Bulletin of the American Physical Society
52nd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 66, Number 6
Monday–Friday, May 31–June 4 2021; Virtual; Time Zone: Central Daylight Time, USA
Session S02: Strong-field and attosecond electron dynamicsLive
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Chair: Alexandra Landsman, Ohio State University |
Thursday, June 3, 2021 10:30AM - 10:42AM Live |
S02.00001: Core-valence attosecond transient absorption spectroscopy of polyatomic molecules Nikolay Golubev, Jiri J Vanicek, Alexander I Kuleff Tracing ultrafast processes induced by interaction of light with matter is often very challenging. In molecular systems, the initially created electronic coherence becomes damped by the slow nuclear rearrangement on a femtosecond timescale which makes real-time observations of electron dynamics in molecules particularly difficult. In this work, we demonstrate theoretically that the attosecond transient absorption spectroscopy (ATAS) can be a very useful technique to trace such ultrafast processes in molecules. We report an extension of the theory underlying ATAS for the case of molecules, including a full account for the coupled electron-nuclear dynamics in the initially created wave packet, and apply it to probe the oscillations of the positive charge created after outer-valence ionization of the propiolic acid molecule. By taking advantage of element-specific core-to-valence transitions induced by X-ray radiation, we show that the resolution of ATAS makes it possible to trace the dynamics of electron density with atomic spatial resolution. |
Thursday, June 3, 2021 10:42AM - 10:54AM Live |
S02.00002: High-Harmonic Spectroscopy of Charge Migration Simulated with TDDFT Kyle A Hamer, Aderonke Folorunso, Francois Mauger, Kenneth Lopata, Kenneth J Schafer, Mette B Gaarde Attosecond spectroscopic tools are now offering perspectives for measuring ultrafast coherent electron dynamics in molecules, such as charge migration, on their natural timescale. However, the corresponding numerical simulations remain extremely challenging since they require both an accurate description of the correlated electron/hole dynamics as well as the associated spectroscopic signals themselves. In this talk, I will present the results of recent high-harmonic-spectroscopy simulations using real-space TDDFT. I will discuss how these harmonic-spectroscopy simulations can be applied to molecular systems that undergo charge migration. Specifically, using the example of a conjugated halogenated alkyne linear molecule, I will show how periodic charge-migration motions in the molecule's pi system shape the harmonic-generation signal. Finally, I will discuss possible implications of these signatures for experimental measurements of molecular charge migration. |
Thursday, June 3, 2021 10:54AM - 11:06AM Live |
S02.00003: Revisiting multi-center interferences in carbon dioxide through high-harmonic spectroscopy Daniel R Tuthill, Timothy Scarborough, Timothy T Gorman, Francois Mauger, Pierre Agostini, Louis F DiMauro We present high-harmonic spectroscopic measurements of the spectral intensity and phase of multi-center interferences in carbon dioxide where we observe a wavelength-dependent change in the interference mechanism. We use molecular impulsive alignment combined with four different driving wavelengths, ranging from the near-infrared to mid-infrared, and a range of intensities at each wavelength for high-harmonic generation. In our measurements we observe an intensity-dependent, multi-orbital interference at near-infrared driving wavelengths and an intensity-independent, two-center interference at mid-infrared driving wavelengths. Our assignment of two different interference mechanisms is supported by a change in sign and intensity-dependence of the spectral phase jump between near-infrared and mid-infrared driving wavelengths. These results resolve a long-standing discrepancy in prior publications and provide new insight about geometric two-center interferences and dynamic multi-orbital interferences. |
Thursday, June 3, 2021 11:06AM - 11:18AM Live |
S02.00004: The Strong Field Simulator: An Attosecond Study of Electron Recollision Andrew J Piper, Qiaoyi Liu, Dietrich Kiesewetter, Jens Baekhoj, Kenneth J Schafer, Pierre Agostini, Louis F DiMauro We report on a novel attosecond study of electron recollision using sub-cycle XUV pulses to ionize noble gases dressed by an infrared field of sufficient intensity to drive recollision, this technique is dubbed the Strong Field Simulator. Previous strong-field experiments have been limited as tunneling can happen at times within the infrared field cycle that are weighted by the instantaneous ionization probability proportional to field strength. Thus, the measurement is confined to trajectories near the extreme of the electric field. In this experiment, the XUV pulses serve to replace the tunneling step of the semi-classical model, while the infrared field still accelerates the electron and drives recollision. Isolating the ionization step from the other two, we select the moment of ionization, and correspondingly the electron's quantum trajectory, by varying the arrival time of the XUV pulse with respect to the infrared field. We observe the outcomes of these singular trajectories by measuring the electron energy spectra in addition to the relative rateThis three-step model assumes that electrons follow distinct classical trajectories in the second step that correspond to specific ionization times in the first. Past work on understanding this phenomenon has been limited, as the measured observables correspond to a superposition of all possible trajectories of single and double ionization. The XUV/infrared delay, infrared and XUV wavelengths, infrared intensity, and target species are all controllable parameters allowing for a robust multi-dimensional study of electron recollision. |
Thursday, June 3, 2021 11:18AM - 11:30AM Live |
S02.00005: The role of electron correlation in strong-field ionization of molecules studied using the molecular R-matrix with time approach Zdenek Masin, Jakub Benda, Jimena D Gorfinkiel, Gregory Armstrong, Andrew Brown, Daniel Clarke, Hugo W van der Hart, Jack Wragg We apply the recently developed molecular R-matrix with time (RMT) approach to show explicitly for the case of water that electron correlation in strong field ionization is more important at lower intensities and we observe the transition from below- to above-barrier ionization regime. |
Thursday, June 3, 2021 11:30AM - 11:42AM Live |
S02.00006: Extracting and launching sub-cycle attosecond dynamics through strong-field ionization Maximilian Hartmann, Veit Stooss, Paul Birk, Gergana Borisova, Shuyuan Hu, Christian Ott, Thomas Pfeifer Strong-field ionization (SFI) is a central theme of attosecond physics. With the declared goal of tracking the motion of electrons in real-time, experimental access to the ultrafast time scales of electron dynamics is commonly facilitated by pump-probe schemes using ultrashort extreme ultraviolet (XUV) and near-infrared (NIR) pulses of light. Here, we present two studies employing SFI of Xenon atoms in the regime where the NIR-pump and XUV-probe pulses overlap. First, the sub-cycle buildup structure in the absorption lines of the strong-field generated ions serves as a timing tool to detect deliberatly induced group delays on the XUV pulses with a precision of five attoseconds [1]. Second, a lineshape analysis of the absorption spectra gives direct insight into the dipole response of the ions. Oscillations in the asymmetry and in the absorption line strength are observed, and their NIR intensity dependence is discussed within the context of the underlying electron dynamics. |
Thursday, June 3, 2021 11:42AM - 11:54AM Live |
S02.00007: Enhancing high-order harmonic generation by controlling the diffusion of the electron wavepacket Travis Severt, Jan Tross, Georgios Kolliopoulos, Itzik Ben-Itzhak, Carlos A Trallero We present a substantial enhancement of the vacuum-ultraviolet photon flux using high-order harmonic generation (HHG) driven by two-color, $\omega$--3$\omega$, laser pulses compared to the fundamental ω driving pulse. Specifically, we achieve over an order-of-magnitude enhancement for 20- to 40-eV photons and up to three-orders of magnitude at lower energies. More importantly, we demonstrate that the $\omega$--3$\omega$ driving field can be used to control the diffusion (also known as "dispersion") of the electron wavepacket. Furthermore, the two-color phase controls the electron excursion time over a factor of two range, enabling one to probe the remaining target at different times. Finally, we show that the divergence angle of the harmonics is reduced by a factor of two, showing that the bichromatic driving fields improve HHG's capability as a light source. |
Thursday, June 3, 2021 11:54AM - 12:06PM Live |
S02.00008: Controllable transient excitation gratings in crossing-beams filament wake channels in a dense gas Dmitri A Romanov, Suyash Bajpai, Robert J Levis When femtosecond laser filamentation occurs in a dense gas, electron-collisional processes lead to extensive excitation of the constituent atoms or molecules. This dense-gas situation corresponds to either high-pressure gases or longer carrier wavelengths. When filamentation takes place in an intersection area of two laser beams crossing at a small angle, the transient interference patterns predicate formation of ionization and excitation gratings, which can be controlled by temporal characteristics of the laser pulses. During the pulse, the strong-field ionization of constituent atoms/molecules competes with impact ionization and collisional excitation by energetic free electrons, which are driven by the oscillating laser field and gain considerable energy via inverse Bremsstrahlung process while scattering on neighboring neutral atoms. The complex interplay of these processes determines the transverse grating profiles of the ionic and excited-atom densities, as well as of the electron density and temperature at the end of the pulse. Using a kinetic model of these processes, we explore sensitivity of the ionization and excitation grating profiles in high-pressure argon gas to the envelope shape of the driving laser pulses. Evolution of such a grating in the filament wake is manifested in specific angular patterns of the Rabi sidebands emission associated with the excited states manifold. |
Thursday, June 3, 2021 12:06PM - 12:18PM Live |
S02.00009: Attosecond time delays near the photoionisation threshold of neon Matteo Moioli, Kathryn Hamilton, Hamed Ahmadi, Dominik Ertel, Marvin Schmoll, Alexei N. Grum-Grzhimailo, Elena V. Gryzlova, Maria M. Popova, Maxim D. Kiselev, David Atri Schuller, Klaus R Bartschat, Gavin P Menning, Robert Moshammer, Thomas Pfeifer, Claus Dieter Schroeter, giuseppe sansone Spectroscopy with attosecond pulse trains is routinely applied for the investigation of electronic dynamics in atoms and molecules [1]. Specifically, the process of photoionization has been investigated using a two-color photoionization scheme, demonstrating the existence of tiny, but measurable, delays in photoionization [2]. In the vast majority of these investigations, extreme ultraviolet harmonics of a fundamental frequency with photon energies above the ionization threshold are used. The time delays can be decomposed in a term inherent to the group delay of the different harmonics (τGD) and an atomic time delay (τa). |
Thursday, June 3, 2021 12:18PM - 12:30PM Live |
S02.00010: Time-dependent fragment emission asymmetry in RABBITT experiments with H2 Farshad Shobeiry, Patrick Fross, Hemkumar Srinivas, Divya Bharti, Thomas Pfeifer, Anne Harth, Robert Moshammer Photo-dissociation in a homonuclear diatomic molecule, here H2, can lead to a preferred direction of the photo-electron emission with respect to the positively charged dissociation fragment, leading to a non-vanishing asymmetry parameter [1]. |
Thursday, June 3, 2021 12:30PM - 12:42PM Live |
S02.00011: Multi-Sideband RABBITT in Ar: Experiment and Theory Divya Bharti, Kathryn Hamilton, Hemkumar Srinivas, Farshad Shobeiry, Thomas Pfeifer, Robert Moshammer, Klaus R Bartschat, Anne Harth
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