XDL2011 Workshop 3 Abstracts

Ultra-fast Science with "Tickle and Probe"
Monday, June 20th - Tuesday, June 21st, 2011

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Organizers: Robert Schoenlein (Lawrence Berkeley National Laboratory), Brian Stephenson (Argonne National Laboratory), Eric Dufresne (APS), & Joel Brock (Cornell University)

Workshop Agenda (html)

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What is the 'Ideal' X-ray Source?

Chi-Chang Kao
SLAC National Accelerator Laboratory

The recent successful commissioning of the Linac Coherent Light Source (LCLS) at SLAC has clearly demonstrated the importance of accelerator physics R&D, and stimulated several new free electron laser (FEL) projects worldwide, including a high repetition rate FEL proposed by LBNL in the US. In parallel, there also has been significant progress made in energy recovery linac R&D, and the design of ultimate storage rings. More importantly, there is a continued growth of photon science in new research directions that demands better sources. So, it is timely to examine the "ideal" x-ray source for the main categories of experimental techniques in spectroscopy, scattering and imaging based on these progresses, and provide critical input to accelerator physics R&D for future sources.

Expected Performance of CW ERL & USR Ultra-fast Hard X-ray Sources

Don Bilderback
Cornell University

ERLs and USRs are under consideration for next generation, high-duty cycle (>MHz rep rates), coherent x-rays sources. They both feature extremely high average spectral brightness, diffraction-limited performance and are the response to 3rd generation storage ring users/developers who would like "more x-ray flux, smaller x-ray beam size, more coherent x-ray flux on sample, higher energy resolution probes and/or short pulses for repetitive probing". In most cases, the x-ray beam should minimally impact the sample under study. We review the general features of ERLs and USRs in the context of the Cornell ERL and PEP-X as two well-developed examples from the x-ray community. Additionally, examples of utilization of the advanced properties of these machines will be given. The first example explores using the timing structure to repetitively probe the response of x-ray excited optical luminescence. The second uses the extremely high average brightness to conceptually develop ideas toward a confocal x-ray microscope that is designed to image single atoms based on either Thompson scattering or x-ray fluorescence.

High-repetition-rate Ultrafast X-ray Experiments with Accelerator-based Sources

Aaron Lindenberg
SLAC National Accelerator Laboratory

High repetition-rate ultrafast x-ray studies at low peak intensities provide new opportunities for investigating dynamical processes in space and time, complementary to existing free electron laser-based sources. I will discuss recent efforts and results in this direction at SSRL with few picosecond x-ray pulses at MHz repetition rates as they relate to the development of ERL and USR sources. Associated scientific opportunities in materials science, and experimental challenges, including sample refreshing, laser technology, and the development of novel pump sources at THz frequencies will also be discussed.

Rapid Chemical and Physical Processes in Solution

Edward Castner
Rutgers University

We will discuss some classic and some hot topics in condensed phase chemical physics that require time-resolved x-ray probing in order to create breakthroughs in the field. The focus will be on chemical processes in solution triggered by an ultrashort laser pulse; the evolution of the system can then be probed by time-resolved x-ray spectroscopy (e.g., XAFS/EXAFS), small-to-wide angle scattering, or inelastic scattering.

Topics will be presented to include a number of problems relevant to solving critical energy needs. These may include confined liquids (crowded cellular environments; battery, fuel cell and ultracapacitor materials; and fluids in nanoporous materials), energy flow and trapping in light-harvesting photosynthetic assemblies and their artificial mimics; and interfacial studies of electrolyte/metal or electrolyte/semiconductor interfaces. Finally, illustrative examples of things that can be learned by time-resolving the existing studies of ionic liquids by SAXS/WAXS and neutron methods will be presented. Topics for discussion will include the challenges presented by the small atomic numbers for most of these systems, and the constraints posed by x-ray pulse repetition rates, as well as optimal ranges of x-ray wavelengths.

Using Optical Knobs to Control Photoinitiated Reactions

Roseanne Sension
University of Michigan

Optical pulses are characterized by bandwidth, spectrum, polarization, mode, phase, and intensity. All of these parameters can be modified in the interaction between the pulse and a material system to manipulate spectroscopic observables and to influence or control photoinitiated reactions. This talk will focus on the examples of optical control in condensed phase molecular systems and will look to potential applications in the future. New ultrafast X-ray sources will provide a tool capable of probing atomic structure directly while also expanding the capacity to probe details of electronic structure. The potential to couple optical control with X-ray probes will enable new important insight into reaction dynamics.

Ultrafast X-ray Studies of Complex Materials: Science Challenges and Opportunities

Robert Schoenlein
Lawrence Berkeley National Laboratory

Ultrafast X-ray science is an emerging new field of research, enabling measurements of matter at fundamental time-scales, spatial-scales, and energy-scales. Understanding and controlling matter at these scales is key for numerous science applications ranging from: developing new approaches to harvesting solar energy; to exploiting emergent phenomena that underpin the exotic properties of complex correlated systems. This talk will focus on ultrafast visible and X-ray studies of complex materials - particularly transition-metal oxides, and will provide an overview of the science that motivates the development of new ultrafast X-ray sources. Understanding the complex interplay between atomic structure and electronic structure and properties in these materials is a fundamental challenge due to the strong coupling between charge, spin, orbit, and lattice vibrational degrees of freedom. Time-resolved measurements provide a new window to this problem by enabling the study of coupled interactions on time scales shorter than the underlying correlations. X-rays are ideal probes of atomic structure, and offer important advantages for probing electronic structure as well (valence states, bonding geometry etc.) The application of X ray structural probes on the ultrafast time scale is becoming an important new tool for understanding complex materials.

Time-resolved Diffuse Scattering

David Reis
SLAC National Accelerator Laboratory

A high repetition rate femtosecond hard x-ray source would be uniquely suited for studying electron-phonon and phonon-phonon coupling near equilibrium by bringing an unprecedented combination of atomic scale time- and momentum resolution. In particular, it opens the possibility of performing inelastic x-ray scattering in the time domain that allows direct access to anharmonic decay channels that cannot be measured by optical or neutron scattering techniques. Here we present recent results on time-resolved x-ray diffuse scattering from non-equilibrium phonons in photoexcited InP. The results show a delayed emission of large-wavevector transverse phonons; however with the current resolution limit from third generation synchrotrons we are unable to capture the earliest stages of energy relaxation between the electrons and phonons and the phonons with themselves.

X-ray Transient Absorption Spectroscopy: A Decade and Beyond

Lin Chen
Northwestern University

During the past decade, x-ray transient absorption spectroscopy (XTA, or LITR-XAS, laser-initiated x-ray absorption spectroscopy) analogous to commonly used optical transient absorption, has been developed to obtain molecular structures along chemical reaction pathways. XTA uses the "pump-probe" approach to gain the time-resolution limited by the x-ray pulse duration, the same approach being used in ultrafast transient optical spectroscopy. Using x-ray pulses with high photon flux from synchrotron sources, transient electronic and molecular structures of metal complexes have been studied in disordered media from homogeneous solutions to heterogeneous solution-solid interfaces. Examples from the studies at synchrotron sources are summarized, including excited state metalloporphyrins, metal-to-ligand-charge-transfer (MLCT) states of transition metal complexes, and charge transfer state of metal complexes at the interface with semiconductor nanoparticles. Prospective in utilizing XTA in x-ray free electron laser (FEL) facilities will be discussed. We envision the new developments at synchrotron x-ray facilities and x-ray FEL will make many breakthroughs in visualizing molecular movies and snapshots, which will ultimately enable the control of chemical reaction pathways.

X-ray Probes of Laser-controlled Molecules in Gases and Solutions

Anne Marie March
Advanced Photon Source

Over the last decade, ultrafast time-resolved laser/x-ray pump-probe experiments have been carried out at 3rd generation synchrotron sources, establishing these techniques as powerful tools for understanding and controlling the behavior of matter at the molecular level. Transient structural changes, both geometric and electronic, of single molecules after excitation by a laser pulse can be probed with high resolution and within complex or disordered environments such as gases and liquids, taking advantage of the superior spatial resolution, elemental specificity and penetration power of x-rays. Technological advances in lasers, x-ray detectors, and x-ray optics are continually improving the quality of such measurements and expanding the possible x-ray measurement methods for time resolved experiments. This talk presents recent efforts implementing high-repetition-rate laser/x-ray pump-probe techniques, which can match the megahertz x-ray pulse repetition rates provided by synchrotron sources and can therefore take full advantage of the high x-ray flux. Important considerations for carrying out pump-probe experiments on gas and liquid phase samples at megahertz repetition rates are highlighted. These advances pave the way for experiments that efficiently utilize future x-ray sources that offer improved temporal resolution, but perhaps at the cost of total flux. This talk will also discuss the possibility of using shaped laser pulses for laser/x-ray pump-probe experiments, where the amplitude and phase of the laser electric field are sculpted and controlled. X-ray spectroscopy techniques could provide novel and versatile feedback mechanisms within coherent control schemes to allow for quantum control of molecular dynamics in complex environments.

Ferroelectrics at the ERL

Carol Thompson
Northern Illinois University

Ferroelectric systems combine technologically interesting functionalities with the accompanying structural responses upon which these properties depend. The structural distortions can be driven with direct electrical stimulation. Such electrical characterization methods are typical approaches for studying polarization reversal and switching at ultra-fast time scales, and they provide the indirect evidence for structural modes active at various time and length scales. Acoustic, mechanical, and thermal, as well as optical stimulations are also able to drive response in ferroelectric systems. It is clear that time-resolved characterization of the underlying structural phenomena are particularly well suited to x-ray techniques as evidenced by the growing number of efforts to apply ultra-fast x-ray scattering and spectroscopy techniques to a study of ferroelectric materials.

We will discuss impacts an ERL might have on studies addressing questions such as polarization reversal and domain dynamics during switching in ferroelectrics, and how particular characteristics of an ERL provide unique tools for investigations on high quality ferroelectric oxide heterostructures.

Molecular Switches and Molecular Machines Investigated with Ultrafast Pulsed X-ray Radiation

Simone Techert
IFG Structural Dynamics of (bio)chemical Systems,
Max Planck Institute for Biophysical Chemistry, Goettingen

Characteristic for all chemical reactions are bond breaking and bond making processes. Our vision is to optimize chemical reactions towards specific product states by a clever combination of chemical site-specificity, self-assembly and state-selectivity which can be "tuned" from orbital control through the structure of the local environment and selective excitation schemes (heat / optical pulses) to bulk structural changes - or to say it in other words - from the simple to the complex. We would like to understand - what are the driving forces of environment tuning chemistry? Most motors are defined through gradients in the chemical potential. Are chemical redox potential changes or the changes of chemical potential the most efficient chemical ways for storing energy?

To do so we need to gain a deeper understanding of the mechanism of chemical reactions from a structural point of view - besides its understanding of energetic. In order to elucidate information about reactions and their pathways multidimensional reaction landscapes are required for their description - not only in the energy coordinate but also in the reaction coordinate. In order to elucidate information about the reaction coordinate of complex systems we apply time-resolved x-ray techniques allowing us to obtain a real-time picture of the structural dynamics of chemical and biochemical systems in the crystalline and in the liquid phase.

Common for all time-resolved x-ray experiments is the applied pump / probe scheme, where an optical pump-laser initiates a reaction whose structural time evolution is then investigated by x-ray probe pulses at various time delays. The x-ray photon-in / photon-out techniques are based on diffraction or spectroscopic techniques like near edge spectroscopy or x-ray emission spectroscopy. Meanwhile x-ray spectroscopic techniques probe the local environment around specific atoms in a molecule such as orbitals, crystallographic experiments (monochromatic or Laue) reveal the structure of the bulk of periodic systems. Time-resolved diffuse x-ray scattering experiments give information about the structure of liquids.

In the current contribution we will present our latest efforts in that respect. We will reflect capabilities and limitations of state-of-the-art x-ray techniques for the investigation of two different kind of chemical reactions in complex environment: addition reactions in the solid state and dissociation reactions in the liquid state. We will shortly discuss our current status in reaching this goal (proof-of-principle experiments with free electron laser radiation [1], [2]) and finish the talk with a reflection how the presented investigations of chemical reactions would benefit by utilizing x-ray generated within an ERL.

[1] I. Rajkovic et al.; "Diffraction Properties of Periodic Lattices under Free Electron Laser Radiation", Phys. Rev. Lett. 104, 125503-6 (2010)
[2] J. Hallmann, et al.; "First Steps towards Probing Chemical Reaction Dynamics with Free Electron Laser Radiation", J. Phys. B 43, 194009-194016 (2010)

Time-resolved X-ray Spectroscopies and Scattering with One Trillion Photons

Christian Bressler
European XFEL GmbH

Structural Dynamics with hard x-radiation offers to add new observables for elucidating the detailed steps of chemical reactivity. X-ray absorption fine structure (XAFS) spectroscpoy reveals details about the locsl geometric and electronic structure around selected atoms, where the x-ray absorption near-edge spectroscopy (XANES) delivers additional information about unoccipied orbitals (LUMO's). X-ray emission spectroscopy (XES, RIXS) reveals details about occupied electronic states including spin-sensitive information, while x-ray diffuse scattering of photoexcited molecules in solution can deliver details about the guest-host interactions and the thermal response around the solute.

We have recently combined these complementary tools into one single experiment in a ttime-resolved pump-probe configuration to unnderstand the combined electronic, nuclear and spin degrees of freedom during dynamic processes in molecular systems. Picosecond studies at 3rd generation synchrotrons at MHz repetition rates permitted to exploit the full flux available at synchrotrons for time-resolved studies, while initial femtosecond XANES studies at the LCLS revealed dynamic processes during the initial steps of photochemical reactivity. Both groups of studies rely on the use of one trillion photons per data point, which is 4-6 oders of magnitude larger than conventional studies with 1 kHz laser systems (100 ps), or femtosecond sliced SR.

This talk will present current and future prospects with new femtosecond x-radiation sources

Toward Fourier-limited X-ray Science

Shin-ichi Adachi
High Energy Accelerator Research Organization, KEK

The future target beyond diffraction-limited X-ray will be Fourier-limited X-ray. Fourier limit is understood as the lower limit for the pulse duration which is possible for a given optical spectrum (or energy resolution) of a pulse. In order to realize Fourier limit in X-ray regime, two schemes seem to be promising, which are seeded XFEL¹ and oscillator-based XFEL². Possible sciences newly opened by Fourier-limited X-ray will be discussed.

Current status of time-resolved X-ray activities and ERL R&D projects at KEK will be mentioned as well.

References:

1. J. Feldhaus, J. Arthur, and J.B. Hastings; J. Phys. B: At. Mol. Opt. Phys. 38 S799-S819 (2005)
2. K.-J. Kim, Y. Shvyd'ko, S. Reicher, Phys. Rev. Lett. 100 244802 (2008)