Bulletin of the American Physical Society
49th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics APS Meeting
Volume 63, Number 5
Monday–Friday, May 28–June 1 2018; Ft. Lauderdale, Florida
Session C07: Photoionization, Photodetachment, and Photodissociation |
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Chair: C. William McCurdy, University of California, Davis and Lawrence Berkeley National Laboratory Room: Grand D |
Tuesday, May 29, 2018 10:30AM - 10:42AM |
C07.00001: Employing Resonance-Enhanced Multi-Photon Dissociation for measuring the dissociation energy of LiO$^{\mathrm{-}}$ Ben Berry, Bethany Jochim, T. Severt, Peyman Feizollah, Kanaka Raju P., K. D. Carnes, B. D. Esry, I. Ben-Itzhak We have evaluated the dissociation energy of LiO$^{\mathrm{-}}$ through photo-fragmentation by measuring the kinetic energy release (KER) upon dissociation into Li$+$O$^{\mathrm{-}}$. Resonance-enhanced two-photon absorption was used to increase the rate of dissociating LiO$^{\mathrm{-}}$ by about two orders of magnitude over single-photon photodissociation. This REMPI-like process involves exciting LiO$^{\mathrm{-}}$ to an intermediate state from which photodissociation is more efficient. Using the measured KER distribution, we set a lower limit on the dissociation energy of 3.0$\pm$0.1 eV, which is 0.3 eV larger than the value based on theory. [Preview Abstract] |
Tuesday, May 29, 2018 10:42AM - 10:54AM |
C07.00002: Photoionization Of Ne III For Astrophysical Applications Sultana Nahar Photoionization is one process that introduces lines for neon ions seen in astrophysical plasmas such as of quasers, planetary nebulae, supernova ramnents. The present study on photoionization of Ne III, Ne III + h$\nu~ \rightarrow$ e + Ne IV, reports features and characteristics in cross sections ($\sigma$) obtained from the relativistic Breit-Pauli R-Matrix method using a close coupling wavefunction expansion of 58 levels of the residual ion Ne IV of configuratioins $2s^22p^3$, $2s2p^4$, $2p^5$, $2s^22p^23s$, $2s^22p^23p$, $2s^22p^23d$, $2s2p^33s$. Along with narrow Rydberg resonances, wide Seaton resonances due to photoexcitation-of-core (PEC) are noted in $\sigma$. Although relatively a low Z ion, it shows distinct relativic effects in the low energy region near the ionization threshold by producing resonances through couplings of fine structure channels. Such resonances, not allowed in LS coupling, can explain the the observed features in the measured $\sigma$ carried out at the sophisticated facility of Advanced Light Source in Berkeley. These features will be demonstrated at the presentation. [Preview Abstract] |
Tuesday, May 29, 2018 10:54AM - 11:06AM |
C07.00003: R-Matrix calculations for Improved Atomic Data for Astrophysical Opacities and Plasma Effects Anil Pradhan, Sultana Nahar A review of a renewed effort to recalculate astrophysical opacities using the R-Matrix method is presented [1]. The computational methods and new extensions are described. Resulting enhancements found in test calculations under stellar interior conditions compared to the Opacity Project could potentially lead to the resolution of the solar abundances problem, as well as discrepancies between recent experimental measurements of iron opacity at the Sandia Z-pinch inertial confinement fusion device and theoretical opacity models. Outstanding issues also discussed are: (i) accuracy, convergence, and completeness of atomic calculations, (ii) improvements in the Equation-of-State of high-temperature-density plasmas, and (iii) redistribution of resonant oscillator strength in the bound-free continuum, and (iv) plasma broadening of auotionizing resonances. Results are reported for Fe XVII and other Fe ions. This work is suported by the National Science Foundation and the Department of Energy. The computational work is carried out at the Ohio Supercomputer Center. [1] S.N. Nahar and A.K. Pradhan, Phys. Rev. Letts., 116, 235003, 2016. [Preview Abstract] |
Tuesday, May 29, 2018 11:06AM - 11:18AM |
C07.00004: Photodetachment of $K^-$ into highly polarizable excited states Matthew Eiles, Chris Greene Photodetachment of atomic negative ions is a robust tool for probing highly non-trivial electron correlation, especially when the photon has sufficient energy to both detach an electron and excite the residual neutral atom into a low-lying Rydberg state. In this scenario, the detached electron induces strong polarization and dipole forces in the residual neutral atom, leading to non-coulombic long-range interactions. These interactions are of particular interest in two regimes. First, excited states of maximal angular momentum can possess huge negative polarizabilities, which lead to repulsive polarization potentials. Secondly, near-degeneracies in atomic states with high angular momenta can lead to the formation of permanent dipole potentials. In order to study the effects of these unusual atomic potentials on the partial photodetachment cross sections observed experimentally, we have performed an extensive R-matrix calculation over this range of highly excited states. [Preview Abstract] |
Tuesday, May 29, 2018 11:18AM - 11:30AM |
C07.00005: Classical trajectory studies of the dynamics of one-photon double photionization of H$_2$O Zachary Streeter, Frank Yip, Robert R. Lucchese, Benoit Gervais, C. William McCurdy The triple differential cross section in the body frame for double photoionization of water, H$_2$O $+ h\nu \rightarrow$ 2e$^-$+ H$^+$ + H$^+$ +O, can be measured in principle by detecting the protons and photoelectrons in coincidence -- but only if the dynamics of dissociation of the doubly charged molecular ion are known. A classical trajectory study of the nine lowest states of the water dication is presented using \textit{ab initio} potential energy surfaces, and sampling from a semiclassical initial distribution of positions and momenta. Excellent agreement with preliminary experimental momentum imaging measurements of double photoionization of water show that eight dication states can be unambiguously identified in the experiment with the aid of theory. The theoretical trajectory results will allow body frame measurements of double photoionization to yield all eight states even though the usual assumption of direct dissociation, the ``axial recoil'' approximation, breaks down for three of the dication electronic states seen in the experiment, opening the door for the first kinematically complete double photoionization experiments on a polyatomic molecule. [Preview Abstract] |
Tuesday, May 29, 2018 11:30AM - 11:42AM |
C07.00006: Recoil frame photoelectron angular distributions for valence photoionization of CF$_4$ Robert R. Lucchese, Cynthia S. Trevisan, C. William McCurdy, Saijoscha Heck, Kirk A. Larsen, Thomas N. Rescigno Results of recent momentum imaging experiments on photoionization of CF$_4$ have shown that upon valence ionization producing several states the CF$_4^+$ cation dissociates to produce CF$_3^+$ + F, thereby allowing the measurement of the photoelectron angular distribution relative to the axis of recoil. For ionization from the 4t$_2$ orbital, the experiment shows a dramatic reversal in the angular distribution of the photoejected electron over a narrow range energies of less than 2 eV. We present the results of photoionization calculations using both the complex Kohn variational and Schwinger variational methods that reveal two overlapping shape resonances of a$_1$ and t$_2$ symmetry whose interference is responsible for this behavior and which require coupling between the ionization channels producing the degenerate components of the A$^2$T$_2$ state of the CF$_4^+$ ion. It is shown that ionization from the 3t$_2$ orbital shows similar resonances but not the dramatic changes in angular dependence with photon energy. Comparison of theory and experiment is also presented for ionization from the 1t$_1$ and 1e orbitals. [Preview Abstract] |
Tuesday, May 29, 2018 11:42AM - 11:54AM |
C07.00007: Photodetachment spectroscopy of cold trapped molecular anions near threshold Roland Wester, Olga Lakhmanskaya, Malcolm Simpson, Simon Murauer, Markus Nötzold, Alice Schmidt-May, Robert Wild, Eric Endres, Viatcheslav Kokoouline Negatively charged molecular ions have drawn a lot of attention in recent years, in particular owing to the detection of several molecular anions in cold interstellar molecular clouds. Cryogenic radiofrequency ion traps are well suited tools to study bound-bound spectroscopy and photofragmentation processes with high resolution and high sensitivity. We have studied the photodetachment of the amide anion NH$_2^-$ and the interstellar anions CN$^-$ and C$_3$N$^-$ near threshold and have compared the spectra to the threshold behavior given by the Wigner threshold laws. These data reveal information about the role of the permanent electric dipole moment of the neutral product, which is important for the description of anion formation by electron attachment. For the NH$_2^-$ anion, the spectra have allowed us to probe the lowest two rotational transitions in this ion near 447 GHz and 934 GHz and the contribution of the different hyperfine states. [Preview Abstract] |
Tuesday, May 29, 2018 11:54AM - 12:06PM |
C07.00008: Electron-Ion 3D momentum imaging of the dissociative single ionization of hydrogen molecules in the presence of a laser field A. Gatton, K. Larsen, E. Champenois, W. Iskander, S. Heck, D. Reedy, D. Call, T. Severt, B. Jochim, R. Strom, J.B. Williams, D. Slaughter, I. Ben-Itzak, A.L. Landers, Th. Weber We present updated results from our 2-color IR laser+synchrotron Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) experiments on the dissociative single ionization of $H_2$. Light from beamline 10.0.1 ($17.9$, $18.34$ and $18.56eV$, $80ps$, $50meV$ resolution) at the Advanced Light Source was used to excite and ionize $H_2$ in the presence of pulsed laser field ($1030nm$, $12ps$, $5\times 10^{11} W/cm^{2}$). We refined the apparatus and collected an order of magnitude more data than our preliminary experiment at two specific laser delays: short (XUV+Laser overlapped) and long ($>200ps$ time offset). We present more evidence for the interplay of photoelectron retro-action with the strong laser field in the dissociation of $H_{2}^{+}$. We also present evidence for the signature of light induced conical intersections in the dissociation of the laser dressed hydrogen cations, as recently reported by Natan et. al. (PRL 116, 143004 (2016)). [Preview Abstract] |
Tuesday, May 29, 2018 12:06PM - 12:18PM |
C07.00009: Nuclear Dynamics of H$^{\mathrm{+}} \quad +$ H$^{\mathrm{+}}$ Fragmentation in NH$_{\mathrm{3}}$ Following Direct Single-Photon Double Photoionization at 61.54eV. Kirk Larsen, Saijoscha Heck, Averell Gatton, Wael Iskandar, Elio G. Champenois, Richard Strom, Travis Severt, Bethany Jochim, Dylan Reedy, Joshua B. Williams, Zachary Streeter, Thomas N. Rescigno, C. William McCurdy, Robert R. Lucchese, Daniel S. Slaughter, Itzik Ben-Itzhak, Thorsten Weber We report measurements on the H$^{\mathrm{+}} \quad +$ H$^{\mathrm{+}}$ fragmentation pathways in neutral NH$_{\mathrm{3}}$ following direct single-photon double photoionization at 61.54eV, where the two photoelectrons and two protons are measured in coincidence using 3-D momentum imaging. We observe four dication electronic states which contribute to H$^{\mathrm{+}} \quad +$ H$^{\mathrm{+}}$ fragmentation, three of which exhibit equal energy sharing between the two protons, while the third exhibits strongly unequal energy sharing. We tentatively attribute the unequal proton energy sharing feature to multi-step fragmentation, while the other three equal proton energy sharing features are attributed to single-step fragmentation. Molecular plane proton momentum distributions for these states provide insight into the nuclear dynamics that emerge following direct single-photon double photoionization. [Preview Abstract] |
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