Clocking transient chemical changes by ultrafast electron diffraction

Williamson, J. Charles; Cao, Jianming; Ihee, Hyotcherl; Frey, Hans‐Martin; Zewail, Ahmed H.

doi:10.1038/386159a0

Cited by 249 publications

(178 citation statements)

References 22 publications

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“…Indeed, developments of the recent past have shown that it is possible to generate pulses of even subpicosecond duration of both X-rays 49-51 and electrons. 29 In time-domain diffraction experiments a short laser pulse initiates a change in a molecular structure. The electron or X-ray pulse arrives with a small time delay at the sample, from which it is diffracted.…”

mentioning

confidence: 99%

“…In the gas phase, pump-probe electron diffraction has been applied to observe the photodissociation of a heavy atom from small molecules. 25,29 Heavy atoms help the diffraction experiments because their atomic scattering factors are large, resulting in large scattering signals.The success of those experiments points to the tremendous potential of pump-probe diffraction techniques in exploring chemical reaction dynamics. However, given the experimental constraints, the molecular systems have been chosen for experimental feasibility, rather than interest in an inherently important chemical reaction.…”

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Ultrafast Diffraction Imaging of the Electrocyclic Ring-Opening Reaction of 1,3-Cyclohexadiene

2001

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Changes in electron diffraction patterns are observed on ultrashort time scales upon irradiation of 1,3-cyclohexadiene with femtosecond laser pulses. 1,3-Cyclohexadiene is known to experience a ring opening reaction to hexatriene upon excitation to the 1 B 2 electronic state. Internal conversion brings the molecule to a saddle point, from where one pathway leads back to cyclohexadiene, while another path generates 1,3,5-hexatriene in one of its isomeric forms. Structural observations are made at picosecond time delays using an ultrashort electron pulse that is diffracted off the nascent product molecules. The diffraction images illustrate that structural observations of prototypical organic reactions can be made in real time, opening a new methodology to study chemical reaction dynamics.Molecular spectroscopy with femtosecond or picosecond time resolution has become tremendously successful in exploring energy relaxation processes and chemical reactions in real time. It is now possible to obtain spectra of molecules just as they cross a transition state during a chemical reaction. 1 Such spectroscopic studies have led to enormous insights about the flow of energy within and between molecules, allowing detailed inferences about the mechanisms of the reactions.Nonetheless, time-resolved spectroscopy is burdened by fundamental constraints. Most mechanistic chemistry is based on structural models, whereas spectroscopy reveals energy levels. Synthetic chemists describe reactions as transformations of molecular structures, with reaction channels that are determined by spatial distributions of functional groups, steric hindrances, or spatial electrostatic charge distributions. In contrast, spectroscopy can measure only energy levels and populations of molecules in energy levels. Thus, time-resolved spectroscopy, however useful, shows only the time dependence of energy level populations.Energy levels and structures are of course connected via potential energy surfaces and quantum mechanics. However, this link is conceptually difficult, and tremendous computational resources must applied to understand even simple chemical reactions from a quantum mechanical perspective. It therefore has been a long-standing goal of experimental physical chemists to observe time-dependent structures of molecules during chemical reactions. Such structural observations carry the promise of a much more direct connection to mechanistic organic chemistry. We report here the investigation of a prototypical organic reaction by time-resolved electron diffraction.The concept of probing time dependent molecular structures by diffraction has been well documented. 2 Provided one succeeds in generating short bursts of electrons or X-rays, both electron diffraction 3-37 and X-ray diffraction 38-49 can be adapted to the time domain. Indeed, developments of the recent past have shown that it is possible to generate pulses of even subpicosecond duration of both X-rays 49-51 and electrons. 29 In time-domain diffraction experiments a short laser pulse initia...

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Ultrafast Diffraction Imaging of the Electrocyclic Ring-Opening Reaction of 1,3-Cyclohexadiene

2001

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“…Furthermore, electron diffraction has found broad application for gas-phase structuredetermination in chemistry [5]. Recent developments have mainly focused on realizing time-resolved experiments in order to study structural dynamics, where xray and electron diffraction served as complementary approaches [6][7][8][9][10][11].To be able to record structural changes during ultrafast molecular processes of small complex molecules in the gas phase, signals from many identical molecules have to be averaged. Gas-phase investigations pose the challenge that the sample might comprise different isomers and sizes [12].…”

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Electron gun for diffraction experiments off controlled molecules

Müller¹,

Trippel²,

Długołȩcki³

et al. 2015

J. Phys. B: At. Mol. Opt. Phys.

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A dc electron gun, generating picosecond pulses with up to 8 × 10 6 electrons per pulse, was developed. Its applicability for future time-resolved-diffraction experiments on state-and conformer-selected laser-aligned or oriented gaseous samples was characterized. The focusing electrodes were arranged in a velocity-map imaging spectrometer configuration. This allowed to directly measure the spatial and velocity distributions of the electron pulses emitted from the cathode. The coherence length and pulse duration of the electron beam were characterized by these measurements combined with electron trajectory simulations. Electron diffraction data off a thin aluminum foil illustrated the coherence and resolution of the electron-gun setup.Diffractive imaging is a promising approach to unravel the microscopic details of chemical processes through the recording of so-called molecular movies in the gasphase, which trace the structural dynamics of individual molecules and nano-particles at the atomic level. Electron and x-ray diffraction are well established tools to investigate the structures of solid state samples [1], for example in transmission electron microscopy [2] or xray crystallography [3,4]. Furthermore, electron diffraction has found broad application for gas-phase structuredetermination in chemistry [5]. Recent developments have mainly focused on realizing time-resolved experiments in order to study structural dynamics, where xray and electron diffraction served as complementary approaches [6][7][8][9][10][11].To be able to record structural changes during ultrafast molecular processes of small complex molecules in the gas phase, signals from many identical molecules have to be averaged. Gas-phase investigations pose the challenge that the sample might comprise different isomers and sizes [12]. In addition, the molecules in the gas phase are typically randomly oriented. It is therefore important to provide samples, which are as clean and defined as possible, to allow for experimental averaging over multiple electron pulses. Clean molecular samples can be generated by their spatial separation according to shape [13][14][15] and size [16]. Controlling the spatial orientation of the molecules leads to an enhancement of the information that can be retrieved from a diffraction pattern, as proposed theoretically [17][18][19][20] and demonstrated experimentally for x-ray [21,22] as well as for electron diffraction [23,24]. Strong alignment or orientation is generally necessary [11,20] for three-dimensional structure reconstruction [24] and can be provided in cold supersonic molecular beams by strong-field laser alignment and mixed-field orientation [25][26][27][28][29]. The low density of these controlled gas-phase samples requires sources of large-cross-section particles or photons with large brightness, while still ensuring atomic resolution. Electron sources can meet these requirements even in table-top setups.The first sources for creating electron pulses short enough to study ultrafast processes in molecules o...

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Determination of rotational constants in a molecule by femtosecond four-wave mixing

Frey

Beaud

Gerber

et al. 2000

J. Raman Spectrosc.

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Femtosecond time‐resolved DFWM experiments were performed on CHCl3 and SO2. The recurrences observed originate from the preparation of rotational coherences within the sample. The determination of rotational constants for a symmetric and an asymmetric top are shown. The simulation of the transients for the symmetric top allows the extraction of the main moment of inertia B and the centrifugal terms perpendicular to the molecule axis. For the asymmetric top, all three rotational constants can be derived. In addition to the transients at 1/2(B + C), two additional types of asymmetry transients at 1/4C and 1/4A arise from this experiment. The first origins from the levels at low K and the second from the levels at high K in the high‐J limit. Copyright © 2000 John Wiley & Sons, Ltd.

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Clocking transient chemical changes by ultrafast electron diffraction

Cited by 249 publications

References 22 publications

Ultrafast Diffraction Imaging of the Electrocyclic Ring-Opening Reaction of 1,3-Cyclohexadiene

Ultrafast Diffraction Imaging of the Electrocyclic Ring-Opening Reaction of 1,3-Cyclohexadiene

Electron gun for diffraction experiments off controlled molecules

Determination of rotational constants in a molecule by femtosecond four-wave mixing

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