KEYWORDS: Molecules, Femtosecond phenomena, Ultrafast phenomena, Diffraction, Solar energy, Space reconnaissance, Solar processes, Photosynthesis, Chemical species
The conversion of light into mechanical and chemical energy, at the level of single molecules, drives many processes in nature such as vision and photosynthesis, and is important for solar energy conversion and storage applications. These changes take place at the atomic level, on femtosecond timescales. We use ultrafast electron diffraction, which probes changes in molecular structure with atomic (sub-Angstrom) resolution in space and femtosecond resolution in time. Here we show that we can retrieve, with a high level of detail, the structural dynamics that take place after photoexcitation of complex molecules.
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 Å spatial resolution, and dynamics were resolved with 200 fs resolution.
We investigated the ultrafast photochemical ring-opening in the molecule α-phellandrene by a combination of megaelecronvolt ultrafast electron diffraction and excited state ab initio multiple spawning wavepacket simulations. α- Phellandrene exhibits a number of different conformers which produce different ring-opening photoproducts according to the Woodward-Hoffmann rules. In our study we image the conversion of a specific conformer of α-phellandrene in the Woodward-Hoffmann predicted photoproduct in real time and space.
Following the coupled motion of electrons and nuclei in molecules is difficult if one uses time-resolved approaches that only provide direct information on one or the other. We combine two complementary measurements, Time- Resolved Photoelectron/PhotoIon Spectroscopy (TRPES and TRPIS) and Ultrafast Electron Diffraction, to follow the electronic and nuclear dynamics of gas phase CH2I2 when exposed to UV light. In order to interpret the measurement, trajectory surface hopping calculations are carried out and all the measurement observables are simulated and directly compared with the measurement signals. Our measurements highlight the coupled electron-nucleus dynamics that allow for electronic potential energy to be converted into nuclear kinetic energy as well as complicated structural rearrangements of the molecule that involve symmetry breaking, dissociation, rotation, and non-local wave-packet dynamics.
The conversion of light into chemical and mechanical energy can take place at the level of single molecules, where the absorption of a photon leads to changes in the molecular structure on ultrafast time scales. Observing these dynamics requires simultaneously reaching atomic (sub-Angstrom) spatial resolution and femtosecond temporal resolution. We have recently showed that we can reach these milestones with ultrafast electron diffraction (UED), capturing structural dynamics in isolated molecules as they take place. We have observed bond breaking, the motion and splitting of nuclear wavepackets in complex photochemical reactions and coherent motions that persist after the reaction is completed.
Thomas Wolf, Jie Yang, David Sanchez, João P. Nunes, Robert Parrish, Xiaozhe Shen, Martin Centurion, Ryan Coffee, James Cryan, Markus Gühr, Kareem Hegazy, Adam Kirrander, Renkai Li, Jennifer Ruddock, Theodore Vecchione, Stephen Weathersby, Peter Weber, Kyle Wilkin, Haiwang Yong, Quiang Zheng, Todd Martinez, Xijie Wang, Michael Minitti
The photoinduced ring opening of 1,3-cyclohexadiene is a prototypical photo-allowed, ultrafast reaction in agreement with the Woodward-Hoffmann rules. Furthermore, it is a model reaction for the biosynthesis of vitamin D. The reaction mechanism involves coupled dynamics of the electrons and nuclei of the molecule in the vicinity of a conical intersection between the excited state and the ground state. It has been intensively investigated by spectroscopic methods, which were primarily sensitive to transient changes in the electronic structure during internal conversion through a conical intersection between the excited state and the ground state. The corresponding structural dynamics have so far only been investigated by a ultrafast x-ray diffraction study. However, spatial resolution down to bond lengths has not been achieved so far. We have investigated the ring opening by gas phase MeV ultrafast electron diffraction with an unprecedented combination of femtosecond temporal and sub Angstrom spatial resolution. The obtained momentum transfer range allows us to follow transient bond length changes by real space transformation of the experimental dataset. Thus, the experimentally observed structural dynamics can be directly compared to quantum molecular dynamics simulations. We, furthermore, can follow the further structural relaxation of the molecule for several hundred femtoseconds beyond relaxation through the conical intersection with the ground state.
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.
Martin Centurion, Jie Yang, Markus Guehr, Xiaozhe Shen, Renkai Li, Omid Zandi, Kyle Wilkin, Theodore Vecchione, Ryan Coffee, Jeff Corbett, Alan Fry, Nick Hartmann, Carsten Hast, Kareem Hegazy, Keith Jobe, Igor Makasyuk, Joseph Robinson, Matthew Robinson, Sharon Vetter, Stephen Weathersby, Chales Yoneda, Xijie Wang
Ultrafast electron diffraction (UED) has the potential to capture changes in the structure of isolated molecules on the natural spatial and temporal scale of chemical reactions, that is, sub-Angstrom changes in the atomic positions that happen on femtosecond time scales. UED has the advantage that electron sources can easily reach sub-Angstrom spatial resolution, but so far femtosecond resolution had not been available for gas phase experiments due to the challenges in delivering short enough electron pulses on a gas target and the velocity mismatch between laser and electron pulses. Recently, we have used relativistic electron pulses at MeV energy to solve these challenges and reach femtosecond resolution. We have, for the first time, imaged coherent nuclear motion in a molecule with UED. In a proof-of-principle experiment, we captured the motion of a laser-excited vibrational wavepacket in iodine molecules. We are currently performing experiments in more complex molecules to capture laser-induced dissociation and conformational changes. We have also developed a table top 100 keV source that relies on a pulse compressor to deliver femtosecond electron pulses on a target and uses a tilted laser pulse to compensate for the velocity mismatch between the laser and the electrons. This source has a high repetition rate that will complement the high temporal resolution of the relativistic source.
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.
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.
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.
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.
We describe a holographic technique capable of sampling dynamic events at 150 femtosecond time resolution. We apply the technique to the study of the nonlinear propagation of high energy pulses through gas and condensed media. The holograms are recorded as a digitized image from a CCD camera and reconstructed numerically to retrieve the refractive index change during the nonlinear optical process. We show dramatic differences in the pulse propagation characteristics depending on the strength of the nonlinear coefficient of the material and it's time response. Both positive and negative index changes have been measured in different media. The holographic technique allows us to distinguish the very fast positive index changes that are generally attributable to the Kerr nonlinearity from the negative index changes that result from free electrons generated by multiphoton ionization.
We present a holographic recording technique with 150 femtosecond time resolution. This technique allows us to capture either a single hologram with fine spatial resolution (4 micrometers), or a time-sequence of multiple holograms with reduced spatial resolution in a single-shot experiment, while preserving amplitude and phase information. The time resolution and the frame rate are limited only by the duration of the laser pulses. The holograms are recorded on a CCD camera and digitally reconstructed. We have used the technique to study the nonlinear propagation of high energy femtosecond pulses through liquids. We have observed dramatic differences in the pulse propagation characteristics depending on the strength of the nonlinear coefficient of the material and it's time response. The fine spatial resolution allows us to zoom in and visualize the spatial profile of the pulses breaking up into multiple filaments while the phase recovered from the holograms helps us identify the nonlinear index changes in the material. We have measured both positive and negative index changes. Very fast positive index changes are generally attributable to the Kerr nonlinearity. The negative index changes can be caused by electron plasma generated by multiphoton absorption.
We have observed the filamentation of optical pulses in carbon disulfide(CS2) using femtosecond time-resolved optical polarigraphy(FTOP). A pump-probe setup is used to capture the propagation of a 150 femtosecond laser pulse in CS2. The probe pulse propagates in a direction perpendicular to the pump. The high intensity of the pump pulse causes a transient index change in the material through the Kerr effect. The induced birefringence is proportional to the intensity of the pump and can be captured by monitoring the polarization of the probe. The probe pulse is imaged on a CCD camera to recover the intensity profile of the pump pulse. We have used this technique to observe the spatial evolution of the pulse as a function of power and propagation distance. Initially, the pulse propagation causes a coarse redistribution of the intensity. The beam then breaks up into stable light filaments which propagate for several millimeters, and finally the beam profile becomes unstable to small fluctuations in the input power.
In this paper we report on a holographic method used to record fast events in the nanosecond time scale. Several frames of the expansion of shock waves in air and in a polymer sample are recorded holographically in a single shot experiment, using a pulse train generated with a single pulse from a Q-switched Nd:YAG laser. The time resolution is limited by the laser pulse width, which is 5.9 ns. The different frames are recorded on the holographic material using angle multiplexing. Two cavities are used to generate the signal and reference pulses at different angles. We also present a method in which the recording material is replaced by a CCD camera. In this method the holograms are recorded directly on the CCD and digitally reconstructed. The holograms are recorded on a single frame of the CCD camera and then digitally separated and reconstructed.
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