Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session C4: Photoionization and Photodetachment Processes I |
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Chair: Norrah Berrah, Western Michigan University Room: Clark Hall 108 |
Wednesday, May 20, 2009 2:00PM - 2:12PM |
C4.00001: Quantitative rescattering theory for laser-induced high-energy plateau photoelectron spectra Zhangjin Chen, A.T. Le, Toru Morishita, C.D. Lin We present a quantitative rescattering (QRS) theory to describe high-energy photoelectrons generated by intense laser pulses. In the QRS, the momentum distributions of these electrons are expressed as the product of a returning electron wave packet with the elastic differential cross sections (DCS) between free electrons with the target ion. We show that the returning electron wave packets are determined mostly by the lasers only, and can be obtained from the strong field approximation. The validity of the QRS model is examined by checking against accurate results from solving the time dependent Schr\"odinger equation. We further show that experimental photoelectron spectra for a wide range of laser intensity and wavelength can be explained by the QRS theory, and that the DCS between electrons and target ions can be extracted from experimental photoelectron spectra. [Preview Abstract] |
Wednesday, May 20, 2009 2:12PM - 2:24PM |
C4.00002: Study of shake-up states in helium by XUV-IR pump-probe experiments S. Nagele, J. Feist, R. Pazourek, E. Persson, J. Burgd\"orfer, B.I. Schneider, L.A. Collins The rapid progress in the creation of attosecond pulses paves the way towards time-resolved control and observation of ultrafast electronic dynamics. In a recent XUV-IR pump-probe experiment Uiberacker \emph{et~al.} [1] studied the ionization dynamics of shake-up states in Neon ions. The overall stepwise structure of the resulting double ionization yield as a function of the delay time between the two pulses results from incoherent tunneling of the excited shake-up states. However, recent theoretical studies [2,3] suggest that coherent effects play an important role as well. In addition, the influence of the IR field on the shake-up process might have significant effects. Since a full \emph{ab initio} treatment of Ne atoms in external fields is not feasible, we will study the process for helium where the full multi-electron dynamics can still be solved numerically. In particular, we will investigate the role of coherent effects, electronic interactions, and the presence of the IR field in the shake-up process. $[1]$ M.~Uiberacker et al., \emph{Nature} \textbf{446}, 627 (2007) $[2]$ A.K.~Kazansky et al., \emph{EPL} \textbf{82}, 13001 (2008) $[3]$ S.~Nagele et al., \emph{to be published} [Preview Abstract] |
Wednesday, May 20, 2009 2:24PM - 2:36PM |
C4.00003: Interference between nonsequential and sequential two-photon double ionization in short XUV pulses J. Feist, R. Pazourek, S. Nagele, E. Persson, J. Burgd\"orfer, L.A. Collins, B.I. Schneider We investigate the effects of electron correlation in the two-photon double ionization of helium for ultrashort XUV pulses with durations ranging from a hundred attoseconds to a few femtoseconds. We perform time-dependent \emph{ab initio} calculations of the two-electron Schr\"odinger equation. For photon energies above $65.4\,$eV, two-photon double ionization can proceed via two different channels: one where the intermediate state after one-photon absorption is the ground state of the He$^+$ ion, and another where it is an excited (shake-up) state. The ``nonsequential'' contribution from the ground state channel can then interfere with the ``sequential'' contribution from the shake-up channel. The relative strength of these contributions is controlled by the pulse duration. This observation may provide a route toward measuring the pulse duration of FEL pulses. [Preview Abstract] |
Wednesday, May 20, 2009 2:36PM - 2:48PM |
C4.00004: Connecting field ionization to photoionization via 17 GHz microwave fields Joshua Gurian, Haruka Maeda, Thomas Gallagher Microwave ionization of hydrogenic Rydberg atoms at frequencies below the classical Kepler frequency occurs by direct field ionization and is well understood. Microwave ionization is less understood as the microwave frequency increases beyond the Kepler frequency, $\omega=1/n^3$, towards the photoionization limit, $\omega \rightarrow 1/n^2$, where ionization by absorption of one microwave photon is possible. Here we present first results connecting field ionization to photoionization using a 17.05 GHz microwave field. Ionization requiring one to seven photons is clearly resolved, but all of these processes require similar microwave fields. In addition the fields required for single photon ionization are approximately two orders of magnitude lower than predicted by perturbation theory. This work has been supported by the National Science Foundation. [Preview Abstract] |
Wednesday, May 20, 2009 2:48PM - 3:00PM |
C4.00005: Photoionization microscopy: experiment and theory Christian Bordas, Mahdi Harb, Samuel Cohen, Franck Lepine, Francis Robicheaux, Marc J.J. Vrakking Photoelectron imaging spectroscopy has emerged as a powerful tool capable of giving increasing insight into microscopic properties of matter. In standard velocity map imaging the recorded image allows a direct reconstruction of the initial 3D velocity distribution of the electrons. When this technique is improved to study meV electrons quantum interferences become visible in the image. When ionization of hydrogenic atoms occurs via a Stark resonance in the continuum, the observed image represents a direct macroscopic projection of the bound component of the electronic wavefunction. This fully justifies the designation of photoionization microscope given to this experiment. Experimental results on xenon and lithium atoms will be presented at the conference. More specifically the high sensitivity to the resonant character of the excitation in lithium, as opposed to xenon, will be illustrated and discussed within the framework of wavepacket propagation calculations. [Preview Abstract] |
Wednesday, May 20, 2009 3:00PM - 3:12PM |
C4.00006: Double Photoionization of Magnesium from Threshold to 70 eV Ralf Wehlitz, Pavle Jurani\'c, Dragan Luki\'c We have determined the double-to-single photoionization ratio of magnesium from 22 to 70 eV photon energy using monochromatized synchrotron radiation and the ion time-of-flight method\footnote{R.\ Wehlitz, P.N.\ Jurani\'c, and D.V. Luki\'c, PRA {\bf 78}, 033428 (2008)}. Our results compare reasonably well with recent theoretical calculations\footnote{A.S. Kheifets and I. Bray, Phys. Rev. A {\bf 75}, 042703 (2007)} in the non-resonant energy region below 54 eV. Here, the photon energy dependence of the ratio can be modeled by a suitably scaled helium double-to-single photoionization ratio. However, to our surprise, a previously proposed scaling model for that ratio does not work in the case of Mg. From the near-threshold double-photoionization cross section we estimate the range of validity of the Wannier threshold law to be about 0.7 eV with a rather small cross section near threshold. At photon energies above 54 eV the ratio is strongly affected by $2p \rightarrow n\ell$ inner-shell excitations and rises sharply from a non-resonant value of about 1\% below the $2p$ thresholds to about 1000\% above thresholds\footnote{R.\ Wehlitz and P.N.\ Jurani\'c, Phys.\ Rev.A {\bf 79}, 013410 (2009)}. [Preview Abstract] |
Wednesday, May 20, 2009 3:12PM - 3:24PM |
C4.00007: Double Photoionization of Be and Mg Atoms using the $R$-Matrix with Pseudo-States Method Donald Griffin, Mitch Pindzola, Connor Ballance, James Colgan We report on $R$-matrix with pseudo-states (RMPS) calculations of the total cross sections for double photoionization of Be and Mg atoms from their singlet ground and triplet metastable terms. Time-dependent close-coupling (TDCC) calculations are also carried out for the double photoionization cross sections of Mg from its ground term. The RMPS total ground-term cross sections for Be and Mg are in good agreement with results from TDCC and convergent close-coupling (CCC) calculations at lower energies, but are above them at higher energies. They are also in good agreement with the results of synchrotron measurements. The total cross sections for double photoionization from the triplet metastable terms of Be and Mg are found to be 2-3 times smaller than those from their singlet ground terms. [Preview Abstract] |
Wednesday, May 20, 2009 3:24PM - 3:36PM |
C4.00008: Photoionization and Electron-Ion Recombination of Neutral Cr Using the Unified Method Sultana Nahar The inverse processes of photoionization and electron-ion recombination of neutral chromium, (Cr~I + h$\nu \leftrightarrow$ Cr~II + e) for the ground and excited states are studied in detail using the unified method. The unified method based on close-coupling approximation and R-matrix method (i) subsumes both the radiative and dielectronic recombinations for the total rates and (ii) provides self-consistent sets of photoionization cross sections $\sigma_{PI}$, and state-specific and total recombination rates $\alpha_{RC}$. The new results for the total ground and excited state cross sections show enhancement in the background at the first excited core threshold, $3d^44s~^5D$ state. Prominent PEC (phot-excitation-of-core) resonance due to the dipole allowed transition in the core is found in photoionization cross sections of most of the excited states. Results are presented for all septet and quintet states with $n\leq$ 10 and $l\leq$ 9 that couple to the core ground state $^6S$ and contribute to the total recombination rate. The total recombination rate shows at relatively low temperature, two recombination peaks, one at 630 K and the other one at 40,000 K. [Preview Abstract] |
Wednesday, May 20, 2009 3:36PM - 3:48PM |
C4.00009: Magnetic field structure in photodetachment from the $^{2}$P$_{1/2 }\to $ $^{3}$P$_{2}$ threshold of the O$^{-}$ ion John N. Yukich, Anne Joiner, Robert H. Mohr Numerous experiments have examined photodetachment in a magnetic field at the $^{2}$P$_{3/2 }\to \quad ^{3}$P$_{2}$ threshold of ions such as S$^{-}$ and O$^{-}$. The threshold energy is known as the atom's \textit{electron affinity}. Many of these experiments have resolved cyclotron and Zeeman structure in the detachment cross section. We report on an experiment that for the first time has visually resolved magnetic field structure in detachment at the lowest-lying threshold for the O$^{-}$ ion, the $^{2}$P$_{1/2 }\to \quad ^{3}$P$_{2}$ threshold. Our experimental apparatus includes a Penning ion trap in which the ions are created, trapped and stored, and a single-mode, amplified diode laser. Our observations also yield a quantitative measurement for the threshold energy. [Preview Abstract] |
Wednesday, May 20, 2009 3:48PM - 4:00PM |
C4.00010: Absolute Single Photoionization Cross Sections of Se$^{3+}$ For the Determination of Elemental Abundances in Planetary Nebulae David Esteves, Nicholas Sterling, Alex Aguilar, A.L. David Kilcoyne, Ronald Phaneuf, Rene Bilodeau, Eddie Red, Brendan McLaughlin, Patrick Norrington, Connor Balance Numerical simulations show that derived elemental abundances in astrophysical nebulae can be uncertain by factors of two or more due to atomic data uncertainties alone, and of these uncertainties, absolute photoionization cross sections are the most important.~ Absolute single photoionization cross sections for Se$^{3+}$ ions have been measured from 42 eV to 56 eV at the ALS using the merged beams photo-ion technique. Theoretical photoionization cross section calculations were also performed for these ions using the state-of-the-art fully relativistic Dirac R-matrix code (DARC). The calculations show encouraging agreement with the experimental measurements. [Preview Abstract] |
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