Ultrafast XUV and X-ray light sources offer new opportunities to unravel fundamental electronic and nuclear dynamics in matter. The Chemical Dynamics program at the Ultrafast X-ray Science Laboratory is focused on the application of emerging laboratory- and accelerator-based X-ray techniques to monitor the flow of charge, mass, and energy in molecules, clusters, and interfacial systems. A brief overview of the program will be followed by the discussion of two particular showcase examples of ongoing research: The study of photoinduced charge-transfer dynamics in dye-sensitized nanocrystals by time-resolved X-ray photoelectron spectroscopy and the investigation of quantized fluid dynamics in nano- to micron-scale superfluid helium droplets by single-shot coherent diffractive imaging.

Interfacial charge transfer studies are performed at the Linac Coherent Light Source (LCLS, SLAC National Accelerator Laboratory) and the Advanced Light Source (ALS, Berkeley). Recent LCLS results demonstrate the potential of time-resolved X-ray photoelectron spectroscopy (TRXPS) to monitor charge migration in complex interfacial systems with femtosecond time resolution, chemical sensitivity and element specificity. The visible light induced transient oxidation state of N3 dye molecules adsorbed to nanocrystalline ZnO is characterized in a concerted effort of TRXPS experiments and ab-initio calculations of the interfacial electronic structure.

The superfluid nature of helium droplets presents a rare opportunity to study the onset of macroscopic quantum phenomena in sub-micron scale systems. Pure and doped helium droplets are studied by coherent diffractive imaging (CDI) using femtosecond X-ray pulses from the LCLS. Single-shot CDI data provide the most direct access yet to droplet size- and shape-distributions as well as fundamental dynamics inside the clusters. The results will be discussed in the context of the onset of quantum vorticity in finite three-dimensional quantum systems.
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The year 2013 marks the 100th anniversary of the publication of Bragg’s Law by W. L. Bragg and the publication of the first crystal structure determination by x-rays, by W.H. Bragg and W.L. Bragg. This presentation will include a brief introductory discussion regarding the knowledge of the generation and scattering of x-rays in this time period, some personal background regarding the Braggs, and how Laue’s famous photograph set the stage for this father and son team to not only win the Nobel Prize for Physics in 1915 "For their services in the analysis of crystal structure by means of X-rays," but to change the world of science for many years to come through their development of x-ray crystallography.
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The Joint Center for Energy Storage Research (JCESR) develops concepts and technologies for portable electricity storage for transportation and stationary electric storage for the electricity grid. Electrified transportation replaces foreign oil with a host of domestic electricity sources such as gas, nuclear, wind and solar, and utility scale electric storage enables the grid to bridge the peaks and valleys of variable wind and solar generation and consumer demand. JCESR looks beyond Li-ion technology to new materials and phenomena to achieve the factor of five increases in performance needed to realize these transformational societal outcomes. JCESR will leave three legacies: a library of fundamental scientific knowledge of materials and phenomena needed for next-generation batteries, demonstration of battery prototypes suitable for scale up to manufacturing for transportation and the grid, and a new end-to-end integrated operational paradigm for battery research and development spanning discovery research, design, and demonstration.
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The ability of coordination compounds to catalyze chemical reactions and absorb visible radiation makes them appealing targets for the development of photocatalysts. One of the attributes that makes transition metals excellent catalysts – a high density of frontier orbitals – can also lead to ultrafast quenching of electronic excited states. Understanding the properties of coordination complexes that dictate the electronic relaxation dynamics has practical, as well as fundamental importance. Most successful photosensitizers and photocatalysts have utilized 4d and 5d metal centers. The significant cost and low abundance of many 4d and 5d metals has inspired attempts to substitute high cost atoms with isoelectronic 3d metal complexes. But the exchange iron for ruthenium increases the charge transfer relaxation rate by roughly a factor of one million. The huge distinction in lifetimes has generally been attributed to differences in the ligand field excite state energies. But we currently still lack a detailed understanding of how ligand field excited states and charge transfer excited states interact and how this depends upon nuclear and electronic structure. We investigated the role of ligand field excited states in the relaxation dynamics of photogenerated charge transfer states in a series of iron(II) coordination compounds with hard x-ray emission spectroscopy (XES). The tremendous sensitivity of XES to the charge and spin state of the transition metal centers make these techniques ideally suited to investigating the electron dynamics in coordination chemistry. By studying mixed cyanide and bipyridine ligands with advanced x-ray spectroscopy, we discovered that the excited state decay pathway can be controlled and adjusted by systematically tuning the ligand field splitting. We demonstrated that changing the iron ligands lend to a 100-fold increase in the charge transfer excited state lifetime, a critical metric for earth abundant photosensitizers.
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Nano-structured materials and their assemblies have generated considerable scientific and industrial interest as a result of new chemical and physical interactions as their size is reduced and they are positioned into well-defined spatial arrangements. Indeed, a grand challenge in nanotechnology is to construct materials comprised of positionally encoded elements (i.e. nanoparticles) with fine control over spacing, symmetry, and composition, with single- or sub-nanometer precision and registry. The ability to exercise such control over multiple length scales and in three dimensions for a single system would, in principle, provide researchers with a route to fabrication “materials by design”, in which one could design and build a functional system with programmed chemical and physical properties, useful in material synthesis, optics, biomedicine, energy, and catalysis. In this talk, I will discuss recent progress towards this goal, by using DNA as a programmable ligand to direct the assembly of nanoparticles into crystalline arrays. DNA is ideally suited for this purpose, as synthetically tunable variations in nucleotide sequence allow for precise engineering of the nanoparticle’s hydrodynamic radius and binding properties. These factors, in turn, dictate the crystallographic symmetry and lattice parameters of the assembly. By further employing a DNA-functionalized substrate, thin-film nanoparticle superlattices can be grown in a layer-by-layer fashion with fine control over the number of particle layers in the assembly (i.e. film thickness). Importantly, the judicious choice of DNA substrate-particle interconnects allows one to tune the interfacial energy between various crystal planes and the substrate, and thereby control crystal orientation. A theoretical framework to understand these results is presented. These nanoparticle superlattices can further be patterned in arbitrary locations on a substrate using molecular printing techniques such as dip-pen nanolithography (DPN) and polymer pen lithography (PPL). The principles developed in this work represent a major advance in the bottom-up synthesis of nanomaterials and a major step towards the integration of nanoscale materials into functional device architectures. Lastly, ultrafast pump-probe studies of third-generation materials for future photovoltaics will be presented. One such novel photovoltaic material uses heavy O doping of ZnTe to generate the formation of an intermediate band within the forbidden gap, in order to improve the matching of semiconductor absorption and solar spectra. This approach is believed to become useful for realization of single junction solar cells with very high efficiencies. However, the implementation of such devices requires advanced characterization techniques. Multiphoton optical pulse excitations are demonstrated to induce multiband charge transfer dynamics in ZnTe:O films as revealed when monitoring the time-resolved photoluminescence signals.
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Presently there is great interest in the application of light in the X-ray regime, produced by high-order harmonics, to investigate novel coherent X-ray optical phenomena. Loh et al. [1] report the observation of EIT-like behavior in the extreme ultraviolet (XUV) by coherent coupling of 2s2p and 2p2 doubly excited states in He, probing with laser-produced high-order harmonics. The EIT-like phenomenon observed in their work is characterized solely by an increase in transmission over the entire unperturbed lineshape. It is the aim of our work [2] to extend the phenomenological theoretical treatment of this effect included in [1]. We present calculations based on the solution of the time-dependent Schrodinger equation in the LS-coupling configuration interaction basis set. The absorbing boundary is represented by the complex absorbing potential and we present here the analysis of the ionization yield obtained. This approach allows for more accurate treatment of the ionization continuum than presented in [1].

[1] Z.H. Loh, C.H. Greene and S.R. Leone, Chem. Phys. 350, 7 (2008).
[2] M. Tarana, C.H. Greene, Phys. Rev. A 85, 013411 (2012).
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A variety of locally written software is in use on Sectors 11 and 12 at the APS. I will describe the available software and some of the organizing principles behind its design in the hope that it may be more widely useful at other beamlines.

If time/audience interest permits I will also present an overview of a number of relevant features of the Qt framework as related to the software I have written.

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Part of the January 2013 Seminar Series Software for Beamlines. These talks present tools for beamline scientists using spec or python to control data collection or automate alignment or similar beamline tasks.
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X-ray microscopy has become a commonly available and often used tool for element specific investigations on a nanometer scale. However, with a constantly growing user community, the demand for a more flexible sample environment grows as well. In my talk, I will describe several approaches that were realized at the SSRL and the ALS to push the limits of x-ray microscopy, e.g. how to improve the resolution of PEEM microscopy without any changes to the microscope, how to measure in large magnetic fields with sub 10ps time resolution, or how to follow chemical reactions in situ.
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The ultrashort-pulse photoexcitation and measurement techniques are of tremendous interest due to their capability to uncover the ultrafast transient response of materials. Among light-based pump-probe techniques, an original approach employing low-power fs-fiber-lasers was developed to acquire, manipulate and modify a wideband spectrum of photoexcitations in thin films and nanostructures.

In epitaxial ferromagnetic films, coherent spin waves are generated with femtosecond laser pulses via thermal excitation mediated by magnon-electron and magneto-elastic coupling. The propagation speeds and attenuation lengths of exchange spin wave modes are determined during the propagation and reflection at the film boundaries, consistent with their dispersion relation. Moreover, photo-thermal excitation could be used to achieve coherent control of the magnetization vector. An optically-induced spin reorientation transition of first-order is revealed and provides a new route to coherent magnetization switching.

Another experimental effort has been focused on phonon dynamics and thermoelectric transport studies. The coherent optical phonon spectroscopy was employed during the fs laser-induced nanostructuring in binary semiconductors such as Sb₂Te₃ and InSb. Nanostructure fabrication process optimization resulted in highly ordered periodic nanostructures without the adverse effects of residual phase separation. In another case, pump-probe measurements are used to understand the behavior of acoustically mismatched thin films to further assist the design of high-Q acoustic resonators at GHz frequencies.

Lastly, ultrafast pump-probe studies of third-generation materials for future photovoltaics will be presented. One such novel photovoltaic material uses heavy O doping of ZnTe to generate the formation of an intermediate band within the forbidden gap, in order to improve the matching of semiconductor absorption and solar spectra. This approach is believed to become useful for realization of single junction solar cells with very high efficiencies. However, the implementation of such devices requires advanced characterization techniques. Multiphoton optical pulse excitations are demonstrated to induce multiband charge transfer dynamics in ZnTe:O films as revealed when monitoring the time-resolved photoluminescence signals.
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Abstract: The strong-field picture of ionization describes the physics of how an isolated atom interacts with an intense ultra-fast laser field. The basic strong-field picture is described as tunnel ionization, which is characterized by the rapid burst of an electron wave packet into the continuum, followed by the classical motion of a quasi-free electron in a strong laser field and recollision with the parent ion. Recollision physics is at the very heart of what makes strong-field science an exciting tool for probing matter on ultrafast time scales. It offers a mechanism to create Attosecond (1 as = 10-18 s) laser pulses through High-Harmonic Generation and it offers a method for controlling electron-ion collisions on sub-femtosecond (1 fs = 10-15 s) time scales.

In my talk I will discuss how wavelength scaling has offered a more robust description of the strong-field picture. In particular, long wavelength lasers provide deep access to tunnel ionization and high energy electrons (several hundred eV) for studying electron recollision. I will discuss two separate aspects of my contributions that have helped to extend the strong-field picture. First, I will discuss inelastic laser driven scattering, or non-sequential ionization, in the long-wavelength limit of a 3.6 μm laser field. Here, large recollision energies (up to 400 eV) driven at modest field strengths result in the impact ionization of charge states up to Xe6+. The multiple ionization pathways are well described by a white electron wave packet and field-free inelastic cross sections, averaged over the intensity-dependent energy distributions for (e,ne) electron impact ionization. Then, I will discuss how wavelength scaling has made possible extending the strong-field picture of ionization to condensed phase systems. Here, we have observed evidence of a dramatic new mechanism for High Harmonic Generation that is unique to crystals yet closely parallels the semi-classical analysis of the strong-field atomic picture.
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Part of the January 2013 Seminar Series Software for Beamlines. These talks present tools for beamline scientists using spec or python to control data collection or automate alignment or similar beamline tasks.
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Bent crystals have been widely used as optical elements (e.g., monochromators, focusing optics and spectrometers) of high energy synchrotron radiation beamlines. The effects of bending on the reflectivity of the crystal are discussed within the dynamical theory with a full treatment of the crystal anisotropy and biaxial bending. Such knowledge and its combination with ray tracing and wave propagation are essential in the beamline design process. Two particular examples are presented to illustrate the usage of bent crystals for modern synchrotron radiation beamlines: the design of the X-ray Powder Diffraction (XPD) beamline at NSLS-II and the optimization of high luminosity spectrometers at ESRF and XFEL.

The XPD beamline uses a sagittally bent double-Laue crystal monochromator to provide horizontally focused x-ray beam over a large energy range (30-70 keV). A multi-lamellar model is introduced and implemented in the ray tracing of the monochromator. The instrumental resolution function of the beamline is also described.

Bent crystals are also utilized for high luminosity X-ray emission detection. This presentation will compare various concepts of dispersive/non-dispersive spectrometers with different crystal geometries by means of ray tracing.
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Part of the January 2013 Seminar Series Software for Beamlines. These talks present tools for beamline scientists using spec or python to control data collection or automate alignment or similar beamline tasks.
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