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The conversion of light into chemical and mechanical energy mediates many important processes in nature, e.g. vision, photosynthesis and DNA photodamage. To understand the structure-function relationships regulating such processes one must strive to study them in their natural environment, i.e. in the liquid-phase. This presentation reports on the design of a novel Ultrafast Electron Diffraction instrument capable of resolving structural dynamics in liquid samples. The capabilities of this instrument are showcased in the study of water, where its structure was resolved up to the 3rd hydration shell with 0.6 A spatial resolution, and dynamics were resolved with 200 fs resolution.

The conversion of light into usable chemical and mechanical energy is pivotal to several biological and chemical processes, many of which occur in solution. To understand the structure–function relationships mediating these processes, a technique with high spatial and temporal resolutions is required. Here, we report on the design and commissioning of a liquid-phase mega-electron-volt (MeV) ultrafast electron diffraction instrument for the study of structural dynamics in solution. Limitations posed by the shallow penetration depth of electrons and the resulting information loss due to multiple scattering and the technical challenge of delivering liquids to vacuum were overcome through the use of MeV electrons and a gas-accelerated thin liquid sheet jet. To demonstrate the capabilities of this instrument, the structure of water and its network were resolved up to the 3 rd hydration shell with a spatial resolution of 0.6 Å; preliminary time-resolved experiments demonstrated a temporal resolution of 200 fs.

The development of ultrafast gas electron diffraction with nonrelativistic electrons has enabled the determination of molecular structures with atomic spatial resolution. It has, however, been challenging to break the picosecond temporal resolution barrier and achieve the goal that has long been envisioned—making space- and-time resolved molecular movies of chemical reaction in the gas-phase. Recently, an ultrafast electron diffraction (UED) apparatus using mega-electron-volt (MeV) electrons was developed at the SLAC National Accelerator Laboratory for imaging ultrafast structural dynamics of molecules in the gas phase. The SLAC gas-phase MeV UED has achieved 65 fs root mean square temporal resolution, 0.63 A spatial resolution, and 0.22 A−1 reciprocal-space resolution. Such high spatial-temporal resolution has enabled the capturing of real-time molecular movies of fundamental photochemical mechanisms, such as chemical bond breaking, ring opening, and a nuclear wave packet crossing a conical intersection. In this paper, the design that enables the high spatial-temporal resolution of the SLAC gas phase MeV UED is presented. The compact design of the differential pump section of the SLAC gas phase MeV UED realized five orders-of-magnitude vacuum isolation between the electron source and gas sample chamber. The spatial resolution, temporal resolution, and long-term stability of the apparatus are systematically characterized.

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 CF 3 I 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

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 CF

Observing the motion of the nuclear wavepackets during a molecular reaction, in both space and time, is crucial for understanding and controlling the outcome of photoinduced chemical reactions. We have imaged the motion of a vibrational wavepacket in isolated iodine molecules using ultrafast electron diffraction with relativistic electrons. The time-varying interatomic distance was measured with a precision 0.07 {\AA} and temporal resolution of 230 fs full-width at half-maximum (FWHM). The method is not only sensitive to the position but also the shape of the nuclear wavepacket.

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.

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 changes in molecular geometries with sufficient temporal and spatial resolution to image nuclei is a critical challenge in the chemical sciences. Here the authors report gasphase Megaelectronvolt electron diffraction with 100 fs temporal resolution and subAngstrom spatial resolution.

Niobium (Nb) superconducting radio-frequency (SRF) cavities have become the technology of choice for many recent and future particle accelerators. Increasing their accelerating gradient beyond the theoretical limit of Nb (~50 MV/m) by coating multilayer thin film superconductors is our ultimate goal. This idea proposed by Gurevich in 2005 is based on the assumption that the lower critical magnetic field parallel to the surface (Bc1//) increases with very thin films having a thickness close to its magnetic penetration depth. We measured the magnetic field Bpen at which a large number of vortices or fluxons start to penetrate into the material using a SQUID magnetometer. While we found that the measurements for the films thinner than ~200 nm is difficult due to insufficient signal strength and alignment accuracy, we also found that even the films with a thickness a few times the penetration depth show significant increase in Bpen, e.g., ~135 mT and ~210 mT for 500 nm and 300 nm films at 4.5 K, respectively, as compared to ~46mT for bulk MgB2. RF measurements of 50.8 mm diameter MgB2(100 nm)/Al2O3(20 nm)/Nb sample using 11.4 GHz pulsed power and a TE013-mode cavity showed quenches at ~42 mT at 4 K and ~34 mT at 10 K. These results were significantly lower than the numbers predicted from the low-power DC measurement results mentioned earlier, but these were found to be thermal quenches due to a high RF surface resistance caused by the inter-diffusion of coated components.

An X‐band RF cryogenic material testing system has been developed in the past few years. This system employs a high‐Q copper cavity with an interchangeable flat bottom working under a TE013 like mode. By measuring the cavity Qs with a network analyzer, the system can characterize the surface resistance of different samples at different temperatures. Using a 50 MW 2μs pulsed klystron, the system can measure the quenching H field for superconducting samples, up to 300–400 mT. In this paper, we will present the most recent developments of the system and testing results.

An experimental station for an X-band Next Linear Collider has been constructed at SLAC. This station consists of a klystron and modulator, a low-loss waveguide system for RF power distribution, a SLED II pulse-compression and peak-power multiplication system, acceleration sections and beam-line components (gun, pre-buncher, pre-accelerator, focussing elements and spectrometer). An extensive program of experiments to evaluate the performance of all components is underway. The station is described in detail in this paper, and results to date are presented. >

Niobium (Nb) Superconducting RF (SRF) cavities have been used or will be used for a number of particle accelerators. The fundamental limit of the accelerating gradient has been thought to be around 50 MV/m due to its RF critical magnetic field of around 200 mT. This limit will prevent new projects requiring higher gradient and compact accelerators from considering SRF structures. There is a theory, however, that promises to overcome this limitation by coating thin (less than the penetration depth) superconductors on Nb. We initiated measurements of critical magnetic fields of Nb coated with various thin film superconductors, starting with MgB2 films deposited using reactive evaporation technique, with the goal to apply this coating to SRF cavities. This paper will present first test results of the RF critical magnetic field of a system consisting of a 10 nm B and a 100 nm MgB2 films deposited on a chemically polished 2-inch single grain Nb substrate.