Ultrafast Electron Diffraction (UED)

Abstract

With properly timed sequences of ultrafast electron pulses, it is now possible to image complex molecular structures in the four dimensions of space and time with resolutions of 0.01 Å and 1 ps, respectively. The new limits of ultrafast electron diffraction (UED) provide the means for the determination of transient molecular structures, including reactive intermediates and non‐equilibrium structures of complex energy landscapes. By freezing structures on the ultrafast timescale, we are able to develop concepts that correlate structure with dynamics. Examples include structure‐driven radiationless processes, dynamics‐driven reaction stereochemistry, pseudorotary transition‐state structures, and non‐equilibrium structures exhibiting negative temperature, bifurcation, or selective energy localization in bonds. These successes in the studies of complex molecular systems, even without heavy atoms, and the recent development of a new machine devoted to structures in the condensed phase, establish UED as a powerful method for mapping out temporally changing molecular structures in chemistry, and potentially, in biology. This review highlights the advances made at Caltech, with emphasis on the principles of UED, its evolution through four generations of instrumentation (UED‐1 to UED‐4) and its diverse applications.

Ultrafast electron diffraction: Excited state structures and chemistries of aromatic carbonyls

“…The third-generation ultrafast electron diffraction apparatus at Caltech has been briefly presented elsewhere, 54,55 and here we provide the detailed methodology that makes these experiments possible. The ultrafast electron diffraction ͑UED͒ experimental apparatus is a combination of several components that will be addressed separately: a femtosecond laser system, a high-vacuum chamber, a high-voltage ultrafast pulsed electron gun, a charge-coupled device ͑CCD͒ detector, and a high-temperature inlet system.…”

“…54 The pulse properties for these experiments were measured to be 8 ϫ 10 4 electrons/ pulse for acetophenone and 3 ϫ 10 4 electrons/ pulse for benzaldehyde. The FWHMs of the electron beams were measured to be ϳ370 and ϳ360 m for benzaldehyde and acetophenone, respectively, and the pulsewidths in these experiments were typically less than 6 or 20 ps depending on the number of electrons per pulse.…”

“…The B3LYP level 60 with the 6-311G͑d , p͒ basis set has been found useful in determining structure, energy, and vibrational frequencies. 54,61 Geometry optimization for the ground state of reactants and possible products of benzaldehyde and acetophenone systems were carried out at the B3LYP level using the 6-311G͑d , p͒ basis set. UB3LYP/ 6-311G͑d , p͒ was used for radical product candidates such as those involving H, methyl, formyl, acetyl, benzoyl, and phenyl radicals as well as for the triplet state of molecules.…”

High-coherence picosecond electron bunches from cold atoms

“…U ltrafast electron diffraction (UED) enables the study of molecular structural dynamics with atomic spatial resolution at picosecond timescales [1][2][3] . Determination of the structure and function of molecules is critical to understand all but the most simple chemical reaction pathways, to understand biochemical dynamics such as protein folding and regulation or to understand the formation of cracks in novel materials 4,5 .…”

Atomic-Scale Imaging in Real and Energy Space Developed in Ultrafast Electron Microscopy

“…[2][3][4][5][6][7][8][9][10][11][12] Typically, for microscopy the electron was accelerated to 120 keV and for diffraction to 30 keV, respectively, and we had to address the issues of group velocity mismatch, in situ clocking (time zero) of the change, and frame referencing. 1,3 One powerful concept implemented is that of "tilted pulses", which allow for the optimum resolution to be reached at the specimen. 13 For ultrafast electron microscopy (UEM), the concept of "single-electron" imaging is fundamental.…”