Bulletin of the American Physical Society
42nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 56, Number 5
Monday–Friday, June 13–17, 2011; Atlanta, Georgia
Session C2: Ultrafast and Intense X-rays |
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Chair: Gilles Doumy, Argonne National Laboratory Room: A602 |
Tuesday, June 14, 2011 2:00PM - 2:30PM |
C2.00001: Manipulating the motion of large molecules: Information from the molecular frame Invited Speaker: Large molecules have complex potential-energy surfaces with many local minima. They exhibit multiple stereoisomers, even at the low temperatures ($\sim$1 K) in a molecular beam, with rich intra- and intermolecular dynamics. Over the last years, we have developed methods to manipulate the motion of large, complex molecules and to select their quantum states. We have exploited this state-selectivity, for example, to spatially separate individual structural isomers of complex molecules [1] and to demonstrate unprecedented degrees of laser alignment and mixed-field orientation of these molecules [2]. Such clean, well-defined samples strongly benefit, or simply allow, novel experiments on the dynamics of complex molecules, for instance, femtosecond pump-probe measurements, X-ray or electron diffraction of molecular ensembles (including diffraction-from-within experiments), or tomographic reconstructions of molecular orbitals. These samples could also be very advantageous for metrology applications, such as, for example, matter-wave interferometry or the search for electroweak interactions in chiral molecules. Moreover, they provide an extreme level of control for stereo-dynamically controlled reaction dynamics. We have recently exploited these state-selected and oriented samples to measure photoelectron angular distributions in the molecular frame (MFPADs) from non-resonant femtosecond-laser photoionization [3] and using the X-ray Free-Electron-Laser LCLS. We have also investigated X-ray diffraction imaging and, using ion momentum imaging, the induced radiation damage of these samples using the LCLS. This work was carried out within a collaboration for which J. K\"upper, H. Chapman, and D. Rolles are spokespersons. The collaboration consists of CFEL (DESY, MPG, University Hamburg), Fritz-Haber-Institute Berlin, MPI Nuclear Physics Heidelberg, MPG Semi-conductor Lab, Aarhus University, FOM AMOLF Amsterdam, Lund University, MPI Medical Research Heidelberg, TU Berlin, Max Born Institute Berlin, and SLAC Menlo Park, CA, USA. The experiments were carried out using CAMP (designed and built by the MPG-ASG at CFEL) [4] at the LCLS (operated by Stanford University on behalf of the US DOE) [5]. \\[4pt] [1] Filsinger, Erlekam, von Helden, K\"upper, Meijer, Phys. Rev. Lett. 100, 133003 (2008); Filsinger, K\"upper, Meijer, Hansen, Maurer, Nielsen, Holmegaard, Stapelfeldt, Angew. Chem. Int. Ed. 48, 6900 (2009)\\[0pt] [2] Holmegaard, Nielsen, Nevo, Stapelfeldt, Filsinger, K\"upper, Meijer, Phys. Rev. Lett. 102, 023001 (2009); Filsinger, K\"upper, Meijer, Holmegaard, Nielsen, Nevo, Hansen, Stapelfeldt, J. Chem. Phys. 131, 064309, (2009); Nevo, Holmegaard, Nielsen, Hansen, Stapelfeldt, Filsinger, Meijer, K\"upper, Phys. Chem. Chem. Phys. 11, 9912 (2009)\\[0pt] [3] Holmegaard, Hansen, Kalh{\o}j, Kragh, Stapelfeldt, Filsinger, K\"upper, Meijer, Dimitrovski, Abu-samha, Martiny, Madsen, Nature Phys. 6, 428 (2010)\\[0pt] [4] Str\"uder et al. Nucl Instrum Meth A 614, 483 (2010)\\[0pt] [5] Emma et al. Nat Photonics 4, 641 (2010) [Preview Abstract] |
Tuesday, June 14, 2011 2:30PM - 3:00PM |
C2.00002: Realization of an atomic inner-shell x-ray laser at the Linac Coherent Light Source Invited Speaker: Since the invention of the laser fifty years ago, laser amplification of atomic transitions have been extended to increasingly high power and shorter wavelength. We report on the first successful realization of an atomic x-ray lasing scheme based on photoionization of inner-shell electrons in Neon by the Linac Coherent Light Source (LCLS). By focusing LCLS pulses of 960 eV photon energy into a dense Neon gas sample to a micrometer sized spot, a long narrow plasma column is created on a fs time scale by photoionization of a K-shell electron. Thereby, a population inversion of the 2p-1s transition in singly ionized Neon is established, lasting for only 2.7 fs due to the subsequent Auger decay of the created core hole. Fluorescence photons emitted at the front end of the plasma column get amplified by stimulated emission, resulting in ultra bright, high-intensity x-ray pulses at 850 eV photon energy of fs duration at the exit of the plasma column. The experimental results will be discussed in conjunction with theory and self-consistent gain calculations. [Preview Abstract] |
Tuesday, June 14, 2011 3:00PM - 3:30PM |
C2.00003: Conical intersection dynamics probed by homodyne high-harmonic spectroscopy Invited Speaker: High-harmonic spectroscopy is now established as a powerful method to probe the structure and dynamics of the valence shell of molecules. Recently, we have extended this technique to the time-resolved observation of chemical reactions. Exploiting the homodyne interference between the excited and unexcited molecules in a transient grating geometry, we were able to characterize the evolution of the electronic structure of Br$_2$ undergoing an adiabatic dissociation [1]. Here, we show that high-harmonic spectroscopy reveals electronic dynamics that occur when a photoexcited nitrogen dioxide molecule (NO$_2$) crosses a conical intersection. The electronic symmetry changes that occur as the molecule oscillates across the conical intersection appear as modulations in the coherently detected high-harmonic signal. Taking the measurement to longer delays, we observe the onset of the statistical dissociation dynamics leading to NO($^2\Pi$) and O($^3$P). \\[4pt] [1] H. J. W\"{o}rner, J. B. Bertrand, D. V. Kartashov, P. B. Corkum and D. M. Villeneuve, Nature {\bf 466}, 604-607 (2010) [Preview Abstract] |
Tuesday, June 14, 2011 3:30PM - 4:00PM |
C2.00004: Atomic photoionization with synchronized X-ray and optical lasers Invited Speaker: Photoionization is the dominant processes after the interaction of atoms with photons of short wavelength. New possibilities to obtain dynamical information about this extremely fast process were opened up in the last years due to the development of Free Electron Lasers, such as FLASH in Hamburg and LCLS in Stanford, with their unprecedented characteristics, especially the ultra-short temporal width of the pulses, which can be as short of a few femtoseconds, and the extremely high number of photons per pulse (about 10$^{12}$-10$^{13}$ photons/pulse) [1,2]. In a series of experiments at FLASH, the combination of XUV FEL radiation and synchronized NIR laser pulses was used to study the Above Threshold Ionization (ATI) in rare gases for the first time in a regime free from unwanted interference effects. Especially, the polarization dependence of the sideband structures in the electron spectra yields detailed insights into the photoionization dynamics, in particular into the distribution of angular momenta for the outgoing electrons [3]. Recent experiments at the LCLS have taken advantage of the very short (2-5fs) pulse durations, which are delivered by this new X-ray Free Electron Laser. This temporal width coincides with the lifetime of core hole states governing the dynamics of the Auger decay, and with the temporal width of one cycle of the electric field in the optical wavelength regime. By applying angle-resolved electron spectroscopy, the KLL Auger decay in atomic Ne was studied after excitation with few-fs X-ray (1 keV) pulses in the presence of an optical (800 nm) dressing field. The experimental spectra are marked by strong interference effects caused by the coherent emission of electrons produced during one cycle of the superimposed optical dressing field, in excellent agreement with recent theoretical work.\\[4pt] [1] C. Bostedt et al., Nucl. Instrum. Meth. A \textbf{601}, 108 (2009).\\[0pt] [2] N. Berrah et al., J. Mod. Opt. \textbf{52}, 1015 (2010).\\[0pt] [3] M. Meyer et al., Phys. Rev. Lett. \textbf{101}, 193002 (2008).\\[0pt] [4] Y. Ding et al., Phys. Rev. Lett. \textbf{102}, 254801 (2009).\\[0pt] [5] A.K. Kazansky, N.M. Kabachnik, J.Phys.B \textbf{42}, 121002 (2009); \textbf{43}, 035601 (2010). [Preview Abstract] |
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