, we have used two different spatial beam profiles, Gaussian and flattop.

Femtosecond X-ray-induced explosion of C60 at extreme intensity

Understanding molecular femtosecond dynamics under intense X-ray exposure is critical to progress in biomolecular imaging and matter under extreme conditions. Imaging viruses and proteins at an atomic spatial scale and on the time scale of atomic motion requires rigorous, quantitative understanding of dynamical effects of intense X-ray exposure. Here we present an experimental and theoretical study of C60 molecules interacting with intense X-ray pulses from a free-electron laser, revealing the influence of processes not previously reported. Our work illustrates the successful use of classical mechanics to describe all moving particles in C60, an approach that scales well to larger systems, for example, biomolecules. Comparisons of the model with experimental data on C60 ion fragmentation show excellent agreement under a variety of laser conditions. The results indicate that this modelling is applicable for X-ray interactions with any extended system, even at higher X-ray dose rates expected with future light sources.