K. Jobe

Diffractive imaging of a rotational wavepacket in nitrogen molecules with femtosecond megaelectronvolt electron pulses

Imaging changes in molecular geometries on their natural femtosecond timescale with sub-Angström spatial precision is one of the critical challenges in the chemical sciences, as the nuclear geometry changes determine the molecular reactivity. For photoexcited molecules, the nuclear dynamics determine the photoenergy conversion path and efficiency. Here we report a gas-phase electron diffraction experiment using megaelectronvolt (MeV) electrons, where we captured the rotational wavepacket dynamics of nonadiabatically laser-aligned nitrogen molecules. We achieved a combination of 100 fs root-mean-squared temporal resolution and sub-Angstrom (0.76 Å) spatial resolution that makes it possible to resolve the position of the nuclei within the molecule. In addition, the diffraction patterns reveal the angular distribution of the molecules, which changes from prolate (aligned) to oblate (anti-aligned) in 300 fs. Our results demonstrate a significant and promising step towards making atomically resolved movies of molecular reactions.

Imaging CF3I conical intersection and photodissociation dynamics with ultrafast electron diffraction

Motion picture of a conical intersection In most chemical reactions, electrons move earlier and faster than nuclei. It is therefore common to model reactions by using potential energy surfaces that depict nuclear motion in a particular electronic state. However, in certain cases, two such surfaces connect in a conical intersection that mingles ultrafast electronic and nuclear rearrangements. Yang et al. used electron diffraction to obtain time-resolved images of CF3I molecules traversing a conical intersection in the course of photolytic cleavage of the C–I bond (see the Perspective by Fielding). Science, this issue p. 64; see also p. 30 Electron diffraction reveals the interplay of electronic and nuclear motion during light-induced scission of a C–I bond. 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.

Single-shot mega-electronvolt ultrafast electron diffraction for structure dynamic studies of warm dense matter

We have developed a single-shot mega-electronvolt ultrafast-electron-diffraction system to measure the structural dynamics of warm dense matter. The electron probe in this system is featured by a kinetic energy of 3.2 MeV and a total charge of 20 fC, with the FWHM pulse duration and spot size at sample of 350 fs and 120 μm respectively. We demonstrate its unique capability by visualizing the atomic structural changes of warm dense gold formed from a laser-excited 35-nm freestanding single-crystal gold foil. The temporal evolution of the Bragg peak intensity and of the liquid signal during solid-liquid phase transition are quantitatively determined. This experimental capability opens up an exciting opportunity to unravel the atomic dynamics of structural phase transitions in warm dense matter regime.

Measurements, system response, and calibration of the SLAC T-510 experiment

The SLAC T-510 experiment provides the first beam-test of radio-frequency radiation from a charged particle cascade in the presence of a magnetic field (up to 970 G), a model system for radio-frequency emission from a cosmic-ray air shower. The primary purpose of this experiment is to provide a suite of controlled laboratory tests to compare to simulations based on particlelevel models of RF emission, making the calibrations of critical importance. We present system calibrations and analysis of the experiment from end to end. Measurements of the beam charge and two-dimensional magnetic field map are fed directly into the simulations using two different formalisms: ZHS and Endpoints. Simulated electric fields are forward-folded with the system response, allowing for direct comparisons of spectra and waveforms with the simulations.

SLAC T-510: A beam-line experiment for Radio emission from particle cascades in the presence of a magnetic field

Astrophysics, National Taiwan University, Taipei, Taiwan. 7 Dept. of Physics, Univ. of Delaware, Newark, DE 19716, USA. 8 Dept. of Physics, Stanford University, Stanford, CA, 94305, USA. 9 Dept. of Physics and Astronomy, Univ. of Hawaii, Manoa, HI 96822, USA. 10SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, USA. 11Karlsruher Institut fur Technologie, Institut fur Kernphysik , 76021 Karlsruhe, Germany. 12Physics Dept., College of William & Mary, Williamsburg VA 23187, USA. 13Dept. of Physics and Astronomy, University College London, London, United Kingdom. 14Karlsruher Institut fur Technologie, Institut fur Experimentelle Kernphysik, 76128 Karlsruhe,