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 T5: Focus Session: Molecular Imaging and ControlFocus
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Chair: Anthony Starace, University of Nebraska--Lincoln Room: 310 |
Friday, June 9, 2017 8:00AM - 8:30AM |
T5.00001: Ultrafast Imaging of Isolated Molecules with Electron Diffraction Invited Speaker: Martin Centurion Capturing molecular dynamics as they take place is essential for understanding and eventually controlling the outcome of chemical reactions. Ultrafast electron diffraction can be used to image the structure of isolated molecules with atomic resolution, however, until recently it was not possible to reach the femtosecond resolution needed to observe the nuclear motion. We have recently imaged a vibrational wavepacket in iodine with a resolution of 0.1 {\AA} in space and 230 fs in time with electron diffraction, using the MeV electron source at SLAC National Laboratory. This result opens the door to imaging structural dynamics in more complex reactions. Work is currently ongoing in diffraction experiments to capture conformational changes in molecules and also towards improving the temporal and spatial resolution. [Preview Abstract] |
Friday, June 9, 2017 8:30AM - 9:00AM |
T5.00002: Laser-induced electron diffraction for dynamic imaging of molecules Invited Speaker: Chii-Dong Lin Electron diffraction is the well-established tool for probing the structure of gas-phase molecules near the equilibrium geometry. To study chemical dynamics ultrashort electron pulses below a few tens of femtoseconds are needed. Laser-induced electron diffraction (LIED) is a method where molecules can be probed with femtosecond temporal resolution and sub-angstrom spatial resolution. In LIED, molecules are exposed to an intense femtosecond laser pulse. The electrons that have been previously removed by the laser field can be driven back later to rescatter with the parent molecular ion. Using diffraction images from large-angle backscattered events, sub-angstrom spatial resolution can be achieved with tens to hundreds eV electrons. Recent LIED experimental results showing bond breaking in molecules will be illustrated $^{\mathrm{1}}$ . Practical issues related to the retrieval of diffraction images from LIED on aligned molecules $^{\mathrm{2}}$ and possibilities of real-time imaging of dissociating molecules using LIED will be presented. $^{\mathrm{1}}$B. Wolter et al, Science 354,308 (2016). $^{\mathrm{2}}$Chao Yu et al, Scientific Rept. 5, 15753 (2015). This work is performed in collaboration with Anh Thu Le (Kansas State University) and the experimental group of Jens Biegert (ICFO, Barcelona). [Preview Abstract] |
Friday, June 9, 2017 9:00AM - 9:12AM |
T5.00003: Simultaneous x-ray imaging of A and B state dynamics in iodine at the LCLS Matthew Ware, Adi Natan, James Cryan, Phil Bucksbaum, James Glownia We will discuss our most recent analysis of the nuclear dynamics of photoexcited iodine from the LCLS. At the LCLS, we pumped a gas cell of iodine with a weak 520nm, 50 fs pulse from the X state into the dissociating A and bound B electronic states, and the nuclear dynamics are then probed with 9 keV, 40 fs x-rays with variable time delay. The A and B electronic states are perpendicular and parallel electronic transitions respectively, so their time-dependent x-ray diffraction signals can be isolated from each other. This work highlights the difficulty of using x-ray diffraction to film the excited state dynamics in molecules: the diffraction signal is an incoherent sum of all populated electronic states and, when the x-ray coherence time is shorter than the electronic coherence time, the signal can potentially include coherent interference between electronic states. Thus, even for simple systems like iodine, distinguishing the nuclear dynamics on different electronic surface s becomes a difficult task. For iodine, the analysis required knowledge of the dipole selection rules between the X and the A and B electronic states as well as the bound and dissociating character of the A and B states in order to separate the total diffraction signal into its A and B state contributions. [Preview Abstract] |
Friday, June 9, 2017 9:12AM - 9:24AM |
T5.00004: Detection of Core Hole Localization in X-ray Photoionization Cynthia Trevisan, Clyde McCurdy, Thomas Rescigno In the quest to find further evidence of the core hole localization phenomenon we recently found in CF$_{\mathrm{4}}$, we present \textit{ab initio} calculations of molecular frame photoelectron angular distributions of electrons ejected from the core orbitals of the fluorine K-edge of various isomers of difluoroethylene (C$_{\mathrm{2}}$H$_{\mathrm{2}}$F$_{\mathrm{2}})$.$_{\mathrm{\thinspace }}$In the case of CF$_{\mathrm{4}}$, the probability of removing a core electron from any of the four F atoms is the nearly the same for all directions of photoejection of the electron. However, we found that for a particular decay channel, detecting an F$^{\mathrm{+}}$ ion makes the probability of having this ion be the atom that was core ionized nearly unity, because of a chemical effect related to the electronegativity of fluorine. C$_{\mathrm{2}}$H$_{\mathrm{2}}$F$_{\mathrm{2}}$ has two symmetry-equivalent fluorine atoms. Our work explores the extent to which the localization of core holes also takes place on one of two of the fluorine equivalent atoms following X-ray photoionization and is clearly visible in an experiment that averages around the axis of recoil of ion fragments after Auger decay. [Preview Abstract] |
Friday, June 9, 2017 9:24AM - 9:36AM |
T5.00005: Imaging many-body Coulomb interactions and ultrafast photoionization and diffraction with cold atom electron and ion sources Robert Scholten, Rory Speirs, Dene Murphy, Joshua Torrance, Daniel Thompson, Benjamin Sparkes, Andrew McCulloch The CAEIS cold-atom electron/ion source, based on photoionisation of laser cooled atoms, provides a powerful tool for investigating fundamental physical processes. The very low temperature of the ions has allowed us to image intra-beam Coulomb effects with unprecedented detail. With ultrafast laser excitation and streak detection we can probe competing ionization processes, particularly via Rydberg states, including sequential excitation, multiphoton excitation, resonance-enhanced multiphoton excitation and two-color multiphoton excitation. Knowledge from these studies has enabled ultrafast single-shot diffractive electron imaging with atomic resolution using a CAEIS. [Preview Abstract] |
Friday, June 9, 2017 9:36AM - 9:48AM |
T5.00006: Laser-induced echoes: from molecular alignment to harmonics generation in free electron lasers Ilya Sh. Averbukh, E. Gershnabel, Y. Prior, G. Karras, E. Hertz, F. Billard, B. Lavorel, O. Faucher, J.-M. Hartmann, G. Siour, K. Lin, P. Lu, J. Ma, X. Gong, Q. Song, Q. Ji, W. Zhang, H.P. Zeng, J. Wu Recently we predicted a novel phenomenon of molecular alignment echoes, and demonstrated it by measuring laser-induced birefringence in a thermal CO$_{\mathrm{2}}$ gas excited by a pair of femtosecond laser pulses [PRL 114, 153601 (2015)]. Here we report a new effect of fractional echoes observed in the same system via the third harmonic generation from a probe pulse [PRA 94, 033404 (2016)]. Moreover, using the technique of coincidence Coulomb explosion imaging (COLTRIMS) for the direct spatiotemporal analysis of molecular angular distribution, we observed a gamut of novel types of echoes, including fractional echoes of high order, spatially rotated echoes, and the counter-intuitive imaginary echoes at negative times [PRX 6, 041056 (2016)]. Remarkably, a phenomenon similar to fractional echoes of high order lies behind the recent demonstration of the efficient generation of high harmonics in free-electron lasers [Nat. Photonics 10, 512 (2016)]. [Preview Abstract] |
Friday, June 9, 2017 9:48AM - 10:00AM |
T5.00007: Control of Rotational Energy and Angular Momentum Orientation with an Optical Centrifuge Hannah M. Ogden, Matthew J. Murray, Amy S. Mullin We use an optical centrifuge to trap and spin molecules to an angular frequency of 30 THz with oriented angular momenta and extremely high rotational energy and then investigate their subsequent collision dynamics with transient high resolution IR spectroscopy. The optical centrifuge is formed by combining oppositely-chirped pulses of 800 nm light, and overlapping them spatially and temporally. Polarization-sensitive Doppler-broadened line profiles characterize the anisotropic kinetic energy release of the super rotor molecules, showing that they behave like molecular gyroscopes. Studies are reported for collisions of CO$_{\mathrm{2}}$ super rotors with CO$_{\mathrm{2}}$, He and Ar. These studies reveal how mass, velocity and rotational adiabaticity impact the angular momentum relaxation and reorientation. Quantum scattering calculations provide insight into the J-specific collision cross sections that control the relaxation. [Preview Abstract] |
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