Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 68, Number 7
Monday–Friday, June 5–9, 2023; Spokane, Washington
Session H04: Focus Session: Dynamics of ElectronsFocus Live Streamed
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Chair: Gilles Doumy, Argonne National Laboratory Room: Conference Theater |
Wednesday, June 7, 2023 8:00AM - 8:30AM |
H04.00001: XUV and X-ray induced ultrafast electron dynamics in gas-phase targets Invited Speaker: Alicia Palacios We exploit the new capabilities of coherent light sources producing sub-femtosecond pulses to manipulate electronic dynamics in atoms and molecules with attosecond resolution. Attosecond pump-probe spectroscopy using high-order harmonic generation has made it possible to retrieve time-resolved images of molecular excitation and ionization processes [1-3], at the heart of electron and charge transfer reactions of multidisciplinary interest. Our most recent theoretical progresses on recent applications of these techniques on polyatomic molecules will be presented. Moreover, the high intensities and brilliance of light pulses produced in free-electron lasers (FELs) and X-ray FELs give access to a manifold of non-linear phenomena that have remained experimentally unexplored until now [4,5], e.g. highly coherent multiphoton ionization processes on small targets. We will discussed the interpretation and outcome of recent, and possible future, experiments at FELs with particular emphasis on non-linear processes strongly dominated by electron-electron and electron-nuclear correlations. |
Wednesday, June 7, 2023 8:30AM - 8:42AM |
H04.00002: Decomposition of the electronic decoherence induced by the nuclear motion into contributions of individual modes Alan Scheidegger, Nikolay Golubev, Jiri Vanicek Ultrafast dynamics of the electron density of a molecule can be initiated by the creation of a superposition of electronic states [1]. The strong correlation between the nuclear and electronic motion typically induces a fast decoherence within just a few femtoseconds, making the experimental observation of electronic oscillations challenging [2]. We recently developed an efficient method [3, 4], based on the semiclassical description of the nuclear motion after a molecular ionization [5], to find molecules with long-lasting electronic coherence and charge migration. We found that the hole oscillations in the but-3-ynal molecule after ionization out of the HOMO lasts for about 10 femtoseconds. Here we show that extending the carbon skeleton to obtain the pent-4-ynal molecule conserves the ionization spectrum and long-lasting charge migration. The semiclassical description of the nuclear motion allows to decompose the overall decoherence into individual normal mode contributions without additional computation. In agreement with the observation made in [6], we find that in molecules with planar symmetry, only the in-plane normal modes contribute to the decoherence. |
Wednesday, June 7, 2023 8:42AM - 8:54AM |
H04.00003: Inner Shell Ionization and Ejection of Relativistic Electrons from High Power Laser Pulses Smrithan Ravichandran, Andrew M Longman, Marine Huault, Roberto Lera Matellanes, Calvin Z He, Robert Fedosejevs, Luis Roso, Wendell T Hill High power lasers have opened new vistas for light-matter interaction. With intensities more than 1019 W/cm2, lasers interacting with rarefied gases can remove electrons from inner shells and accelerate them to relativistic speeds. For non-relativistic intensities, the ponderomotive force causes the ejection of electrons normal to the laser axis. At relativistic intensities, these electrons gain significant momentum along the direction of laser propagation and are ejected over a range of angles, θ, relative to the axis. There is strong evidence that as the intensity is increased, the electrons gain more energy and θ decreases. However, the exact relationship between the electron energy and the laser intensity is still complicated and not well known. Further, theoretical studies predicted a uniform and symmetric electron distribution around the laser axis for an ideal Gaussian focus and past studies have only measured it for varying θ along fixed azimuthal angles. In this presentation, we review the relationship between θ, the electron energy and the laser intensity, and share our recent work that, to the best of our knowledge, produced the first two-dimensional image of the ring-like electron ejection distribution from rarefied gas. We show that this distribution is highly asymmetric, plausibly due to aberrations in the laser focus. Our results may possibly enable a direct way to assess these aberrations and measure the intensity in the focus at full power. |
Wednesday, June 7, 2023 8:54AM - 9:06AM |
H04.00004: Probing charge migration in bromoacetylene using frequency-matched ionization Denawakage D Jayasinghe, Aderonke S Folorunso, Mengqi Yang, Francois Mauger, Kyle A Hamer, Kenneth J Schafer, Mette B Gaarde, Kenneth Lopata We present how strong field ionization (SFI) can be used as a coherent probe to measure the charge migration (CM) in bromoacetylene. This simple “two-site” molecule has shown CM between Br and carbon/carbon triple bond sites on a femtosecond time scale in [A. S. Folorunso et al. Phys. Rev. Lett. 126, 133002 (2021)]. The dynamics were simulated using real-time time dependent density functional theory (RT-TDDFT) approach with auxiliary basis sets, complex absorbing potentials, and tuned range-separated functionals. To initiate the CM, we create a localized hole on Br site using constrained density functional theory, and to probe, a strong AC field was applied perpendicular to the CM axis (molecular axis), i.e. field-free CM. We observe that time evolution of the hole via CM affects the ionization yields in multiple ways. First, the yield is increased when the field frequency and the CM frequency are commensurate. Second, in the commensurate case, the ionization yields are enhanced when the field time delay is such that the hole is on the triple bond when the field is a maximal. Additional calculations with different electron donating and withdrawing groups were also studied to determine the effect of hole contrast on yield. These results demonstrate how SFI is a promising technique for observing hole dynamics during CM. |
Wednesday, June 7, 2023 9:06AM - 9:36AM |
H04.00005: Electronic quantum coherence in glycine molecules probed with ultrashort x-ray pulses in real time Invited Speaker: Tim Laarmann Since the pioneering experiments by Weinkauf and Schlag on electron mobility and dissociation in peptide cations [1], the interplay between local ionisation and molecular reactivity is of considerable interest in many areas of physics, chemistry and biology [2]. The amino acid glycine is an abundant basic building block of proteins and plays a part in the recognition sites on cell membranes and enzymes. When energetic radiation hits a glycine molecule, often one of its electrons gets knocked out. In the resulting glycine ion, the remaining electronic charge begins to redistribute itself, resulting in a time-dependent oscillation of the charge density. To ionise glycine, we used the ultrashort soft X-ray pulses from the free-electron laser FLASH at DESY, each lasting less than five femtoseconds. With these flashes and by applying sophisticated post-analysis data processing algorithms, we could look at the behaviour of one specific of glycine’s 40 electrons from a particular orbital. In the applied time-resolved Auger electron spectroscopic study, events featuring electrons from the 10a’ inner-valence orbital are of interest. The 10a’ orbital spans nearly the full molecular backbone, and in consequence, the transient local electron hole density moves to the same extent [3], thus making this orbital also an excellent candidate for the study of charge-induced chemical dynamics [4]. While the initial knockout of the electron results in a positive charge at a specific atom in the molecule, the following charge oscillation creates a force field that makes the nuclei move as well.The time-frequency spectra of the many-body quantum mechanical wave packets represented by coherent superpositions of electronic states dressed by vibrational excitations have been measured for the first time along different reaction coordinates in the glycine cation. We could show that the observed coherences reveal rich information on the many-body quantum system including ultrafast decay and site-specific couplings that differ in phase. |
Wednesday, June 7, 2023 9:36AM - 9:48AM |
H04.00006: Time-delay control of reversible electron spirals using arbitrarily chirped attopulses Muhammad Aqmar Haziq Md Yusoff, Nathaniel Joseph Jr Strandquist, Jean Marcel Ngoko Djiokap Photoionization through single photon absorption by two synchronized, linearly chirped, counter-rotating circularly polarized, attosecond pulses is known [1] to create reversible spiral patterns in the momentum distribution of the ejected electron when the two pulses have equal but opposite chirp rates. Here we extend this study by demonstrating how this reversible spiral pattern can be controlled by varying the chirp rates as well as the time delay between the pulses [2]. For two synchronized pulses, we find that using arbitrary chirp rates of each attosecond pulses can create a reversible spiral pattern identical to the one produced by equal but opposite chirp rates. Having two pulses with arbitrary chirp rates demonstrates the interference between two electron wavepackets with different full width half maximum (FWHM). By adding a nonzero time delay between the pulses, we find that the reversible spiral pattern can be controlled by using the linear Ramsey spectral phase to manipulate the chirp-induced linear or quadratic spectral phases of the photoelectron. Possible applications of such exquisite manipulation of both linear and quadratic spectral phases include manipulation of the photoelectron wavepacket group delay. |
Wednesday, June 7, 2023 9:48AM - 10:00AM |
H04.00007: Attochemistry Principles of Charge Migration Aderonke S Folorunso, Francois Mauger, Kyle A Hamer, Denawakage D Jayasinghe, Imam S Wahyutama, Justin R Ragains, Robert R Jones, Louis F DiMauro, Mette B Gaarde, Kenneth J Schafer, Kenneth Lopata Attosecond charge migration (CM) is a coherent process where an electron density hole moves across a molecule in a particle-like manner. Despite intense experimental and theoretical interest, many questions remain about how charge migration can be modulated by systematic variation in the chemical structure of a molecule, and how this can be understood mechanistically. Building on our previous studies of halogen effects and bonding, we present systematic time-dependent density-functional theory (TDDFT) simulations of CM in para-functionalized bromobenzene derivatives (Br-C6H4-R). We observe enhanced hole contrast for the strong electron donors (e.g., R = N(CH3)2, -NH2), and reduced contrast for electron acceptors (e.g., -CF3). The trend is quantitatively described by the Hammett sigma value of the group, which is a metric describing the chemical reactivity of benzene derivatives. We also present results demonstrating how the initial hole location affects CM, and how more complex molecular geometries can result in steering of CM. Collectively, our observations demonstrate that simple attochemistry principles and a density-based picture are useful tools for predicting and understanding CM, without resorting to an ambiguous interpretation in terms of a complicated beating of many states. |
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