TY - JOUR A1 - Wilkin, Kyle J. A1 - Parrish, Robert M. A1 - Yang, Jie A1 - Wolf, Thomas J. A. A1 - Nunes, J. Pedro F. A1 - Gühr, Markus A1 - Li, Renkai A1 - Shen, Xiaozhe A1 - Zheng, Qiang A1 - Wang, Xijie A1 - Martinez, Todd J. A1 - Centurion, Martin T1 - Diffractive imaging of dissociation and ground-state dynamics in a complex molecule JF - Physical review : A, Atomic, molecular, and optical physics N2 - We have investigated the structural dynamics in photoexcited 1,2-diiodotetrafluoroethane molecules (C2F4I2) in the gas phase experimentally using ultrafast electron diffraction and theoretically using FOMO-CASCI excited-state dynamics simulations. The molecules are excited by an ultraviolet femtosecond laser pulse to a state characterized by a transition from the iodine 5p perpendicular to orbital to a mixed 5p parallel to sigma hole and CF2 center dot antibonding orbital, which results in the cleavage of one of the carbon-iodine bonds. We have observed, with sub-Angstrom resolution, the motion of the nuclear wave packet of the dissociating iodine atom followed by coherent vibrations in the electronic ground state of the C2F4I radical. The radical reaches a stable classical (nonbridged) structure in less than 200 fs. Y1 - 2019 U6 - https://doi.org/10.1103/PhysRevA.100.023402 SN - 2469-9926 SN - 2469-9934 VL - 100 IS - 2 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Yang, Jie A1 - Zhu, Xiaolei A1 - Wolf, Thomas J. A. A1 - Li, Zheng A1 - Nunes, João Pedro Figueira A1 - Coffee, Ryan A1 - Cryan, James P. A1 - Gühr, Markus A1 - Hegazy, Kareem A1 - Heinz, Tony F. A1 - Jobe, Keith A1 - Li, Renkai A1 - Shen, Xiaozhe A1 - Veccione, Theodore A1 - Weathersby, Stephen A1 - Wilkin, Kyle J. A1 - Yoneda, Charles A1 - Zheng, Qiang A1 - Martinez, Todd J. A1 - Centurion, Martin A1 - Wang, Xijie T1 - Imaging CF3I conical intersection and photodissociation dynamics with ultrafast electron diffraction JF - Science N2 - Conical intersections play a critical role in excited-state dynamics of polyatomic molecules because they govern the reaction pathways of many nonadiabatic processes. However, ultrafast probes have lacked sufficient spatial resolution to image wave-packet trajectories through these intersections directly. Here, we present the simultaneous experimental characterization of one-photon and two-photon excitation channels in isolated CF3I molecules using ultrafast gas-phase electron diffraction. In the two-photon channel, we have mapped out the real-space trajectories of a coherent nuclear wave packet, which bifurcates onto two potential energy surfaces when passing through a conical intersection. In the one-photon channel, we have resolved excitation of both the umbrella and the breathing vibrational modes in the CF3 fragment in multiple nuclear dimensions. These findings benchmark and validate ab initio nonadiabatic dynamics calculations. Y1 - 2018 U6 - https://doi.org/10.1126/science.aat0049 SN - 0036-8075 SN - 1095-9203 VL - 361 IS - 6397 SP - 64 EP - 67 PB - American Assoc. for the Advancement of Science CY - Washington ER - TY - GEN A1 - Yang, Jie A1 - Guehr, Markus A1 - Vecchione, Theodore A1 - Robinson, Matthew Scott A1 - Li, Renkai A1 - Hartmann, Nick A1 - Shen, Xiaozhe A1 - Coffee, Ryan A1 - Corbett, Jeff A1 - Fry, Alan A1 - Gaffney, Kelly A1 - Gorkhover, Tais A1 - Hast, Carsten A1 - Jobe, Keith A1 - Makasyuk, Igor A1 - Reid, Alexander A1 - Robinson, Joseph A1 - Vetter, Sharon A1 - Wang, Fenglin A1 - Weathersby, Stephen A1 - Yoneda, Charles A1 - Wang, Xijie A1 - Centurion, Martin T1 - Femtosecond gas phase electron diffraction with MeV electrons N2 - We present results on ultrafast gas electron diffraction (UGED) experiments with femtosecond resolution using the MeV electron gun at SLAC National Accelerator Laboratory. UGED is a promising method to investigate molecular dynamics in the gas phase because electron pulses can probe the structure with a high spatial resolution. Until recently, however, it was not possible for UGED to reach the relevant timescale for the motion of the nuclei during a molecular reaction. Using MeV electron pulses has allowed us to overcome the main challenges in reaching femtosecond resolution, namely delivering short electron pulses on a gas target, overcoming the effect of velocity mismatch between pump laser pulses and the probe electron pulses, and maintaining a low timing jitter. At electron kinetic energies above 3 MeV, the velocity mismatch between laser and electron pulses becomes negligible. The relativistic electrons are also less susceptible to temporal broadening due to the Coulomb force. One of the challenges of diffraction with relativistic electrons is that the small de Broglie wavelength results in very small diffraction angles. In this paper we describe the new setup and its characterization, including capturing static diffraction patterns of molecules in the gas phase, finding time-zero with sub-picosecond accuracy and first time-resolved diffraction experiments. The new device can achieve a temporal resolution of 100 fs root-mean-square, and sub-angstrom spatial resolution. The collimation of the beam is sufficient to measure the diffraction pattern, and the transverse coherence is on the order of 2 nm. Currently, the temporal resolution is limited both by the pulse duration of the electron pulse on target and by the timing jitter, while the spatial resolution is limited by the average electron beam current and the signal-to-noise ratio of the detection system. We also discuss plans for improving both the temporal resolution and the spatial resolution. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 326 Y1 - 2016 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-394989 ER -