Bulletin of the American Physical Society
48th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 62, Number 8
Monday–Friday, June 5–9, 2017; Sacramento, California
Session C5: Time-Resolved Electron Dynamics and Attosecond Spectroscopy |
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Chair: Johan Mauritsson, Lund University Room: 310 |
Tuesday, June 6, 2017 2:00PM - 2:12PM |
C5.00001: Effects of electron correlation on the time-delay in photoionization of atomic beryllium Lars Bojer Madsen, Juan Omiste, Wenliang Li We report on the effects of electron correlation on the relative time-delay in photoionization of atomic beryllium in the channels Be[($1s^22s^2$) $^1$S$^e$] $\rightarrow$ Be$^+$[($1s^22s$) $^2$S$^e$] + $e^-$ ($p$ electron), and Be[($1s^22s^2$) $^1$S$^e$] $\rightarrow$ Be$^+$[($1s^22p$) $^2$P$^o$] + $e^-$ ($s$ or $d$ electron). We use our recent three-dimensional implementation of the time-dependent restricted-active-space self-consistent-field method and study the changes in the value obtained for the time-delay when including more and more correlation. We find that the mean-field, time-dependent Hartree-Fock theory does not account accurately for the time-delay. A larger active orbital space is needed. We find that the relative time-delay between ionization into Be$^+$[($1s^22s$) $^2$S$^e$] and Be$^+$[($1s^22p$) $^2$P$^o$] is around 7-8 attoseconds. [Preview Abstract] |
Tuesday, June 6, 2017 2:12PM - 2:24PM |
C5.00002: Spatio-Temporal Control of XUV FID (xFID) Emission in Helium E. R. Simpson, S. Bengtsson, N. Ibrakovic, S. Camp, K. J. Schafer, M. B. Gaarde, L. Rippe, J. Mauritsson Precise spatio-temporal control of extreme ultra-violet (XUV) light resulting from free induction decay (FID) has recently been demonstrated through opto-optical modulation in argon [1]. Here we present an extension of this technique, exploring precision control over XUV induced FID (xFID) in helium. By tuning the frequency content of the coherent XUV excitation pulse, we probe the resonant excitation of a number of bound excited states. The directionality of the xFID signal is controlled by a Stark shift-induced phase gradient applied using a non-coaxial, variable delay, IR control pulse. In this way both the spatial and temporal properties of the xFID signal can be controlled through the intensity and delay of the applied control pulse. We observe the direction of the xFID signal from the 2p state to be opposite to that for the higher np manifold, as expected from the direction of the applied Stark shift. In addition the 2p state splits, emitting FID in both directions. This forms an effective beam splitter, opening the door for `which way' interference. By shaping, or increasing the number of control pulses, possibilities for xFID control could include opto-optical switching and focussing of the xFID emission. [1] S. Bengtsson et al. arXiv:1611.04836v1 [Preview Abstract] |
Tuesday, June 6, 2017 2:24PM - 2:36PM |
C5.00003: Attosecond time-resolved photoemission from Cu(100) and Cu(111) surfaces Marcelo Ambrosio, Uwe Thumm Motivated by the striking dependence of the valence electronic structure of transition metal surfaces on their crystallographic orientation, and by very recent experiments [1,2] on laser-assisted extended ultraviolet (XUV) photoemission from solid surfaces, we calculated photoemission spectra from Cu(100) and Cu(111) surfaces as a function of the photoelectron final kinetic energy and the delay between an ionizing attosecond XUV pulse train and assisting infrared (IR) laser pulse [3]. Our numerical simulations predict distinct differences in delay-dependent photoelectron energy distributions and photoemission time delays for Cu(100) and Cu(111) surfaces that can be scrutinized experimentally in a suggested \textit{in situ} comparative RABBITT configuration, by placing the two surfaces on a sliding platform while keeping all optical components and pathlengths fixed. In addition, our numerical results also show that the inclusion of the Fresnel-reflected incident IR pulse at the metal-vacuum interface modifies photoelectron spectra and photoemission time delays in a characteristic way that reveals the degree of spatial location of the initial electronic states. [1] R. Locher \textit{et al.,} Optica \textbf{2}, 405 (2015). [2] Z. Tao \textit{et al.,} Science \textbf{353}, 62 (2016). [3] M. J. Ambrosio and U. Thumm, A \textbf{94}, 063424 (2016). [Preview Abstract] |
Tuesday, June 6, 2017 2:36PM - 2:48PM |
C5.00004: Controlling electronic couplings with tunable long wavelength pulses: Study of Autler-Townes splitting and XUV emission spectra Nathan Harkema, Chen-Ting Liao, Arvinder Sandhu Attosecond transient absorption spectroscopy (ATAS) enables the study of excited electron dynamics with unprecedented temporal and energy resolution. Many ATAS experiments use an extreme ultraviolet (XUV) pump pulse and a near-infrared (NIR) probe fixed at the fundamental laser frequency ($\sim$ 800 nm) to study the light induced effects on electronic structure of atoms and molecules. We extend the technique by using an optical parametric amplifier in one arm of our setup, which allows us to independently tune the frequency of the probe pulse from 1200 to 1800 nm. These long-wavelength pulses allow us to explore a new regime, where we can control the couplings between nearby electronic states to alter the transient absorption lineshapes in atoms. We use this technique to investigate the 4p-3s detuning dependent Autler-Townes splitting of the 4p state in Helium. Light induced Floquet structures extending into the continuum are observed in our study. We demonstrate new tunable XUV emission channels from four-wave mixing processes, and the efficiency of these emissions can be strongly enhanced through resonant couplings. The tunable IR induced electronic couplings are also used to influence the autoionization dynamics in Argon. [Preview Abstract] |
Tuesday, June 6, 2017 2:48PM - 3:00PM |
C5.00005: Pulse-parameter dependence of nuclear ``attosecond time delays" Greg Armstrong, D. Ursrey, J. V. Hernandez, F. Anis, T. Severt, M. Zohrabi, Ben Berry, Peyman Feizollah, Bethany Jochim, Kanaka Raju P., J. McKenna, B. Gaire, K. D. Carnes, I. Ben-Itzhak, B. D. Esry One of the main goals of strong-field photodissociation is the control of chemical reactions. Recent experiments [1] have successfully controlled the spatial asymmetry in D$_2^+$ using two-color interferometry. These experiments achieved vibrational resolution, and so were able to determine the spatial asymmetry of a number of vibrational states as a function of two-color delay. The relative phase in the delay-dependent spatial asymmetry obtained in these experiments may be used to define a time delay in dissociation from adjacent vibrational states --- a technique used previously to produce relative time delays in atomic ionization from the photoelectron spectrum [2]. Further two-color measurements in this direction are being planned. As a guide to these experiments, we aim to determine theoretically the dependence of such delays on laser intensity, pulse length, and pulse shape. We also identify the parameters that maximize the contrast in the delay-dependent spatial asymmetry. [1] M. Zohrabi, Ph.D thesis, Kansas State University (2014). [2] K. Kl\"{u}nder et al., Phys. Rev. Lett. {\bf 106} 143002 (2011) [Preview Abstract] |
Tuesday, June 6, 2017 3:00PM - 3:12PM |
C5.00006: An Attosecond Transient Absorption Spectroscopy Setup with a Water Window Attosecond source Andrew Chew, Yanchun Yin, Jie Li, Xiaoming Ren, Yang Wang, Yi Wu, Zenghu Chang Attosecond transient absorption, or time-resolved pump-probe spectroscopy, are excellent tools that can be used to investigate fast electron dynamics for a given atomic or molecular system. Recent push for high energy long wavelength few cycle laser sources has resulted in the production of x-ray spectra that would allow the probing of electron dynamics at the carbon k-edge in molecules such as CH$_{\mathrm{4}}$ and CO$_{\mathrm{2}}$. The motion of charges can be caused by photo-dissociation and charge migration. We present here the first results from our experimental setup where we produce a broadband attosecond pulse with spectra that stretches into the water window. [Preview Abstract] |
Tuesday, June 6, 2017 3:12PM - 3:24PM |
C5.00007: A comparative study of attosecond photoelectron streaking spectroscopy of metallic nanospheres Jianxiong Li, Erfan Saydanzad, Uwe Thumm We present new numerical results for streaked photoemission from Au, Ag, and Cu nanospheres by an extreme ultraviolet (XUV) and an infrared (IR) or visible streaking pulse, based on a quantum-mechanical model [1]. We discuss significant plasmonic streaking oscillation-amplitude enhancements and phase shifts for all three metals, relative to the results excluding the induced plasmonic field near the nanoparticle surface. Based on our streaked spectra, we demonstrate the reconstruction of the plasmonic-field enhancement and phase shift for each material, suggesting the use of attosecond streaking spectroscopy to reveal the dielectric plasmonic response of nanoparticles in the IR and visible spectral range. [1] J. Li, E. Saydanzad, and Uwe Thumm, Phys. Rev. A 94, 051401(R) (2016). [Preview Abstract] |
Tuesday, June 6, 2017 3:24PM - 3:36PM |
C5.00008: Control of photoemission delay in resonant two-photon transitions L. Argenti, A. Jimenez Galan, R. Taieb, J. Caillat, A. Maquet, F. Martin The emission time delay $\tau$ in one-photon absorption, which coincides with half the Wigner scattering delay $\tau_W$, is a fundamental descriptor of the photoelectric effect. While it is hard to access $\tau$ in a direct way, it is possible to extrapolate it from the delay in two-photon transitions, $\tau^{(2)}$, measured with attosecond pump-probe schemes, provided that the contribution of the probe stage can be factored out. In absence of resonances, $\tau$ can be expressed as the energy derivative of the dipole ionization amplitude, $\tau= \partial_E \arg D_{Eg}$, and $\tau \simeq \tau^{(2)} - \tau_{cc}$ where $\tau_{cc}$ is associated to the dipole transition in the continuum. Here we show that in the presence of a resonance the correspondence between $\tau$ and $\partial_E \arg D_{Eg}$ is lost. Furthermore, while $\tau^{(2)}$ still coincides with $\partial_E \arg D^{(2)}_{Eg}$, it does not have any scattering counterpart. Indeed, $\tau^{(2)}$ can be much larger than the lifetime of an intermediate resonance in the two-photon process, or more negative than the lower bound imposed on scattering delays by causality. Finally, $\tau^{(2)}$ is controlled by the probe frequency. By varying $\omega_{IR}$, therefore, it is possible to radically alter a photoelectron group delay. [Preview Abstract] |
Tuesday, June 6, 2017 3:36PM - 3:48PM |
C5.00009: Electron Matter-Wave Vortices in Double Photoionization of Helium by Attosecond Pulses Jean Marcel Ngoko Djiokap, Alexei V. Meremianin, Nikolai L. Manakov, Suxing Hu, Lars B. Madsen, Anthony F. Starace Double photoionization of helium by a pair of time-delayed circularly-polarized attosecond pulses is shown to produce two-electron momentum distributions that exibit \emph{two}-start spiral vortex structures. These structures originate from Ramsey interference of the created pair of two-electron wavepackets, each carrying a total angular momentum of unity. The predicted vortex-shaped patterns occur for any energy partitioning between electrons, and are exquisitely sensitive to the time delay between the two pulses, their relative phase, their ellipticity and handedness. Moreover, these kinds of vortices occur for both in-plane and out-of-plane detection geometries; however, they only take place when the angular separation $\hat{p}_1\cdot\hat{p}_2$ between the electron momenta is held fixed. Our results are obtained by solving \emph{ab initio} the seven-dimensional two-electron time-dependent Schr\"{o}dinger equation and are analyzed using a perturbation theory. Such vortices are thus general phenomena, as similar patterns have been reported following single-electron ionization in both atomic\footnote{J.M. Ngoko Djiokap \emph{et al.}, Phys.~Rev.~Lett. \textbf{115}, 113004 (2015).} and molecular\footnote{K.-J. Yuan \emph{et al.}, Phys.~Rev.~A \textbf{93}, 053425 (2016).} processes. [Preview Abstract] |
Tuesday, June 6, 2017 3:48PM - 4:00PM |
C5.00010: Spectral Signatures of Resonantly Enhanced High Harmonic Generation and the Influence of Quantum Seth Camp, Samuel Beaulieu, Yann Mairesse, Kenneth Schafer, Mette Gaarde Near the ionization threshold of argon, a new feature has been observed in the high harmonic generation (HHG) spectrum. This feature is due to emission occurring much later in time than the driven HHG response at the peak of the driving pulse, and is red-shifted relative to the main peak of harmonic 13. We present a theoretical framework for understanding this delayed emission process in terms of multiphoton resonances between the ground state and Stark shifted excited states, leading to resonantly enhanced HHG. We numerically solve the TDSE for an Ar atom in the single active electron approximation to calculate the HHG spectrum. By investigating the temporal properties of resonant harmonics as functions of the driving pulse duration and peak intensity, we find that the emission of such resonantly enhanced harmonics will temporally shift so that it occurs at a particular (constant) intensity. This can lead to late, red-shifted, harmonic emission, as well as early, blue-shifted, harmonic emission. We also find that the resonantly enhanced emission is delayed in time at the sub-cycle level relative to the non-resonant emission and we discuss the interplay between the pictures of semi-classical electron trajectories leading to HHG and multiphoton, resonantly excited states. [Preview Abstract] |
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