Ultrafast electron diffraction, femtosecond time

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. 

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],... 

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. 

Fe(CO)4 proceeds, that is, Fe(CO)s— excited Fe(CO)s— Fe(CO)4— Fe(CO)4, or Fc(CO)5— excited Fe(CO)5 — excited Fe(CO)s— Fe(CO)4. Fe(CO)4 was not observed in the low-temperature matrix experiments. Femtosecond UV laser excitation with time-of-flight mass spectrometric detection was able to show that photolysis of Fe(CO>5 at 267 nm proceeds via an excited singlet state to yield (in ca. 30 fs) Fe(CO)4, initially in an excited singlet state, which then rapidly decays to the lowest singlet state.Ultrafast electron diffraction experiments were able to determine a more precise structure of Fe(CO>4 and confirm the singlet pathway. 

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