Bulletin of the American Physical Society
41st Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 55, Number 5
Tuesday–Saturday, May 25–29, 2010; Houston, Texas
Session Q2: Experiments with the World's First X-ray Free Electron Laser |
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Chair: Lew Cocke, Kansas State University Room: Imperial Center |
Friday, May 28, 2010 8:00AM - 8:30AM |
Q2.00001: AMO Instrumentation for the LCLS Free Electron Laser Invited Speaker: The Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory is the world's first x-ray free electron laser (FEL) providing short (3-300 fs) intense ($\le $10$^{13}$ photons) pulses of x-rays in the 800-8000eV photon energy range. The x-ray FEL beam is produced when a short pulse of electrons from the SLAC linac is passed through up to 100m of undulator. A suite of instrumentation for AMO studies using the LCLS beam was developed and installed in the first experiment hutch of the LCLS. The first component of this instrumentation is a set of focusing optics that are used to produce an extremely high intensity in the $\sim $1$\mu$m focus of the x-ray beam. Two experimental chambers follow. The first has a pulsed supersonic gas jet with a set of electron and ion time-of-flight spectrometers arrayed around the intersection between the gas jet and the focused x-ray FEL beam. The second chamber is built to support a high collection efficiency magnetic bottle electron spectrometer designed to measure a full electron energy spectrum on each FEL pulse. Additional diagnostics in the second chamber are designed to measure the pulse intensity and beam profile. The instrumentation was successfully commissioned in the summer of 2009 and first user experiments began in October 2009. [Preview Abstract] |
Friday, May 28, 2010 8:30AM - 9:00AM |
Q2.00002: Multiphoton absorption processes using the LCLS x-ray free-electron laser Invited Speaker: Multiphoton absorption processes in the soft x-ray regime ($\sim$800 - 2000 eV) were observed at the Linac Coherent Light Source free-electron laser. Microfocusing produced very high photon flux densities, e.g., $\sim$10$^{13}$ photons/$\sim$100 fs in a $\sim$1$\mu$m focal spot, corresponding to a peak intensity of $\sim$10$^{18}$ W/cm$^2$. Ion-charge-state spectra and photoelectron/Auger-electron spectra of atomic neon were recorded as functions of x-ray energy, pulse duration, and pulse energy. As predicted [1], sequential x-ray absorption processes resulted in multiple photoelectron lines and high ion charge states, including fully-stripped Ne$^{10+}$ from six-photon absorption. At these high flux densities, two-photon ionization of both 1s electrons competes with Auger decay, as evidenced by \textit{KK--KLL} Auger-hypersatellite lines. The complex electron spectra could be disentangled using the electron energetics, angular-distributions, and measurements as a function of the pulse duration. Below the 870-eV \textit{K}-edge of neutral Ne, a two-photon resonant process was observed in which 1s $\rightarrow$ 2p excitation follows 2p ionization. At 2000 eV x-ray energy and 2 mJ pulse energy, the yields of the highest charge states were observed to decrease when the pulse duration was decreased from 230 fs to 80 fs, i.e., when the peak intensity was increased by a factor $\sim$3. The observed phenomena will be compared with results of theoretical modeling.\\[4pt] [1] N. Rohringer and R. Santra, Phys. Rev. A \textbf{76}, 033416 (2007). [Preview Abstract] |
Friday, May 28, 2010 9:00AM - 9:30AM |
Q2.00003: First Results on Ultrafast and Ultraintense X-Ray Studies of Molecular Photoabsorption using the LCLS Free Electron Laser Invited Speaker: The study of atomic and molecular inner-shell photoionization with conventional x-ray sources is dominated by processes involving the production of single core holes. However, the unprecedented short pulses and peak power at x-ray wavelengths of the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory provides new research opportunities and opens the door to study ultra fast, nonlinear x-ray physics. We have used the LCLS to investigate fundamental questions concerning laser pulse duration dependent ionization as well as examine the creation and decay of multiple core-holes. In particular, we focused on double core-holes ionization in N$_{2}$. We measured the photoelectron, Auger and secondary electron relaxation pathways subsequent to multiple core vacancies in molecules as well as the fragmentation patterns and the charge-state distributions of the resulting ions as a function of wavelength, pulse duration and intensity. The new light source allows the characterization of the molecular ionization and dissociation dynamics and provides new insight into the interaction of matter with intense short pulses. In addition we expect our results to contribute to the foundation for future imaging experiments on molecules. The LCLS photon beam was focused to about 2$\mu $m$^{2}$ area producing an intense x-ray laser beam of up to 10$^{18}$ W/cm$^{2}$. We have used x-ray pulses with duration from about 7fs to 280 fs and a photon energy of 1.1 keV to investigate the production of multiple core holes. We have observed the multiple ionization of N$_{2}$ resulting in fragment ions of up to bare N$^{7+}$ [1]. Furthermore, evidence for double core hole has been observed. The experiment was performed at the LCLS AMO beamline which is equipped with an ion time-of-flight spectrometer to determine the ion charge state distribution as well five angle and energy resolving electron time-of-flight spectrometers to detect the emitted photoelectrons and Auger electrons. \\[4pt] [1] Work done in collaboration with L. Fang, M. Hoener, O. Kornilov, M. Guehr, O. Gessner, S.T. Pratt, C. Blaga, C. Bostedt, J.D. Bozek, P. Bucksbaum, C. Buth, R. Coffee, J. Cryan, L. DiMauro, J. Glownia, E. Hosler, E. Kanter, , E. Kukk, B. Murphy and D. Rolles and N. Berrah. [Preview Abstract] |
Friday, May 28, 2010 9:30AM - 10:00AM |
Q2.00004: Coherent Diffractive Imaging at LCLS Invited Speaker: Soft x-ray FEL light sources produce ultrafast x-ray pulses with outstanding high peak brilliance. This might enable the structure determination of proteins that cannot be crystallized. The deposited energy would destroy the molecules completely, but owing to the short pulses the destruction will ideally only happen after the termination of the pulse. In order to address the many challenges that we face in attempting molecular diffraction, we have carried out experiments in coherent diffraction from protein nanocrystals at the Linac Coherent Light Source (LCLS) at SLAC. The periodicity of these objects gives us much higher scattering signals than uncrystallized proteins would. The crystals are filtered to sizes less than 2 micron, and delivered to the pulsed X-ray beam in a liquid jet. The effects of pulse duration and fluence on the high-resolution structure of the crystals have been studied. Diffraction patterns are recorded at a repetition rate of 30 Hz with pnCCD detectors. This allows us to take 108,000 images per hour. With 2-mega-pixel-detectors this gives a data-rate of more than 400 GB per hour. The automated sorting and evaluation of hundreds of thousands images is another challenge of this kind of experiments. Preliminary results will be presented on our first LCLS experiments. This work was carried out as part of a collaboration, for which Henry Chapman is the spokesperson. The collaboration consists of CFEL DESY, Arizona State University, SLAC, Uppsala University, LLNL, The University of Melbourne, LBNL, the Max Planck Institute for Medical Research, and the Max Planck Advanced Study Group (ASG) at the CFEL. The experiments were carried out using the CAMP apparatus, which was designed and built by the Max Planck ASG at CFEL. The LCLS is operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. [Preview Abstract] |
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