Bulletin of the American Physical Society
50th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics APS Meeting
Volume 64, Number 4
Monday–Friday, May 27–31, 2019; Milwaukee, Wisconsin
Session Q09: Time-Resolved Electron Scattering Processes |
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Chair: Anthony Starace, University of Nebraska, Lincoln Room: Wisconsin Center 103DE |
Thursday, May 30, 2019 2:00PM - 2:30PM |
Q09.00001: Attosecond Electron Imaging. Invited Speaker: Mohammed Hassan Ultrafast Electron Microscopy and Diffraction has been demonstrated to be an effective table-top technique for imaging the atomic motion in real time and space. However, imaging the faster motion of electron dynamics has remained beyond reach due to the lack of temporal resolution and the time jittering between the pump``laser'' and the probe ``electron'' pulse. Recently, we demonstrated the ability to control the temporal profile of the electron pulses using ultrashort laser pulses, the approach we called the optical gating. The recent advancement in attosecond physics and the generation of optical attosecond pulses open the door for generating sub-femtosecond electron pulses by the attosecond optical gating to attaining the desired temporal resolution in electron microscopy and diffraction imaging experiment, to establish what we so-called ``Attomicroscopy'', which enable the imaging of electron motion in the act. Such electron imaging would reveal the quantum physics of complicated systems. Also, it will provide real-time access to electron dynamics in atoms and molecules and improve our understanding of chemistry. [Preview Abstract] |
Thursday, May 30, 2019 2:30PM - 3:00PM |
Q09.00002: Imaging electronic and molecular motions by ultrafast electron diffraction and impact ionization Invited Speaker: Hua-Chieh Shao Owing to advances in ultrafast electron technologies, it is now possible to generate and manipulate fs electron pulses with sub-\aa ngstr\"om de Broglie wavelengths. Therefore, it is now feasible to directly image and visualize electronic and molecular transient motions during reactions. We have theoretically studied and explored various schemes for utilizing ultrafast electron pulses to image electronic and molecular motions [1-6]. In particular, time-resolved $(e, 2e)$ electron momentum spectroscopy has been considered as a means of probing the momentum profile of electronic motion in lithium atoms [6]. Specifically, we have studied the adiabatic population transfer of the electronic state of the Li atom from the ground state ($2s$) to the first excited state ($2p$) driven by a chirped ps laser pulse. During the population transfer, the time-varying valence electronic motion is imaged by time-delayed 100- and 1-fs electron pulses through the mechanism of high-energy impact ionization. The simulations show that the momentum distribution of the valence electron at the moment of collision can be retrieved from the time-resolved spectrum. However, the level of detail of the information about the motion depends on the pulse duration and the time scale of the electronic motion.\\ \\Recently, we have studied ultrafast electron diffraction as a means of imaging the oriented ro-vibrational motions of deuterated lithium hydride (LiD) and hydrogen (HD) molecules. The molecular motion is assumed to be initiated by a pump pulse that impulsively excites an electron from the ground state to some excited electronic state. Then the ensuing molecular motion in the excited state is imaged by 1-fs electron pulses. The simulated diffraction images show a delay-dependent ring pattern owing to the interfering scattering amplitudes from the constituent atoms as the molecule vibrates. Moreover, the centrosymmetry of the diffraction images is violated in time-resolved measurements, which exhibit asymmetric angular distributions that relate to the direction of motion of the atoms.\\ \\This work is supported in part by the U.S. National Science Foundation under grant No. PHY-1505492. This work was completed utilizing the Holland Computing Center of the University of Nebraska.\\ \noindent[1] H.-C. Shao and A.F. Starace, Phys. Rev. A \textbf{87}, 050701(R) (2013); [2] H.-C. Shao and A.F. Starace, Phys. Rev. A \textbf{88}, 062711 (2013); [3] H.-C. Shao and A.F. Starace, Phys. Rev. A \textbf{90}, 032710 (2014); [4] H.-C. Shao and A.F. Starace, Phys. Rev. A \textbf{94}, 030702(R) (2016); [5] H.-C. Shao and A.F. Starace, Phys. Rev. A \textbf{96}, 042706 (2017); [6] H.-C. Shao and A.F. Starace, Phys. Rev. A \textbf{97}, 022702 (2018). [Preview Abstract] |
Thursday, May 30, 2019 3:00PM - 3:12PM |
Q09.00003: Scattering Processes using Unique Particle Wave Forms Allison Harris, Alexander Plumadore, Zoryana Smozhanyk Recent experimental work has succeeded in producing electron wave packets with unique features. One such case is that of an electron vortex beam in which the electrons carry discrete amounts of orbital angular momentum. Another new electron wave packet in the form of an Airy function has also been generated experimentally. Airy wave packets are minimally dispersive, exhibit force-free acceleration, and can self-heal. All of these newly generated particle wave forms open the door to countless applications such as the control and rotation of nanoparticles, improved resolution in electron microscopy, characterization of chiral structures, and many others. In order for possible applications to be realized, it is necessary to understand how these newly generated wave forms interact with matter at a fundamental level. Unfortunately, there is very little work, either experimentally or theoretically, regarding the basic interactions of these electron wave packets with individual atoms or molecules. We present here theoretical studies of electron vortex and Airy beam scattering processes, including ionization and tunneling. [Preview Abstract] |
(Author Not Attending)
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Q09.00004: Theoretical study of vibrational excitation and dissociative electron attachment of NO2 by an electron impact Hainan LIU, Samantha Fonseca dos Santos, Chi Hong Yuen, Pietro CORTONA, Viatcheslav Kokoouline, Mehdi AYOUZ The NO2 molecule plays a critical role in modeling atmospheric processes. However, the theoretical description of the vibrational excitation and dissociative electron attachment (DEA) for this open-shell molecule is still an extremely challenging task to date. In this study, we use a theoretical approach that combines the normal modes approximation, the R-matrix formalism, and the vibrational frame transformation to compute the cross sections for electron-impact vibrational excitation of NO2. The cross sections for DEA to NO2 are estimated through a simplified approach which is based on the fact that the resonance energy only varies substantially over a subset of normal coordinates, and compared to available experimental data. Thermally-averaged rate coefficients are obtained from the cross sections for the temperatures 10 K-10000 K. [Preview Abstract] |
(Author Not Attending)
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Q09.00005: Vibronic excitation cross sections and rate coefficients of CH$+$ by electron impact Xianwu Jiang, Chi Hong Yuen, Pietro Cortona, Viatcheslav Kokoouline, Mehdi Ayouz The vibronic excitations of CH$+$ by electron impact are of great importance in astrophysical and technological plasmas. However, this process is far from being precisely modeled in theory. We propose a new model that combines the multichannel quantum defect theory (MQDT) and the UK R-matrix code to compute cross sections and thermally-averaged rate coefficients for vibronic (de-)excitation of CH$+$ by an electron impact. In this model, the R-matrix formalism is employed to evaluate the electron-ion scattering matrix for a fixed geometry of the ion. The scattering matrix describing the vibronic transition is obtained from the vibronic frame transformation and close-channel-elimination procedure which are applied at high scattering energies where the scattering matrix is smooth. For the obtained rate coefficients, fitting formulas are derived. The interval of applicability of the formulas is from 40 to 10,000 K. [Preview Abstract] |
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