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 N4: Photoionization of Atomic Systems |
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Chair: Robert Jones, University of Virginia Room: Garden 1-2 |
Thursday, June 7, 2012 10:30AM - 10:42AM |
N4.00001: ABSTRACT WITHDRAWN |
Thursday, June 7, 2012 10:42AM - 10:54AM |
N4.00002: Quantum Interferences in Helium ionization in the Presence of Attosecond Pulses and Strong Laser Fields Niranjan Shivaram, Henry Timmers, Xiao-Min Tong, Arvinder Sandhu In the presence of strong near-infrared (NIR) laser fields and attosecond extreme-ultraviolet (XUV) pulses containing discrete harmonics, interferences can occur between different resonance mediated ionization pathways. The phase of the final ion/electron yield depends on the phases of the light fields and quantum phases of the transition matrix elements playing a role in the ionization process. We use attosecond pulse trains to excite Helium atoms to components of laser dressed Floquet states. As the laser pulse intensity ramps on femtosecond timescales, we observe transitions between ionization channels mediated by different atomic resonances. The quantum phase of interfering paths is extracted for each channel and compared with TDSE simulations. Our results elucidate photoionization mechanisms in strong-fields and open doors for photo-absorption/ionization control schemes. In the case of degenerate Floquet resonances the electron yields and angular distributions are also strongly modified by these interferences. We also demonstrate that a two-color ionization measurement can be used to extract the time-of-birth of attosecond pulses with respect to the driving NIR field. [Preview Abstract] |
Thursday, June 7, 2012 10:54AM - 11:06AM |
N4.00003: Double photoionization of doubly-excited lithium G. Armstrong, M.S. Pindzola, A. Kheifets, M. Schuricke, G. Veeravalli, Ch. Dornes, G. Zhu, K. Joachimsmeyer, R. Treusch, A. Dorn, J. Colgan We present triple differential cross sections and recoil ion momentum distributions for double photoionization of the 1s2s2p state of lithium. Double ionization of lithium may be treated as a two-active-electron process, where the ``active'' 2s and 2p electrons move in the field of the ``frozen-core'' Li$^{2+}$ 1s state.The time-dependent close-coupling (TDCC) method is used to solve the two-electron time-dependent Schr\"{o}dinger equation in full dimensionality. This work is motivated by recent FLASH experiments, which have obtained recoil-ion momentum distributions at a photon energy of 59 eV, where the 1s2s2p state is first reached via a 1s-2p photoexcitation from the initial ground state, and may then be doubly-ionized after the absorption of a second photon. The TDCC calculations in this work treat the subsequent photoionization of this doubly-excited state. The results are compared to those obtained by the convergent close-coupling method and to measurement, and provide a first comparison between theory and experiment in this fundamental few-photon few-body problem. [Preview Abstract] |
Thursday, June 7, 2012 11:06AM - 11:18AM |
N4.00004: Angular and Ellipticity Dependence of Cesium Photoelectrons in a Strong Field Emily Sistrunk, Anthony DiChiara, Urszula Szafruga, Pierre Agostini, Louis DiMauro The ionization of atoms driven by a strong infrared (IR) laser has yielded many interesting results in noble gases in the near IR (NIR). The Keldysh parameter, $\gamma $, provides a useful metric for ionization dynamics. Utilizing the scaling of $\gamma $, experiments in cesium (Cs) in the mid-IR (MIR) can be compared to noble gases in the NIR to evaluate the effect of the atoms' electronic structure on the strong field dynamics. Cs is an attractive atomic system for strong field studies since it has only one valence electron. In conditions such that $\gamma \sim $1, resonant-like enhancements have been observed in the plateau of the photoelectron energy spectrum. These enhancements have been attributed to a combination of multiphoton excitation and rescattering dynamics. In the strong field regime, rescattering effects are expected to change rapidly with ellipticity. These plateau enhancements are present in Cs driven in the MIR and are relatively insensitive to ellipticity. Understanding this unusual insensitivity to ellipticity could yield clues as to the origin of the enhancements. [Preview Abstract] |
Thursday, June 7, 2012 11:18AM - 11:30AM |
N4.00005: Precision measurement of carrier-envelope phase dependence of ATI spectra for the noble gases using phase-tagging A.M. Sayler, T. Rathje, S. Fasold, D. Adolph, W. M\"{u}ller, D. Hoff, G. Paulus Presented are the carrier-envelope phase (CEP) and energy dependent few-cycle above-threshold ionization (ATI) spectra for Xenon, Argon, Krypton and atomic Hydrogen. This data was obtained by a phase tagging technique which is based on a novel robust, real-time, every-single-shot technique for determining the CEP via a stereographic ATI setup. The CEP is calculated and output in real-time and this information is then used to tag ATI spectra to investigate their dependence on the relative CEP. This technique along with calculations of the CEP dependence of atomic Hydrogen allow for the determination of the absolute CEP and energy dependent ATI spectra of all gasses measured. [Preview Abstract] |
Thursday, June 7, 2012 11:30AM - 11:42AM |
N4.00006: Potential Barrier Features of Three-Photon Ionization Processes in Atoms Liang-Wen Pi, Anthony Starace We report here model potential results on the frequency dependence of three-photon generalized ionization cross sections from closed subshells of rare gas and other atoms. We find dramatic, resonance-like effects in three-photon ionization processes, which can be explained by potential barriers in the effective radial potential experienced by a photoexcited electron. In the case of Ar and Xe, our calculations show that such potential barriers may affect not only the final state of the electron, but also the intermediate-state electron wave packet at energies in the vicinity of the barrier. Such effects have been demonstrated numerically as a function of frequency in two-photon ionization processes [1]. Here we show that these effects are quite general by considering the multiphoton cross sections for ionization of Ar and Xe within a single-active-electron, central-potential model.\\[4pt] [1] L.W. Pi and A.F. Starace, Phys. Rev. A \textbf{82}, 053414 (2010). [Preview Abstract] |
Thursday, June 7, 2012 11:42AM - 11:54AM |
N4.00007: Photoionization and Electron-Ion Recombination of Ar~XVI and Ar~XVII Sultana Nahar Results on photoionization and electron-ion recombination of Ar~XVII and Ar~XVII obtained from unified method will be reported. The method, based on relativistic Breit-Pauli R-matrix method and close-coupling approximation, (i) subsumes both the radiative and dielectronic recombination and (ii) provides self-consistent sets of photoionization and recombination cross sections, $\sigma_{PI}$ and $\sigma_{RC}$. Important features for level-specific $\sigma_{PI}$ and recombination rate coefficients ($\alpha_R$), such as for diagnostic w, x, y, z X-ray lines of Ar~XVII in the ultraviolet region of astrophysical spectra will be illustrated. Monochromatic decay dominates the low energy photoionization and low temperature recombination rates. However, high energy resonances in $\sigma_{PI}$ introduce a DR bump at high temperature recombination. While the 1s-2p core excitations enhance the background of $\sigma_{PI}(nSLJ)$ at n=2 thresholds. the resonances become much weaker beyond them. The extensive sets of results correspond to fine structure levels with $n \leq 10$ and $0 \leq l \leq 9$. They include 98 levels of Ar~XVI of total angular momenta 1/2 $\leq J \leq$ 17/2 and 191 levels of Ar~XVII of $0 \leq J \leq 9$. The present $\alpha_R(T)$ with temperature show good agreement with ava [Preview Abstract] |
Thursday, June 7, 2012 11:54AM - 12:06PM |
N4.00008: Ionization of Excited Atoms in Intense, Low Frequency Single-Cycle Fields Sha Li, Robert Jones We have employed intense THz pulses to explore strong-field, single-cycle ionization of low-lying Rydberg states in the low-frequency limit. In contrast to the ground-state atoms commonly used as targets for intense laser experiments, excited atoms and polar molecules can exhibit large linear Stark shifts in the presence of static or slowly varying fields. These shifts can have a significant effect on the ionization probability and dynamics in the field. In the experiments, sodium Rydberg atoms are laser-excited using two ns dye lasers and then exposed to a single-cycle, ps THz pulse produced via optical rectification in LiNbO3 of a tilted pulse front, 100fs, 780nm laser pulse. A time-of-flight spectrometer is used to record the ionization yield and energy distribution of the ejected electrons as a function of principal quantum number and the THz field strength. The measurements are compared with those of previous experiments on long-pulse microwave and ramped-field ionization of higher-lying Rydberg states and with the results of classical Monte Carlo simulations. [Preview Abstract] |
Thursday, June 7, 2012 12:06PM - 12:18PM |
N4.00009: Interatomic Coulombic Decay (ICD) in deep inner-shell vacancy cascades D. Ray, R.W. Dunford, S.H. Southworth, E.P. Kanter, B. Kraessig, L. Young, D.A. Arms, E.M. Dufresne, D.A. Walko, O. Vendrell, S.-K. Son, R. Santra The photoionization of an inner-shell electron in a heavy atom triggers a vacancy cascade with the emission of x-ray fluorescence and Auger electrons leading to its final charge states. If the atom is part of a molecule or cluster, the decay process may involve removal of the valence electrons on the neighboring atoms thereby forming several charge centers and resulting in the Coulomb explosion of the system. This phenomenon in molecules where the valence electrons on the neighboring atoms play a significant role in the decay process is called Interatomic Coulombic Decay (ICD) [1]. The focus of this work is to explore the ICD effect in XeF$_2$ [2] following K-shell ionization of the Xe atom near 34.5 keV. We compare the total charge produced following Xe K$\alpha$ or K$\beta$ fluorescence decay from atomic Xe and from Xe in XeF$_2$ molecules. We present both experimental and calculational evidence that the fluorine atoms get involved in the decay process and the molecules start undergoing structural changes during the vacancy cascade.\\[4pt] [1] L. S. Cederbaum et al., Phys. Rev. Lett. \textbf{79}, 4778 (1997).\\[0pt] [2] C. Buth et al., J. Chem. Phys. \textbf{119}, 10575 (2003). [Preview Abstract] |
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