Bulletin of the American Physical Society
2013 Joint Meeting of the APS Division of Atomic, Molecular & Optical Physics and the CAP Division of Atomic, Molecular & Optical Physics, Canada
Volume 58, Number 6
Monday–Friday, June 3–7, 2013; Quebec City, Canada
Session J7: Invited Session: XFEL Science |
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Chair: Oliver Gessner, Lawrence Berkeley National Laboratory Room: 303 |
Wednesday, June 5, 2013 2:00PM - 2:30PM |
J7.00001: Multiwavelength anomalous diffraction at high x-ray intensity Invited Speaker: Robin Santra The coherent x-ray scattering pattern of a molecule is connected to the modulus squared of the Fourier transform of the electron density of the molecule. The phase of this Fourier transform is not measured. As a consequence, a reconstruction of the electron density---and thus of the molecular structure---is not immediately possible. In x-ray crystallography at storage-ring-based synchrotron radiation sources, the multiwavelength anomalous diffraction (MAD) method is used to determine phase information by employing anomalous scattering from heavy atoms. X-ray free-electron lasers (FELs) provide the extremely high x-ray intensity required for revealing the structure of single molecules or nanocrystals, but the phase problem remains largely unsolved. A particular challenge is that, at high x-ray intensity, samples experience severe electronic radiation damage, especially to heavy atoms, which hinders direct implementation of MAD with x-ray FELs. In the first part of the talk, I will discuss how MAD phasing can be extended to high x-ray intensity [1]. The proposed technique relies on the existence of a Karle-Hendrickson-type equation in the high-intensity regime and requires the ability to computationally predict the x-ray-induced ionization dynamics of heavy atoms. In the second part of the talk, this ability will be put to the test. I will review x-ray FEL experiments that have been carried out on atomic xenon and will compare the observations to extensive first-principles calculations [2,3]. At sufficiently high photon energies, there is good agreement between experiment and theory. However, close to inner-shell edges, which play a key role for MAD phasing, specific discrepancies are found. A strategy will be discussed that is expected to allow us to eliminate these discrepancies. \\[4pt] [1] S.-K. Son, H. N. Chapman, and R. Santra, Phys. Rev. Lett. {\bf 107}, 218102 (2011).\\[0pt] [2] B. Rudek {\em et al.}, Nature Photonics {\bf 6}, 858 (2012).\\[0pt] [3] H. Fukuzawa {\em et al.}, submitted. [Preview Abstract] |
Wednesday, June 5, 2013 2:30PM - 3:00PM |
J7.00002: Self-referencing time-domain measurements of femtosecond inner-shell dynamics Invited Speaker: Gilles Doumy X-ray radiation has been long used to address selectively atoms and to yield structural information with atomic precision. Recently, much effort has been put into extending those measurements to the fourth dimension, time, using ultrashort x-ray pulses to access the dynamics of the systems under study. The advent of X-ray Free Electron Lasers (XFEL) has quickly revolutionized the field of time resolved x-ray techniques. The availability of tunable pulses ranging from the soft to the hard x-ray region, and lasting only few tens of femtoseconds is enabling access to unprecedented temporal resolution. In addition, the huge increase in pulse brightness compared to 3rd generation synchrotron sources has opened the field of intense x-ray interaction with matter, where much is still unknown. Temporal resolution limits arise from the pulse durations of both pump and probe pulses, group velocity mismatch, as well as the timing jitter that rises inevitably between the two independent sources. A significant effort has been put into gaining the ability to measure the timing jitter for every shot, in order to be able to tag the shots depending on the relative delay, and perform post sorting analysis of the data. A precision around 25 fs (FWHM) has been demonstrated, already a considerable improvement over the uncorrected jitter around 400-500 fs (FWHM). However, if one hopes to follow the very fast dynamics of the electronic configuration, often crucial to understand physical and chemical properties, another breakthrough is needed. Importing the laser streaking techniques developed by the attophysics community, one can greatly improve the temporal resolution of pump probe experiments where electrons are collected to follow the processes. Laser streaking has been extensively used to obtain time domain information, using an intense laser field to modify the final energy of a photoelectron created during the x-ray pulse. In certain conditions, there is a one-to-one relationship between the final energy and the time of ionization during a half optical cycle of the intense laser (i.e the streaking ramp), allowing for a direct reconstruction of the temporal profile from the measured energy spectrum. Our scheme consists in measuring simultaneously the streaking of photoelectrons (PE) created by direct photoionization during the x-ray pulse itself and the streaking of other electrons corresponding to the process under study. Efficient collection of the PE streaked spectrum allows for a shot-to-shot positioning of the x-ray pulse against the streaking ramp, so that any streaked electron of interest can then be positioned relative to both the streaking ramp and the x-ray pulse. Demonstration of such a technique was done at LCLS at SLAC, attempting to measure for the first time, in a time resolved fashion, the Auger decay lifetime of light atoms (Ne and Carbon). Using x-ray pulses as short as a few femtoseconds long, and a streaking laser operating in the infrared around 17$\mu$m, femtosecond resolution should be possible in the determination of the lifetimes. [Preview Abstract] |
Wednesday, June 5, 2013 3:00PM - 3:30PM |
J7.00003: Probing nucleobase photo protection with soft x-rays Invited Speaker: Markus G\"uhr We [1] present a new method for ultrafast spectroscopy of molecular photoexcited dynamics. The technique uses a pair of femtosecond pulses: a photoexcitation pulse initiating excited state dynamics followed by a soft x-ray (SXR) probe pulse that core ionizes certain atoms inside the molecule. We observe the Auger decay of the core hole as a function of delay between the photoexcitation and SXR pulses. The core hole decay is particularly sensitive to the local valence electrons near the core and shows new types of propensity rules, compared to dipole selection rules in SXR absorption or emission spectroscopy. We apply the delayed ultrafast x-ray Auger probing (DUXAP) method to the specific problem of nucleobase photoprotection to demonstrate its potential. The ultraviolet photoexcited $\pi\pi*$ states of nucleobases are prone to chemical reactions with neighboring bases. To avoid this, the single molecules funnel the $\pi\pi*$ population to lower lying electronic states on an ultrafast timescale under violation of the Born-Oppenheimer approximation. The new type of propensity rule, which is confirmed by Auger decay simulations, allows us to have increased sensitivity on the direct relaxation from the $\pi\pi*$ state to the vibrationally hot electronic ground state. For the nucleobase thymine, we measure a decay of the $\pi\pi*$ state and a subsequent filling of the vibrationally hot ground state in 300 fs. This work was supported by the AMOS program within the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy.Portions of this research were carried out at the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory. LCLS is an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. Other portions of this research were carried out at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
Wednesday, June 5, 2013 3:30PM - 4:00PM |
J7.00004: X-ray and optical wave mixing Invited Speaker: Ernest Glover Light-matter interactions have advanced our understanding of atoms, molecules, and materials while also being central to a number of applied areas. Though optical interactions have been heavily studied, their microscopic details are often poorly understood. To date it has not been possible to directly probe the microscopic details of light-matter interactions. X-ray and optical wave mixing, specifically sum frequency generation, was proposed nearly a half century ago as an atomic-scale probe of light-matter interactions. The process is, in essence, optically modulated x-ray diffraction : x-rays inelastically scatter from optically induced charge oscillations and probe optically polarized charge in direct analogy to how conventional x-ray diffraction probes ground-state charge. Here we use an x-ray free electron laser to demonstrate x-ray/optical sum frequency generation through nonlinear interaction of the two fields in single crystal diamond. Optically modulated x-ray diffraction from the (111) planes generates a sum (x-ray $+$ optical) frequency pulse. The measured conversion efficiency ($\sim$ 10$^{-7})$ determines the (111) Fourier components of the optically induced charge and associated microscopic field that arise in the illuminated sample. To within experimental error bars the measured charge density is consistent with first principles calculations of microscopic optical polarizability in diamond. The measurements and calculations indicate that light predominantly perturbs chemical bonds in the diamond lattice. This finding should be generally applicable to covalent semiconductors and closely related materials such as graphene. A simple bond charge model reproduces the measured charge density to within $\sim$ 50{\%}, suggesting that these models can provide reasonably accurate estimates of microscopic optical polarizability in, for instance, photonic and photovoltaic devices based on silicon. The ability to measure atomic-scale charges and fields induced by light should contribute to a better understanding of materials while also creating new ways to study phototriggered dynamics. [Preview Abstract] |
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