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 Z01: Science with XUV and X-ray free-electron lasersLive
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Chair: Anne Marie March, Argonne National Lab |
Friday, June 4, 2021 10:30AM - 10:42AM Live |
Z01.00001: Imaging Electronic Fluxes via Time-Resolved X-Ray Scattering Gopal Dixit, Jean Christophe Tremblay, Gunter Hermann, Vincent Pohl Imaging of electron charge distribution is crucial to understand different instances during chemical reactions such as conformational changes, charge migration, and bond formation and breakage. According to the quantum continuity equation, the flow of electrons is accompanied by associated electronic fluxes. The latter offers a wealth of information and has played a decisive role for understanding chemical reaction mechanisms. However, the notion of electronic fluxes has been restricted to theoretical modelling and there is no general way to probe them directly in experiment. In this work, we demonstrate theoretically real-space and real-time imaging of electronic fluxes associated with charge migration using time-resolved x-ray scattering. For this purpose, we consider oriented benzene as a test system in which a pump pulse induces adiabatic charge migration and ultrashort x-ray pulses probe the electronic fluxes accompanying charge migration. |
Friday, June 4, 2021 10:42AM - 10:54AM Live |
Z01.00002: Optimized XFEL pulses for angle-resolved photoelectron spectroscopy: probing the role of the molecular potential in N2 double ionization Siddhartha Chattopadhyay, Ludger Inhester, Robin Santra, Artem Rudenko, Daniel Rolles, Loren Greenman The attosecond core-hole wave packet dynamics can be probed through the angular correlation in two-site double core hole photoelectron angular distributions. The theoretical questions that remain unanswered are whether the core hole dynamics can be observed after taking into account the scattering phase shift with two photoelectron continuum states, and whether the present experimental capability at LCLS-II can observe such ultrafast dynamics. To address those questions, we have developed a range of approximations that uses a combination of time-dependent perturbation theory with variational scattering theory and the scattering phase shift by considering a plane wave and Coulomb wave continuum states with an optimized XFEL pulse. The phase information, which is encoded in the two photoelectron Coulomb continuum states shows the complex nature of the core hole oscillations due to coherent superposition from the multicenter potential. To address the second question, we introduce a dynamical parameter to optimize the FEL pulse. Our simulations predict a wide photon energy range where core hole dynamics can be observed experimentally that is consistent with the current capabilities of LCLS-II. |
Friday, June 4, 2021 10:54AM - 11:06AM Live |
Z01.00003: Strong-field ionization of FEL-prepared doubly excited states in the helium atom Gergana Borisova, Severin Meister, Hannes Lindenblatt, Florian Trost, Patrizia Schoch, Veit Stooß, Markus Braune, Rolf Treusch, Harald Redlin, Nora Schirmel, Christian Ott, Robert Moshammer, Thomas Pfeifer Electron correlation plays a fundamental role in light-matter interaction. To gain new insights into the role of the initial state for the ionization process, we prepared doubly excited states in helium with extreme ultraviolet (XUV) light provided by the free-electron laser in Hamburg (FLASH), and followed their decay in the presence of an additional infrared (IR) field. The XUV photon energy was scanned over the broad region of doubly excited states between 59 eV and 80 eV, reaching as high as the double ionization continuum of helium. The synchronized IR pulses (800 nm wavelength) strong-field ionize the selectively prepared doubly excited states. In a fully differential measurement of the charged ionization products using a reaction microscope (ReMi) we studied the strong-field dynamics as a function of the XUV photon energy. Both single- and double-ionization events have been observed and the impact of different ionization mechanisms will be discussed, also in comparison with simple model calculations. |
Friday, June 4, 2021 11:06AM - 11:18AM Live |
Z01.00004: Towards observing site-selective chemistry in real time: controlling the formation of molecular double-core-hole states Dimitris Koulentianos, Kai Li, Stephen Southworth, Adam E Fouda, Phay J Ho, Linda Young, Taran Driver, Siqi Li, Jordan T O'Neal, Ming Fu Lin, Thomas J Wolf, Peter Walter, James P Cryan, Xuechen Zheng, Lan Cheng, Gilles Doumy The development of x-ray free electron laser (XFEL) light sources, and third generation synchrotron radiation (SR) facilities allowed for the experimental observation of the elusive double-core-hole (DCH) continuum states. More specifically, for molecules, when two electrons bound at two different core-shells have been ejected to the continuum, enhanced chemical shifts can be measured [1]. Using an XFEL the creation of a DCH state relies on sequential multi-photon absorption [2], while their formation using SR can be achieved through electron correlations [3]. The newly offered ω/2ω x-ray pump-probe scheme, offered by the Linac Coherent Light Source (LCLS), can achieve a minimum delay (0 fs) between the pump and the probe pulses, and allows for more control in the creation of a DCH state by optimizing the ionization cross-sections. By varying the delay between the pump and the probe pulses, the time-evolution of the chemical shift along with the processes at play upon the creation of a core-hole can be studied, both before and after Auger decay takes place. Experiments were performed in the newly commissioned TMO endstation equipped with a Velocity Map Imaging (VMI) electron spectrometer for efficient collection of the photoemission signals and a secondary Time-of-Flight (ToF) electron spectrometer used for single shot characterization of the pump and probe pulses. The experimental results have been supported by ab initio calculations performed at the Δ-CCSD(T) level of theory. First studies on a class of fluorocarbon molecules will be presented. |
Friday, June 4, 2021 11:18AM - 11:30AM Live |
Z01.00005: Probing the ultrafast ring-reconfiguration reaction from quadricyclane to norbornadiene Kurtis D Borne, Surjendu Bhattacharyya, Matteo Bonanomi, Carlo Callegari, Michele Di Fraia, Joao Pedro Figueira Nunes, Shashank Pathak, Oksana Plekan, Anbu S Venkatachalam, Martin Centurion, Artem Rudenko, Peter M Weber, Daniel Rolles, FERMI QD collaboration We report the results of an experiment studying the UV-induced ring-reconfiguration reaction of a multicyclic hydrocarbon, quadricyclane, using time-resolved XUV photoelectron spectroscopy (TRPES). Utilizing a time-of-flight magnetic bottle spectrometer and 19-eV photons from the seeded free-electron laser FERMI, we probe the excitation to 3p and 3s Rydberg states induced by the absorption of a 200-nm photon, the subsequent ultrafast relaxation dynamics to the electronic ground state, and the isomerization dynamics to the bicyclic norbornadiene. The results are compared to a previous UV-TRPES study conducted at a slightly different excitation wavelength of 208 nm. |
Friday, June 4, 2021 11:30AM - 11:42AM Live |
Z01.00006: High-Accuracy Calculations of X-ray Spectra via Relativistic Coupled-Cluster Techniques Xuechen Zheng, Lan Cheng, Stacey L Sorensen, Gilles Doumy, Stephen Southworth, Linda Young We present a systematic route to high accuracy for ab initio calculations of near edge x-ray absorption fine structure (NEXAFS) spectra using core-valence separated equation-of-motion coupled-cluster methods to systematically approach the full configuration interaction limit, an edge-specific scheme for capturing the diffuse nature of core-excited Rydberg states, and the exact two-component theory for treating relativistic effects. Benchmark calculations show that triple excitations not only play important roles in obtaining accurate absolute values for core excitation energies, but also make significant contributions to relative shifts between valence and Rydberg excitations. Calculations of gas-phase water molecule with the inclusion of quadruple excitations and adequate treatments of vibrational structures are shown to reach <0.2 eV accuracy in terms of absolute energies. We also report a joint experimental-computational study of the NEXAFS spectrum for ethyl-trifluoroacetate, as an example of a fairly complex molecule. |
Friday, June 4, 2021 11:42AM - 11:54AM Live |
Z01.00007: Time-Resolved Dynamics on the Giant Plasmon Resonance of C60 Debadarshini Mishra, Aaron C LaForge, Razib Obaid, Shashank Pathak, Florian Trost, Hannes Lindenblatt, Severin Meister, Philipp Rosenberger, Rupert Michiels, Edwin Kukk, Shubhadeep Biswas, Matteo McDonnell, Krishna Saraswathula, Frank Stienkemeier, Daniel Rolles, Francesca Calegari, Markus Braune, Marcel Mudrich, Matthias F Kling, Thomas Pfeifer, Ulf Saalmann, Jan M Rost, Robert Moshammer, Nora Berrah We have investigated the time-resolved ionization and fragmentation dynamics of C60 using intense, ultrashort radiation from the FLASH free-electron laser tuned to the giant plasmon resonance near 20 eV. The dynamics were probed using a one-color pump-probe technique in combination with a reaction microscope. We will present a systematic study of the time-dependent kinetic energies of the atomic and molecular ionic fragments in addition to ion-ion coincidence channels as a function of photon energy, pulse intensity, and pulse length. Our results indicate a strong pulse intensity dependence of the kinetic energies and yields of the carbon monomer and dimer ions, along with a photon energy dependence in the yield of the carbon dications. We will discuss the possible mechanisms for the observed dynamics. |
Friday, June 4, 2021 11:54AM - 12:06PM Live |
Z01.00008: The TMO Instrument: Opportunities, First Results, and Plans for Time-resolved Atomic, Molecular and Optical Science at LCLS-II Peter Walter, James P Cryan, Ming-Fu Lin, Thomas J Wolf, Xiang Li, Ryan Coffee, Taran Driver, Andrei Kamalov, Razib Obaid, Jordan T O'Neal, Anna L Wang, David Fritz, Niranjan Shivaram The new designed Time-resolved Atomic, Molecular and Optical Science end station, will be configured to take full advantage of both the high per pulse energy from the copper accelerator (120 Hz) as well as high average intensity and high repetition rate (1 MHz) from the superconducting accelerator. TMO will support many experimental techniques not currently available at LCLS and will have two X-ray beam focus spots. Thereby, TMO will support AMO science, strong-field and nonlinear science and a new dynamic reaction microscope. |
Friday, June 4, 2021 12:06PM - 12:18PM Live |
Z01.00009: Roadmap to ultrafast quantum degenerate free electron beams Sam Keramati, Timothy J Gay, Herman Batelaan While degenerate Fermi gases have been extensively studied, and neutron and pion antibunching due to the Pauli force in Hanbury Brown-Twiss experiments demonstrated, quantum degenerate free electron beams have yet to be produced. Electron antibunching has been reported using a high-brightness DC field emitter [1], but the effects of Coulomb repulsion were neglected. We have shown theoretically [2] that pulsed, ultrafast photoemission from nanotips may lead to partial to complete quantum coherences, paving the way for the observation of macroscopic effects due to quantum degeneracy. We report here a coincidence experiment with an ultrafast W nanotip photoemitter that observed an antibunching signal of 1 part in 4, stronger by a factor of 300 compared with [1]. We attribute this sub-Poissonian distribution to transverse Coulomb repulsion of photoemitted electrons belonging to the same pulse. We present a roadmap, based on theory constrained by our experimental data, and with electron pulse duration, nanotip apex radius of curvature, and the coherence time and length as parameters, that can guide us to the achievement of quantum degenerate free electron beams. |
Friday, June 4, 2021 12:18PM - 12:30PM Live |
Z01.00010: Free-Electron Laser interacting with Atoms and Molecules Antonis Hadjipittas Free-Electron lasers have a very high photon energy, allowing inner electron ionisation of atoms and molecules. Leaving a hole in an inner orbital makes the system unstable leading to its decay until it reaches a more stable configuration. The system can alter its electronic configuration and energy by many processes such as Auger decay, shake-off or dissociation. At the same time, other photo-ionisations can occur, leading to a variety of possible paths the system can follow until it becomes stable |
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