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 P6: Probing Dynamics with XUV and X-rays |
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Chair: Steve Southworth, Argonne National Laboratory Room: 302 |
Thursday, June 6, 2013 2:00PM - 2:12PM |
P6.00001: Probing the limitations of using adiabatic passage with short XUV pulses in a Raman process Xuan Li, C. William McCurdy, Daniel J. Haxton With the aid of recently developed \textit{ab initio} implementation of the multiconfiguration time-dependent Hartree Fock method, we investigate quantum control of population transfer with a Raman process between two bound electronic states via intermediate states that are doubly excited core hole states well above the ionization threshold. We treat the example of the Li atom with intermediate states of the configuration 1s2s2p. A $\pi + \pi$ pulse approach is shown to yield a population transfer efficiency of 53\% at relatively low laser intensities, while, surprisingly, the Stimulated Raman Adiabatic Passage (STIRAP) approach leads to much lower transfer efficiency ($< 5$\%), and its characteristic robustness at high laser intensities disappear. Further analysis of competing photo-ionization processes reveals the mechanism behind the failure of the STIRAP approach in this case. [Preview Abstract] |
Thursday, June 6, 2013 2:12PM - 2:24PM |
P6.00002: Multi-electron correlation effects in atomic photoemission time delay within a classical model Q. Liao, U. Thumm A recent attosecond-streaking experiment [M. Schultze, \textit{et al}., Science \textbf{328}, 1658 (2010)] revealed a delay of 21 attoseconds between photoemission from 2s and 2p levels of neon atoms. Several independent subsequent theoretical approaches based on the time-dependent Schrodinger equation include electronic correlation at various levels of approximation, but all significantly underestimate the measured streaking delay. We propose a classical model that describes correlated photoelectron emission following the absorption an XUV photon. This model reproduces the measured delay, suggesting that the interaction between the photoelectron and other bound electrons is responsible for the large observed delay. [Preview Abstract] |
Thursday, June 6, 2013 2:24PM - 2:36PM |
P6.00003: Toward Femtosecond Time-Resolved Inner-Shell Transient Absorption Spectroscopy of Ultrafast Dynamics in Sulfur-Containing Molecules Ming-Fu Lin, Daniel Neumark, Stephen Leone, Oliver Gessner Sulfur-containing compounds play an important role in many applications such as polythiophene-based organic solar cells or the removal of sulfur compounds by hydrodesulfurization in the petroleum industry. Ultrafast relaxation dynamics (e.g. dissociation, internal conversion and intersystem crossing) of sulfur-containing molecules after photoexcitation have attracted considerable attention as a pathway to a better understanding of the fundamental chemistry of these systems. The novel technique of extreme ultraviolet (XUV) femtosecond transient absorption spectroscopy provides real-time access to the time-dependent structure and transient electronic states of molecules in the vicinity of a specific atom. The usable photon energy range of a high-order harmonic based XUV transient absorption setup has been extended up to 180~eV, enabling measurements in the vicinity of the sulfur 2p edge (165~eV). This new capability opens the route to monitor ultrafast intramolecular dynamics from the unique perspective of well-localized sulfur atoms. Preliminary results will be presented on the photoinduced ultrafast ring-opening, dissociation and vibrational relaxation dynamics in gaseous thiophene (C$_{\mathrm{4}}$H$_{\mathrm{4}}$S) and carbon disulfide (CS$_{\mathrm{2}})$. [Preview Abstract] |
Thursday, June 6, 2013 2:36PM - 2:48PM |
P6.00004: Electron dynamics in superfluid helium nanodroplets monitored by femtosecond XUV photoelectron imaging Michael Ziemkiewicz, Camila Bacellar, Stephen Leone, Daniel Neumark, Oliver Gessner Superfluid helium nanodroplets, size selected in the range from 10$^{\mathrm{4}}$ to 10$^{\mathrm{6}}$ atoms, are photoionized using a two photon pump-probe scheme. A femtosecond extreme ultraviolet (XUV) pulse electronically excites the droplet at 23.8 eV, close to but below the 24.6 eV ionization energy of a free helium atom. A second femtosecond infrared (IR) beam detaches an electron at an adjustable time delay. Due to the complexity of the droplets and the large amount of energy deposited by the XUV photon, several relaxation channels are available to the system including the ejection of single Rydberg atoms from the droplet, the escape of very low energy (ZEKE) electrons, and the formation of electronically excited helium dimers and trimers. By varying the timing of the pump and probe lasers while simultaneously analyzing the full energy distribution of product photoelectrons, we examine the effect of cluster size on electronic structure and dynamics in helium droplets. The results are discussed in the context of a differentiation between dynamics that proceed primarily in the surface-region and the bulk of the superfluid clusters. [Preview Abstract] |
Thursday, June 6, 2013 2:48PM - 3:00PM |
P6.00005: Collective Autoionization of Helium Nanodroplets Induced by Intense VUV Light Pulses Aaron LaForge Ionization dynamics of helium nanodroplets was studied by the seeded VUV free electron laser (FEL), FERMI@Elettra. Using the unique tunability of the seed laser along with the high intensity of the FEL, it is possible to resonantly excite the nanodroplets such that multiple atoms within the droplet are simultaneously excited. In this state, neighboring excited atoms can decay via energy transfer similar to a doubly excited state decaying by autoionization. As a result of multiple atoms being ionized, the droplet is coulomb exploded. This effect leads to an enhancement in the abundance of ions compared to nonresonant two photon ionization. To observe the effects, one compares the power dependencies on the ion yield for various excited states to those of non-resonant ionization and direct ionization. [Preview Abstract] |
Thursday, June 6, 2013 3:00PM - 3:12PM |
P6.00006: Ultrafast X-ray Coherent Diffractive Imaging of Pure and Doped Helium Droplets C. Bacellar, L. Gomez, J. Cryan, K. Ferguson, S. Schorb, R. Tanyag, C. Jones, J. Kwok, M. Seifrid, B. Poon, E. Malmerberg, F. Sturm, K. Siefermann, F. Weise, S. Marchesini, D. Anielski, A. Rudenko, S. Epp, L. Foucar, D. Rolles, L. Englert, M. Huth, C. Bostedt, A. Vilesov, O. Gessner Coherent diffractive imaging (CDI) experiments were performed on pure and doped helium droplets using femtosecond X-ray pulses from the Linac Coherent Light Source. The superfluid nature of helium droplets presents a rare opportunity to study the onset of macroscopic quantum phenomena in finite, sub-micron scale systems. Despite the small X-ray scattering cross sections of atomic helium, high-quality single-shot CDI data were obtained that give direct access to droplet size- and shape-distributions, which have only been determined indirectly in the past. The diffraction patterns from droplets doped with xenon atoms differ starkly from the patterns from pure droplets. Strong indications for the formation of complex xenon structures inside the superfluid helium environment are observed, giving access to information about the structure and aggregation dynamics of the dopant species. The reconstruction of real-space images from the diffraction patterns is ongoing. The results will provide new information on the dynamics of finite superfluid systems and of nanostructure formation in the extreme environment of a cryogenic nanomatrix. [Preview Abstract] |
Thursday, June 6, 2013 3:12PM - 3:24PM |
P6.00007: ABSTRACT WITHDRAWN |
Thursday, June 6, 2013 3:24PM - 3:36PM |
P6.00008: Pumping K$_{\alpha}$ Resonance Fluorescence by Monochromatic X-Ray Sources Anil Pradhan, Sultana Nahar We demonstrate the correspondence between theoretically calculated K-shell resonances lying below the K-edge in multiple ionization states of an element\footnote{A. K. Pradhan, S. S. N. Nahar, et al, J. Phys. Chem. A \textbf{113}, 12356 (2009).} (Pradhan et al 2009), and recently observed K$_{\alpha}$ resonances in high-intensity X-ray free-electron laser (XFEL) plasmas\footnote{S. M. Vinko et al, Nature \textbf{482}, 59 (2012).} (Vinko et al 2012). Resonant absorptions due to K$_{\alpha}$ transitions in aluminum ions are computed and found to reproduce experimentally observed fluorescence features. Calculated fluorescence features for titanium are presented for possible observation of K$_{\alpha}$ resonances in the 4.5-5.0 keV energy range. A possibly sustainable excitation mechanism for K$_{\alpha}$ resonance fluorescence might be implemented using two monochromatic X-ray beams tuned to the K-edge and the K$_{\alpha}$ resonant energies simultaneously. This targeted ionization/excitation would create inner-shell vacancies via Auger decay, as well as pump K$_{\alpha}$ resonances. The required X-ray fluence to achieve resonance fluorescence would evidently be much less than in the XFEL experiments, and might enable novel biomedical applications. [Preview Abstract] |
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