Bulletin of the American Physical Society
43rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 57, Number 5
Monday–Friday, June 4–8, 2012; Orange County, California
Session C4: Invited Session: Frontiers in Ultrafast X-ray Physics |
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Chair: Linda Young, Argonne National Laboratory Room: Garden 1-2 |
Tuesday, June 5, 2012 2:00PM - 2:30PM |
C4.00001: Optical laser-based THz streaking for full FEL pulse characterization Invited Speaker: Adrian Cavalieri Full temporal characterization of ultrashort, high brilliance x-ray pulses at Free Electron Laser (FEL) facilities, while elusive, will underpin their future use in experiments ranging from single-molecule imaging to extreme timescale x-ray science. This issue is especially acute when confronted with the characteristics of current generation FELs operating on the principle of self-amplified spontaneous emission, as most parameters fluctuate from pulse to pulse. We have achieved this crucial characterization by extending the techniques of photoelectron streaking originally developed for attosecond spectroscopy. In our experiments, high-intensity, optical laser generated single-cycle THz pulses were used to broaden and shift -- or streak -- the photoelectron spectrum of a noble gas target ionized by the incident FEL pulse. Due to the relatively long rise time of the THz streaking field ($\sim $600 fs), these measurements allow for the arrival-time and temporal profile of femtosecond to hundred-femtosecond FEL pulses to be determined simultaneously and on a single-shot basis. Optical laser-based THz streaking is suited for use over the full range of photon energies and pulse durations produced at FELs, from XUV to the hard x-ray regime. Experiments have now been performed at the hard x-ray Linac-Coherent Light Source at the SLAC National Accelerator Laboratory as well as at the XUV Free Electron Laser in Hamburg. Distinct temporal features as short as 50 fs FWHM have been observed in the raw pulse profile prior to any correction for instrument resolution. While these first measurements have been resolution-limited, the potential for improvement to access the sub 10-fs range has also been demonstrated, which would allow for characterization and effective application of the shortest predicted, few-femtosecond x-ray pulses in the near future. [Preview Abstract] |
Tuesday, June 5, 2012 2:30PM - 3:00PM |
C4.00002: Capturing ultrafast molecular dynamics with time-resolved x-ray absorption, x-ray emission, and x-ray scattering Invited Speaker: Anne Marie March Ultrafast, time-resolved, laser-pump, x-ray-probe experiments are powerful tools for understanding and controlling the behavior of matter at the molecular level. Transient structural changes, both geometric and electronic, of single molecules after excitation by a laser pulse can be probed with high resolution and within complex or disordered environments, such as gases and liquids, taking advantage of the superior spatial resolution, elemental specificity and penetration power of x-rays. Third generation synchrotron sources, particularly the Advanced Photon Source (APS), provide x-rays with a unique combination of properties that are well suited for precision time-resolved measurements. These include a high flux (10$^{13}$ photons/second/0.01\% bandwidth) that is distributed in short pulses ($\sim$100 ps) with moderate intensity ($\sim$10$^{6}$ photons/pulse) at a high repetition rate (MHz). Over the last decade laser-pump, x-ray-probe studies have been carried out at synchrotrons but a major challenge has been the low repetition rate (kHz) of standard amplified lasers resulting in underutilization of the synchrotron's high flux. This limitation has recently been removed with the installation of a high repetition rate laser system at 7ID-D at the APS. In this talk I will discuss measurements on the light-induced switching of Fe(II) complexes at 3.26 MHz pump-probe repetition rates which efficiently use the available x-ray flux. This efficiency enabled the complementary techniques x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES) and liquid phase x-ray scattering (XRS) to be used simultaneously to collect information on the structural and electronic dynamics on the picosecond time scale. [Preview Abstract] |
Tuesday, June 5, 2012 3:00PM - 3:30PM |
C4.00003: Time-resolved photoelectron emission from atoms and surfaces: the photoeffect revisited Invited Speaker: Uwe Thumm Streaking spectroscopy experiments enable the resolution in time of photo-ionization processes at the natural time scale (tens of attoseconds, 1 as = 10$^{-18}$ seconds) of the motion of valence electrons in atoms and solids. This ultrahigh time resolution allows the observation of an apparent ``delay-time'' \textit{difference} between the release and detection of photoelectrons from different initial states of atoms and solids. These delays are typically of the order of tens of attoseconds and are a measure of the net quantum phase that is accumulated during the \textit{entire} photoemission process, including the release, propagation, and detection of the photoelectron. I will discuss different interpretations of and contributions to photoemission delay times based on the comparison of calculated time-resolved photo-electron spectra with recent experiments [1,2]. In particular, for time-resolved photo-emission from metal surfaces [3,4], we find our calculated electron spectra to be very sensitive to details in the modeling of dielectric-response and electron-propagation effects during the laser-assisted XUV excitation and emission process [5]. The sensitivity of photoemission time delays to the plasmonic response of solid surfaces suggests the time-resolved observation of collective (plasmonic, excitonic, etc.) excitations in atoms, nano-particles, and solids. \\[4pt] [1] C.-H. Zhang and U. Thumm,\textit{ Phys. Rev}. A \textbf{82}, 043405 (2010);\\[0pt] [2] \textit{Phys. Rev}. A \textbf{84}, 033401 (2011);\\[0pt] [3] \textit{Phys. Rev. Lett}. \textbf{102}, 123601 (2009);\\[0pt] [4] \textit{Phys. Rev}. A \textbf{84}, 065403 (2011);\\[0pt] [5] \textit{Phys. Rev}. A \textbf{84}, 063403 (2011). [Preview Abstract] |
Tuesday, June 5, 2012 3:30PM - 4:00PM |
C4.00004: Clusters in intense x-ray pulses Invited Speaker: Christoph Bostedt Free-electron lasers can deliver extremely intense, coherent x-ray flashes with femtosecond pulse length, opening the door for imaging single nanoscale objects in a single shot. All matter irradiated by these intense x-ray pulses, however, will be transformed into a highly-excited non-equilibrium plasma within femtoseconds. During the x-ray pulse complex electron dynamics and the onset of atomic disorder will be induced, leading to a time-varying sample. We have performed first experiments about x-ray laser pulse -- cluster interaction with a combined spectroscopy and imaging approach at both, the FLASH free electron laser in Hamburg (Germany) and the LCLS x-ray free-electron laser in Stanford (California). Atomic clusters are ideal for investigating the light - matter interaction because their size can be tuned from the molecular to the bulk regime, thus allowing to distinguish between intra and inter atomic processes. Imaging experiments with xenon clusters show power-density dependent changes in the scattering patterns. Modeling the scattering data indicates that the optical constants of the clusters change during the femtosecond pulse due to the transient creation of high charge states. The results show that ultra fast scattering is a promising approach to study transient states of matter on a femtosecond time scale. Coincident recording of time-of-flight spectra and scattering patterns allows the deconvolution of focal volume and particle size distribution effects. Single-shot single-particle experiments with keV x-rays reveal that for the highest power densities an highly excited and hot cluster plasma is formed for which recombination is suppressed. Time resolved infrared pump -- x-ray probe experiments have started. Here, the clusters are pumped into a nanoplasma state and their time evolution is probed with femtosecond x-ray scattering. The data show strong variations in the scattering patterns stemming from electronic reconfigurations in the cluster plasma. The results will be compared to theoretical predictions and discussed in light of current developments at free-electron laser sources. [Preview Abstract] |
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