Ultrafast electron diffraction

The immediate impact of this research will be a clearer understanding of ligand motions during photoelimination reactions. In particular, comparative studies of molecular motions in the gas phase (using ultrafast electron diffraction) and in the liquid phase should become a source of very detailed understanding of the influence of solvation on chemical processes. Such combined studies in collaboration with Peter Weber, Dept, of Chemistry, Brown University are planned. 

Several approaches to modifying femtosecond experiments are being developed so that structures, or at least structural information, as functions of time may be secured. One tactic implements an ultrafast electron diffraction strategy. 

Ultrafast electron diffraction was used to define the structure of the cyclopenta-dienyl radical formed through photodissociation of CpCo(CO)2. The structure obtained reflected the Jahn-Teller distortion from D h symmetry, a dynamic structure thanks to pseudorotations converting dienylic and elongated conformations. ... 

A still more demanding ultrafast electron diffraction study of the electrocyclic ring-opening isomerization of 1,3-cyclohexadiene to l,3(Z),5-hexatriene was... 

Ultrafast Electron Diffraction and Transient Complex Structures — From Gas Phase to Crystallography... 

This article highlights the recent development of ultrafast electron diffraction at Caltech. This development has made it possible to resolve transient structures both spatially (0.01 A) and temporally (picosecond and now femtosecond) in the gas phase and condensed media, surfaces and crystals, with wide ranging applications. We also present some advances made in the studies of mesoscopic ionic solvation and biological dynamics and function. 

Comparison between ultrafast electron diffraction (UED), x-ray diffraction (UXD), and the prospective X-ray Free Electron Laser (XFEL)... 

As a closing example of the powerful application of the concept of coherence in structures and dynamics, we point to its importance in obtaining molecular structural changes with time using ultrafast electron diffraction (UED) (Fig. 12). The UED technique has been developed, so far with -1-ps resolution (Fig. 12). We have reported recently that the introduction of rotational orientation (Section D above) to the diffraction in real time can provide a three-dimensional image of the structure, instead of the conventional two-... 

Ultrafast Electron Diffraction. IV. Molecular Structures and Coherent Dynamics, J. C. Williamson and A. H. Zewail, J. Phys. Chem. 98, 2766 (1994). 

A. H. Zewail Prof. Yamanouchi is correct in pointing out the relevance of ultrafast electron diffraction to the studies of vibrational (and rotational) motion. In fact, Chuck Williamson in our group [1] has considered precisely this point, and we expect to observe changes in the radial distribution functions as the vibrational amplitude changes and also for different initial temperatures. The broadening in our radial distribution function presented here is limited at the moment by the range of the diffraction sampled. 

Abstract This work describes the ultrafast processes preceding the photoinduced decarbonylation of the simple metal carbonyl complexes Cr(CO)6, Fe(CO)5, and Ni(CO)4. Models for their electronic structure are presented based on recent ab initio quantum chemical calculations and these models are used to describe initial excited-state dynamics leading to the expulsion of one CO ligand. Experimental support for the proposed excited-state dynamics is presented, as obtained from ultrafast pump-probe experiments using mass-selective detection, ultrafast electron diffraction, and luminescence studies. The results of some steady-state experiments are also presented where they support the proposed dynamic model. 

Keywords Carbonyl Chromium Decarbonylation Electronic structure Excited state dynamics Iron Mass selective detection Nickel Photophysics Ultrafast electron diffraction... 

Two ultrafast electron diffraction studies on the Fe(CO)5 system have been published [69, 70], In these experiments fs pump pulses excite Fe(CO)5 in the gas phase in a free expansion jet. Two photon excitation was used in these experiments... 

Fig. 23 The ultrafast electron diffraction radial distribution curves obtained using 10 ps electron pulses prior to excitation with a 310 nm fs pulse (top) and the difference signals (f(r)) obtained at 10, 40, 70, and 270 ps delay times. No significant change to the f(r) function was observed after 10 ps. Adapted from [70]...

Fig. 24 Ultrafast electron diffraction radial distribution functions (solid lines -calculated broken lines- experimental) showing excelling agreement for the 3AX Fe(CO)4 (left) compared to the 3B2 structure (right). Bonds represented by broken lines correspond to nonbonding distances. Adapted from [69]...

A methodological breakthrough in the elucidation of catalytic mechanisms comes from the ultrafast electron diffraction (UED) technique. Even though only the most simple models are accessible as yet, it is possible in principle to view hot reaction intermediates on a multi-picosecond (and femtosecond [101]) time-scale after their formation, as shown for CO elimination from Fe(CO)s [101],... 

In section II.A, an apparatus built by Zewail and coworkers and using ultrafast electron diffraction (UED) was described and later in Section III.C a successful application of this technique to the photodissociation of CF3I was mentioned [34]. More recently, the same technique was applied to a study of the photodissociation of CH2I2 [335]. Related to ultrafast diffraction measurements in general, the many papers on ultrafast X-ray diffraction published by Wilson and coworkers [336-340] are of interest, the X-ray and electron diffraction cases being very similar. 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

Baum, P., Zewail, A. H., Breaking Resolution Limits in Ultrafast Electron Diffraction and Microscopy, Proc. Natl. Acad. Sci. USA 2006, 103, 16105 16110. 

Fig. 18. (a) Ultrafast electron diffraction apparatus consisting of an electron gun chamber, a diffraction chamber, and a detector chamber. Two fs laser pulses are used, one to initiate the chemical change and the second to generate the electron pulse, (b) Detector system incident electrons either directly bombard a small CCD or strike a phosphor-coated fused fiber-optic window. Light emitted from the phosphor is amplified by an image intensifies and brought to a scientific-grade CCD. Both CCDs are thermo-electrically cooled [reproduced with permission from (96), p. 1601. 

Ultrafast electron diffraction and transient complex structures - From gas phase to crystallography... 

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