Martin Centurion

Imaging of molecules in the gas phase with ultrafast electron diffraction

A two-step algorithm is developed that can reconstruct the full 3-D molecular structure from diffraction patterns of partially aligned molecules in gas phase. This method is applicable to asymmetric-top molecules that do not need to have any specific symmetry. This method will be important for studying dynamical processes that involve transient structures where symmetries, if any, can possibly be broken. A new setup for the diffraction experiments that can provide enough time resolution as well as high currents suitable for gas phase experiments is reported. Time resolution is obtained by longitudinal compression of electron pulses by time-varying electric fields synchronized to the motion of electron pulses.

Ultrafast 3D imaging of isolated molecules with electron diffraction

Three-dimensional imaging of molecules in the gas phase has been an important but challenging task, since the randomly oriented molecules only provide one-dimensional structural information. In this work, we show that a three-dimensional structure can be reconstructed from ultrafast electron diffraction from impulsively laser-aligned molecules. The diffraction pattern is taken at the maximum degree of alignment, around two picoseconds after the excitation of the laser. An iterative retrieval algorithm is developed to resolve the problem generated by imperfect alignment and a holographic algorithm is used to reconstruct molecular structure.

High-coherence relativistic electron probes for ultrafast structural dynamics

We report on experimental activities on HiRES, a novel ultrafast electron diffraction beamline under development at LBNL. The instrument provides high-flux of relativistic electron pulses with sub-picosecond duration, which are then shaped in transverse and longitudinal phase space producing small spot sizes with femtosecond resolution. Alternatively beam shaping can be used to achieve large lateral coherence lengths for chemical and biological applications.

Femtosecond electron pulse generation and measurement for diffractive imaging of isolated molecules

We have constructed an electron gun that delivers highly charged femtosecond electron pulses to a target with kHz repetition rate. Electron pulses are generated by femtosecond laser pulses in a photoemission process and are accelerated up to 100 kV and compressed to sub-picosecond duration. Compression is essential to compensate for the space charge effect that increases the size of electron pulses in all directions significantly. The pulses are compressed transversely by magnetic lenses and longitudinally by the longitudinal electric field of a radio-frequency cavity. The longitudinal compression is achieved by decelerating the electrons in the leading edge of the pulse, and accelerating the electrons in the trailing edge of the pulse. This results in the pulse compressing and reaching the minimum pulse duration at a known distance from the compression cavity. The short pulse duration and high repetition rate will be essential to observe subpicosecond dynamic processes in molecules in gas phase with a good signal to noise ratio. A streak camera, consisting of a millimeter-sized parallel plate capacitor, was used to measure the pulse duration in situ.

Relativistic ultrafast electron diffraction from molecules in the gas phase(Conference Presentation)

Ultrafast electron diffraction (UED) is a powerful technique that can be used to resolve structural changes of gas molecules during a photochemical reaction. However, the temporal resolution in pump-probe experiments has been limited to the few-ps level by the space-charge effect that broadens the electron pulse duration and by velocity mismatch between the pump laser pulses and the probe electron pulses, making only long-lived intermediate states accessible. Taking advantage of relativistic effects, Mega-electron-volt (MeV) electrons can be used to suppress both the space-charge effect and the velocity mismatch, and hence to achieve a temporal resolution that is fast enough to follow coherent nuclear motion in the target molecules. In this presentation, we show the first MeV UED experiments on gas phase targets. These experiments not only demonstrate that femtosecond temporal resolution is achieved, but also show that the spatial resolution is not compromised. This unprecedented combination of spatiotemporal resolution is sufficient to image coherent nuclear motions, and opens the door to a new class of experiments where the structural changes can be followed simultaneously in both space and time.

Vibrational and condensed phase dynamics: general discussion

R. J. Dwayne Miller opened a general discussion of the paper by Junko Yano: Assuming one can obtain the structure of the Mn/Ca water splitting site, is this sufficient information to understand the water splitting mechanism? I would think one would want to catch the actual water splitting event in the barrier crossing region. The Mn/Ca positions could be strongly modulated by the protein environment to drive this process. Synthetically, it would be relatively easy to do this but the dynamic coupling to the surrounding protein and the charged environment could be the difference in the low efficiency so far of synthetic photocatalysts. Put another way, what is the key question that will be answered by getting the structure of the water splitting complex in the fully primed S4 state?

Structural dynamics: general discussion

Piero Decleva, Andrew J. Orr-Ewing, Markus Kowalewski, Oleg Kornilov, Jon P. Marangos, Hans Jakob Wörner, Allan S. Johnson, Ruaridh Forbes, Daniel Rolles, Dave Townsend, Oliver Schalk, Sebastian Mai, Tom J. Penfold, R. J. Dwayne Miller, Martin Centurion, Kiyoshi Ueda, Wolfgang Domcke, Peter M. Weber, Kyoung Koo Baeck, Oksana Travnikova, Chelsea Liekhus-Schmaltz, João Pedro Figueira Nunes, Daniel M. Neumark, Oliver Gessner, Albert Stolow, Artem Rudenko, Pankaj Kumar Mishra, Adam Kirrander, Danielle Dowek, Fernando Mart́ın, Ágnes Vibók, Michael P. Minitti, Brian Stankus and Christian Burger

Development of MeV Ultrafast Electron Scattering Instruments at SLAC National Accelerator Laboratory

Recent years have witnessed rapid development of ultrafast electron and x-ray scattering instruments, aimed at enabling the studies of structural dynamics on the fundamental time and length scales [1, 2]. Ultrafast electron diffraction (UED) and microscopy (UEM) using a few mega-electron volts (MeV) electron beams is a promising new R&D area which has the potential to open many opportunities for groundbreaking science [3-7].

Ultrafast electron diffraction from aligned molecules

The aim of this project was to record time-resolved electron diffraction patterns of aligned molecules and to reconstruct the 3D molecular structure. The molecules are aligned non-adiabatically using a femtosecond laser pulse. A femtosecond electron pulse then records a diffraction pattern while the molecules are aligned. The diffraction patterns are then be processed to obtain the molecular structure.

Ultrafast Electron Diffraction from Aligned Molecules

We present experimental results on ultrafast electron diffraction from transiently aligned molecules in the absence of external (aligning) fields. The molecules are aligned selectively through a photodissociation reaction using a femtosecond laser pulse.