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 G6: Strong Field Processes I |
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Chair: Wendell Hill, University of Maryland Room: 302 |
Wednesday, June 5, 2013 8:00AM - 8:12AM |
G6.00001: Exploring trends in strong-field dissociation of small polyatomic molecules Bethany Jochim, M. Zohrabi, U. Ablikim, B. Berry, T. Severt, K.D. Carnes, I. Ben-Itzhak Using a 3-D coincidence momentum imaging technique, we examine trends in intense ultrafast laser-induced two-body dissociation and dissociative ionization of an N$_2$O$^+$ ion beam. As an example, it is observed that in both dissociation and dissociative ionization, the N-N bond is more likely to break than the N-O bond for typical laser pulses (Fourier transform-limited, 800 nm, $\sim$30 fs, $\sim$10$^{15}$ W/cm$^2$). This finding is perhaps counterintuitive in light of the stronger bond between the two N atoms in the ground state of N$_2$O$^+$. Utilizing our measured kinetic energy release and angular distribution spectra and the molecule's published electronic structure, we explain this and other trends in bond cleavage and charge localization preference. We also draw comparisons to other triatomic molecular ions, such as CO$_2$$^+$, and attempt to gain insight into the relative importance of chemical bond strength and the spacing of the relevant potential energy surfaces. [Preview Abstract] |
Wednesday, June 5, 2013 8:12AM - 8:24AM |
G6.00002: Angle-Resolved Strong Field Ionization of Polyatomic Molecules: More than the Orbitals Matters Oumarou Njoya, Spiridoula Matsika, Thomas Weinacht We compare the time and angle-dependent strong field ionization yields of three molecules with very similar electronic structure. A pump pulse in the deep ultraviolet excites the molecules to their first bright excited state (HOMO-LUMO excitation). We then measure the strong field ionization yield due to a near infrared probe pulse as a function of delay and angle between pump and probe polarization vectors. \textit{ab initio} electronic structure calculations allow us to associate the parent ion yields with removal of an electron from the LUMO orbital (occupied after excitation by the pump). Despite the fact that the orbitals are very similar, the yields are very different, indicating that it is not the orbital shape alone which determines angle-dependent ionization yields. [Preview Abstract] |
Wednesday, June 5, 2013 8:24AM - 8:36AM |
G6.00003: Dynamic Orientation of OCS Induced by Intense THz Pulses Kisra Egodapitiya, Sha Li, R.R. Jones We have used intense, picosecond THz pulses to dynamically orient OCS molecules. Rotationally cooled molecules in a pulsed super-sonic expansion enter a time of flight mass spectrometer where they are exposed to a 50 fs, 780 nm ``pre-alignment'' pulse. This pulse coherently broadens the rotational probability distribution within each molecule, creating a rotational wavepacket which undergoes initial alignment along the laser polarization axis. After one-quarter of the fundamental rotational period, approximately 20 ps after the pre-alignment pulse, the molecules have rotated out of alignment and are maximally misaligned. At this time they are exposed to an intense THz pulse generated by tilted-pulse-front optical rectification of a 100 fs 780 nm laser pulse in Li:NiO$_{\mathrm{3}}$. The THz field drives transitions between rotational states of opposite parity, creating a rotational wavepacket that exhibits periodic head vs. tail orientation along the THz polarization direction. The time-dependent orientation is probed by detecting the up vs. down asymmetry in the S$^{\mathrm{3+}}$ ion fragments released during the Coulomb explosion of the OCS molecules in an intense 40 fs, 780 nm pulse. Substantial orientation is observed at a delay of one-half the fundamental rotational period after the THz pulse. [Preview Abstract] |
Wednesday, June 5, 2013 8:36AM - 8:48AM |
G6.00004: Multi-Electron Effects in Charge Asymmetric Molecules Induced by Asymmetric Laser Fields Vincent Tagliamonti, Hui Chen, George Gibson Using a 45 fs pump pulse at 800 nm, a wavepacket is created in a charge asymmetric dissociation channel of Iodine, I$_{2}^{2+}$ $\rightarrow$ I$^{2+}$ + I$^{0+}$ (2,0). As the molecule dissociates, a two-color (1$\omega$2$\omega$) probe pulse is used to study the dynamics as a function of internuclear separation $R$. We find a critical region of $R$ in which there is spatially asymmetric enhanced ionization of the (2,0) channel to a counter-intuitive (1,2) channel. In this region the I$^{0+}$ is ionized such that one electron is released to the continuum and another is transfered to the I$^{2+}$ resulting in I$^{0+}$ $\rightarrow$ I$^{2+}$ and I$^{2+}$ $\rightarrow$ I$^{1+}$. At larger $R$, the ionization is consistent with simple one-electron ionization in a double-well where I$^{0+}$ $\rightarrow$ I$^{1+}$. We find qualitative agreement between simulations and experiment further highlighting the importance of multi-electron effects in the strong-field ionization of molecules. [Preview Abstract] |
Wednesday, June 5, 2013 8:48AM - 9:00AM |
G6.00005: Numerical basis-state method for strong-field excitation and ionization Andrew Spott, Shaohao Chen, Andreas Becker, Agnieszka Jaron-Becker Numerical basis state methods offer some advantages over their analytical counterparts in calculations of binding energy and geometries of large molecular systems, primarily with respect to computational efficiency. Toward this end, we have developed a finite-space, single active electron numerical energy basis state method for performing ab-initio TDSE calculations of the interaction of atoms, including spin-orbit coupling, with intense ultrashort laser pulses. Results for the hydrogen atom [1] and other atoms regarding recently reported phenomena such as the dependence of excitation and ionization probabilities on the carrier-envelope phase will be presented.\\[4pt] [1] Chen, S. et al Phys.Rev.A 86, 013410 (2012). [Preview Abstract] |
Wednesday, June 5, 2013 9:00AM - 9:12AM |
G6.00006: Recollision with circular polarization Francois Mauger, Adam Kamor, Andre D. Bandrauk, Cristel Chandre, Turgay Uzer Since its identification in the 90s, the recollision scenario has revealed to be very helpful in explaining many phenomena in atomic and molecular systems subjected to strong and short laser pulses, and it is now at the core of the strong field physics and attosecond science. For linearly polarized laser fields, the recollision scenario has been able to explain nonsequential double ionization (NSDI), high harmonic generation (HHG) and laser induced diffraction (LIED), just to cite them. The same scenario also predicts the absence of recollision when the field is circularly polarized, therefore leading to the absence of NSDI, HHG or LIED. Recently, the influence of the ellipticity of the laser has drawn an increasing level of interest in the strong field community as it is seen as a way to control the electronic dynamics and, for instance, HHG. Using classical models, the common belief of the absence of recollision with circularly polarized laser fields has been proven wrong [1]. In my talk I will present classical and quantum evidence of the presence of recollision with circular polarization. I will discuss the conditions under which such recollisions happen and what they imply.\\[4pt] [1] Phys. Rev. Lett. - {\bf 105}, 083002 (2010). [Preview Abstract] |
Wednesday, June 5, 2013 9:12AM - 9:24AM |
G6.00007: Subcycle interference dynamics of ultrafast photoelectron holography, UPEH, and its applications in molecular imaging XueBin Bian, Andre Bandrauk Ultrafast photoelectron holography, UPEH, is studied by solving a 3-D time-dependent molecular Schr\"odinger equation and a corresponding classical model. We report subcycle interference dynamics between direct and rescattered photoelectrons. Four different interference patterns are predicted by the classical model, and used to interpret the quantum simulations, involving forward and backward rescattering in above threshold ionization (ATI). The numerical simulations provide attosecond time-resolved tools for the electronic imaging of molecular structure [1]. Dynamic imaging of a linear symmetric molecule H$_{2}^{+}$ and dipolar molecule HeH$^{2+}$ are studied as examples.\\[4pt] [1] Xue-Bin Bian and Andre D. Bandrauk, Phys. Rev. Lett. 108, 263003 (2012). [Preview Abstract] |
Wednesday, June 5, 2013 9:24AM - 9:36AM |
G6.00008: Streaking induced finite-range time delays Jing Su, Hongcheng Ni, Andreas Becker, Agnieszka Jaron-Becker We theoretically study time delays for photoionization in a streaking experiment by numerically solving the time-dependent Schr\"odinger equation. Our results show that the time delay in the numerical simulations is solely induced by the coupling between the streaking field and the atomic potential, and can be well approximated by a classical approach. The analysis implies that the time delay is related to a finite range in space, over which the emitted electron propagates during the interaction with the streaking field along its polarization direction. We also demonstrate that, for a short-range potential, in certain limiting cases the streaking induced finite-range time delay approaches the Wigner-Smith time delay. [Preview Abstract] |
Wednesday, June 5, 2013 9:36AM - 9:48AM |
G6.00009: Identifying isotopic effects in intense ultrafast laser-driven D$_2$H$^+$ fragmentation K.D. Carnes, A.M. Sayler, J. McKenna, B. Gaire, Nora G. Kling, B.D. Esry, I. Ben-Itzhak The triatomic hydrogen molecular ion is instrumental as a benchmark toward understanding the strong-field dynamics of polyatomic molecules. Using a crossed-beams coincidence three-dimensional momentum imaging method, we demonstrate clear isotopic effects in the fragmentation of D$_2$H$^+$ induced by 7 fs (40 fs), 790 nm laser pulses at an intensity of 10$^16$ W/cm$^2$ (5$\times$10$^{15}$ W/cm$^2$). Our experiment uniquely separates all fragmentation channels and provides kinematically complete information for the nuclear fragments. We show that for dissociative ionization of D$_2$H$^+$ there is a large difference in branching ratios of the two-body channels, where H$^+$+D$_2^+$ dominates D$^+$+HD$^+$, and the three-body channels, where H$^+$+D$^+$+D dominates D$^+$+D$^+$+H. In contrast, the dissociation channels display minimal differences. [Preview Abstract] |
Wednesday, June 5, 2013 9:48AM - 10:00AM |
G6.00010: Mechanism of delayed double ionization in a strong laser field Turgay Uzer, Francois Mauger, Adam Kamor, Cristel Chandre Nonsequential Double Ionization (NSDI), for linearly polarized strong laser fields, has been successfully explained with the recollision model. In this model, an electron, after being ionized by a strong linearly polarized laser field is hurled back and collides with its parent ion and triggers the ionization of a second electron. The picture for NSDI usually yields little delay between the recollision and the subsequent ionization. Experimental and theoretical results have revealed an alternative route in which there is a substantial delay between the recollision and the subsequent ionization. This mechanism has been coined as Recollision Excitation with Subsequent Ionization (RESI). The recollision model usually attributes a passive role to the secondly ionized (inner) electron. In my talk, I will show that the inner electron's role is very far from being a passive one, waiting for a recollision to ionize [1]. I will show that delayed double ionization stems from the inner electron being promoted to a sticky region. I will identify the mechanism that traps and releases the electron from this region and will show a signature of it as oscillations in relative RESI yields.\\[4pt] [1] Phys.~Rev.~Lett. - {\bf 108}, 063001 (2012); Phys.~Rev.~A - {\bf 85}, 066205 (2012) [Preview Abstract] |
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