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 L5: Photoionization and Photodetachment Processes II |
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Chair: Linda Young, Argonne National Laboratory Room: Clark Hall 107 |
Thursday, May 21, 2009 2:00PM - 2:12PM |
L5.00001: Time-Dependent B-Spline R-Matrix Approach to Double Ionization of Atoms by XUV Laser Pulses Xiaoxu Guan, Oleg Zatsarinny, Klaus Bartschat, Clifford Noble, Barry Schneider We have extended our {\it ab initio\/} and non-perturbative time-dependent approach to describe the effect of intense XUV laser pulses on a general atom~[1] to handle the double ionization process. After using a highly flexible $B$-Spline $R$-matrix method~[2] to generate field-free Hamiltonian and electric dipole matrices, the initial state is propagated in time using an efficient Arnoldi-Lanczos scheme. Test calculations for double ionization of He by a single laser pulse yield good agreement with benchmark results obtained with other methods, such as recent finite-element discrete-variable approaches~[3,4]. The method is then applied to two-color pump-probe processes, for which momentum and energy distributions of the two outgoing electrons are presented. [1]~X.~Guan {\it et al.}, Phys. Rev. A~{\bf 76}, 053411 (2006). [2]~O.~Zatsarinny, Comp. Phys. Commun. {\bf 174}, 273 (2006). [3]~J.~Feist {\it et al.}, Phys. Rev. A~{\bf 77}, 043420 (2007). [4]~X.~Guan {\it et al.}, Phys. Rev. A~{\bf 77}, 043421 (2007). [Preview Abstract] |
Thursday, May 21, 2009 2:12PM - 2:24PM |
L5.00002: Validity of nonrelativistic-form expression for calculation of Compton scattering doubly differential cross sections. L.A. LaJohn, R.H. Pratt The nonrelativistic impulse approximation expression for Compton scattering doubly differential cross sections (DDCS) is the product of a separate kinematic factor K and the Compton profile J (i.e. DDCS=KJ). This form allows one to easily obtain the Compton profile from measured or predicted DDCS. At high photon energies or for heavy atoms, it becomes necessary to use the relativistic impulse approximation (RIA). However in the full RIA expression, the integral over the initial electron momentum p for J is weighted by a p-dependent K. Over 30 years ago Ribberfors [1](as well as others), argued that this expression can be placed in a DDCS=KJ form by assuming that the initial electron energy $E(p)=(p^2+m^2)^{1/2}\approx m$. Further, he argued that this relativistic KJ (RKJ) approximation would be good if the scattering angle $\theta $ is $180^o$, but that small errors would occur away from the Compton peak maximum at other angles. We will show that the assumption $E(p)\approx m$ breaks down for K-shell DDCS for atoms with $Z>30$, due to broadening of the Compton profile, and that the maximum error of the RKJ approximation occurs when $\theta =180^o$, where at the Compton peak maximum the values can be over $50\%$ too large. The results of this study provide a useful guide for when one can use the RKJ approximation to obtain accurate DDCS. \noindent [1.] R. Ribberfors, Phys. Rev. B, {\bf 12} 2067 (1975) [Preview Abstract] |
Thursday, May 21, 2009 2:24PM - 2:36PM |
L5.00003: Chirp effect on the kinetic energy release upon dissociation of H$_{2}^{+}$ in intense ultrashort laser pulses M. Zohrabi, J.J. Hua, J. McKenna, A.M. Sayler, B. Gaire, Nora G. Johnson, P.Q. Wang, K.D. Carnes, B.D. Esry, I. Ben-Itzhak The effect of chirp on the kinetic energy release (KER) of the vibrational states of H$_{2}^{+}$ was measured using a coincidence 3D momentum imaging technique. The data shows significant shifts of the KER peaks associated with the dissociating vibrational levels in intense 70 and 120 fs pulses chirped positively/negatively from the 30 fs transform limited pulses. Our calculations show that this energy shift is due to the dissociation of the H$_{2}^{+}$ early in the pulse -- when the photon energy is shifted to either higher or lower energy depending on the chirp. This effect is reduced by intensity averaging, but it is still a measurable effect. [Preview Abstract] |
Thursday, May 21, 2009 2:36PM - 2:48PM |
L5.00004: Two-center Effects in One-photon Single Ionization of H$_2^+$, H$_2$ and Li$_2^+$ with Circularly Polarized Light Frank L. Yip, Jorge Fern\'andez, Fernando Mart\'In, Thomas N. Rescigno, C. William McCurdy Circularly polarized one-photon single ionization of H$_2^+$ and H$_2$ with energies of a few hundred eV and of Li$_2^+$ with tens of eV allows the investigation of two-center interference effects due to confinement and double-slit diffraction. Accurate theoretical results of Exterior Complex Scaling calculations are presented showing that the angular distributions for circularly polarized light are very similar to those obtained by averaging the corresponding angular distributions for parallel and perpendicular linearly polarized light, implying that coherence between $\Pi_u$ and $\Sigma_u^+$ amplitudes is unimportant in determining those distributions. Consequently, confinement and double-slit effects observed in the case of linear polarization are also present in the case of circularly polarized light. For Li$_2^+$, because the effects appear at electron energies that are significantly lower, the results are more sensitive to details of the molecular potential. [Preview Abstract] |
Thursday, May 21, 2009 2:48PM - 3:00PM |
L5.00005: Global nonresonant vibrational-photoelectron coupling in molecular photoionization Erwin Poliakoff, Aloke Das, David Hardy, John Bozek, Alex Aguilar, Robert Lucchese Using photoelectron spectroscopy and Schwinger variational scattering theory, we have investigated the coupling between vibrational motion and the exiting photoelectron over extended ranges of photoelectron kinetic energy. Photoelectron spectroscopy is performed with vibrational resolution over uncommonly large ranges of energy (\textit{ca.~}200~eV). We find clear and significant changes in vibrational branching ratios as a function of photon energy, in direct contradiction to predictions of the Franck-Condon principle. While it is well known that resonances lead to coupling between electronic and vibrational degrees of freedom, nonresonant mechanisms that result in such coupling are not expected or well-documented. Photoelectron spectra are presented for several electronic states of N$_{2}^{+}$, CO$^{+}$, and NO$^{+}$, and we find that valence isoelectronic channels behave very differently, which is also surprising. Theoretical results indicate that Cooper minima are the underlying cause of these effects, and we are currently working to understand the reasons for the sensitivity of the Cooper minima on bond length. [Preview Abstract] |
Thursday, May 21, 2009 3:00PM - 3:12PM |
L5.00006: Asymmetric Molecular-Frame Photoelectron Angular Distribution for K-shell Photoemission from CO$_2$: a Theoretical Study Shungo Miyabe, C. William McCurdy, Thomas N. Rescigno Coincidence measurements can now be made of the momenta of the electrons and fragment ions that emerge from a molecule following absorption of an energetic photon to obtain the angular distribution of the ejected electrons in the molecular-frame. Recent measurements [1] of molecular frame photoelectron angular distributions (MFPADs) for C 1s photoemission from CO$_2$ show a rapid change over a narrow range of photon energies as well as weak, but definite definite asymmetry with respect to the O$^+$ and CO$^+$ fragment ions for certain energies. Here we report results of complex Kohn calculations of MFPADs for this process and show how asymmetries can arise from the sensitivity of the photoejection process to the instantaneous position of the nuclei and the rapidity of subsequent Auger decay. We show that these effects are most prominent over the narrow range of photon energies where photoelectron confinement effects are present. [1] X.- J. Liu et al. Phys. Rev. Lett. B 101, 083001 (2008). [Preview Abstract] |
Thursday, May 21, 2009 3:12PM - 3:24PM |
L5.00007: Angular Distribution of Ethylene Molecules from the Photo Double Ionization: A kinematically complete experiment Sun Lee, Thorsten Weber, Timur Osipov, C. William McCurdy, Ali Belkacem, Reinhard Doerner The direct Photo Double Ionization (PDI) is a process that arises essentially because of the electron correlation. After successfully studying the PDI of hydrogen molecules in the past our goal is to extend our knowledge from a sigma bonding system to a pi bonding system. Ethylene is an ideal candidate, because it is the simplest hydrocarbon with a pi bond. Thus we investigated the PDI of ethylene near its threshold in order to unravel unique and important intertwining of electron correlation in inter-shell and intra-shell cases and the subsequent molecular dynamics. We focused on the following reaction: C$_{2}$H$_{4}$ + hv -$>$ C$_{2}$H$_{4} \quad ^{2+}$ + 2 e$^{-}->$ 2 e$^{-}$ + CH$_{2}^{+}$ + CH$_{2}^{+ }$ Using the COLTRIMS (COLd Target Recoil Ion Momentum Spectroscopy) technique, we were able to detect two ejected electrons and nascent photo-fragments, i.e. the recoil CH$_{2}^{+ }$ions in coincidence, thus a kinematically complete experiment was performed. From this we derived the angular distributions of the emitted photoelectrons in reference to the fixed in space molecular axis and the linear polarization of the incoming light (40eV). In addition we were able to determine the electronic energy sharing as well as the internuclear distance at the instant of photo absorption. [Preview Abstract] |
Thursday, May 21, 2009 3:24PM - 3:36PM |
L5.00008: Using vibrational branching ratios to probe shape resonances in molecular photoionization Robert Lucchese, Aloke Das, Erwin Poliakoff, John Bozek The measurement of vibrational branching ratios in molecular photoionization can be used as a probe of the nature of resonant states, since such states are often sensitive to the geometry of the molecule. Recent computed results for BF$_{3}$ and C$_{6}$F$_{6}$ will be presented. In C$_{6}$F$_{6}$, we consider the excitation of the two symmetric stretching modes in the photoionization leading to the $C \quad ^{3}B_{2u}$ state of the ion. Two prominent shape resonances at photon energies between 18 and 20 eV respond quite differently to the excitation of the symmetric ring-breathing mode and to the symmetric C-F stretching mode. In BF$_{3}$, the excitation of both the symmetric stretching and the degenerate asymmetric stretching modes are considered in the photoionization leading to the $E \quad ^{2}$A$_{1}$' state of the ion. The symmetric stretching mode shows a relatively weak resonant enhancement in the branching ratio, whereas the asymmetric stretching mode has a much more prominent feature. [Preview Abstract] |
Thursday, May 21, 2009 3:36PM - 3:48PM |
L5.00009: Fourier analysis of subshell photoionizations: Pristine C$_{60}$ and atoms endohedrally confined in C$_{60}$ Matt McCune, Mohamed Madjet, Himadri Chakraborty Theory and experiment have confirmed oscillations in the photoionization from two highest occupied orbitals of free C$_{60}$ [1]. It is however not known if the oscillations from inner orbitals also show the same spectral ``morphology''. We found using a Fourier analysis approach that the oscillation spectacularly alters by the centrifugal barrier created by a high angular momentum inner electron [2]. Theory has also predicted oscillations in the photo cross section of atoms confined in C$_{60}$. Most of the studies however modeled the confining shell by a simplistic one-active-electron potential. A method has recently been established that treats the C$_{60}$ electrons in a sophisticated multi-electron frame which we use to perform calculations of closed-shell atoms in C$_{60}$. Employing the Fourier analysis we unravel the unique interplay between specific ionization modes that induces oscillations in the cross section of the confined atom. [1] Ruedel et al., Phys. Rev. Letts. \textbf{89}, 125503 (2002); [2] McCune et al., J. Phys. B FTC \textbf{41}, 201003 (2008). [Preview Abstract] |
Thursday, May 21, 2009 3:48PM - 4:00PM |
L5.00010: Xe@C$_{60}$: A ``melting pot'' of multiple photo-effects Himadri Chakraborty, Mohamed Madjet, Thomas Renger, Jan-Michael Rost, Steve Manson Considering a Xe atom endohedrally confined in C$_{60}$, we calculate the photoionization cross section of all the subshells of the compound. Calculations are performed by a time-dependent density functional method that includes important correlations [1]. Multiple effects characteristic to the endohedral photo response are found to coexist in the spectrum. The low energy cross section of Xe 5p is found to draw substantial oscillator strength from the plasmon active C$_{60}$ channels. But the high energy 5p cross section exhibits simple confinement oscillations as in the Xe innershell cross sections. Xe 5s and C$_{60}$ 2s hybridize to yield two novel states of mixed atom-fullerene character whose cross sections show very rich structures. Over the Xe 4d giant resonance region the 4d channel dominates and transfers its own oscillations to other weaker atomic channels. Finally, the cross sections of the intermediate angular momentum C$_{60}$ states greatly modify from the interplay of the central Coulomb and the centrifugal barrier potentials. [1] Madjet et al., J. Phys. B \textbf{41}, 105101 (2008). [Preview Abstract] |
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