Ultrafast Electron Diffraction

Molecular dynamics is now routinely studied on femtosecond time scales using various spectroscopies. However, direct structural information of all nuclear coordinates involved in such dynamical processes requires resolution in time by x-ray or electron diffraction. The focus of this laboratory has been the development of ultrafast electron diffraction (UED) for recording structures in motion, which exploits the six-orders-of-magnitude higher scattering cross section of electrons compared with x-rays. Conventional electron diffraction has been developed over the last several decades to become an enormously powerful technique for determining the static structures of molecules in the gas phase; the subsequent plementation of pulsed electron sources has added a temporal dimension to such studies. In UED, a femtosecond (fs) laser pulse is used to initiate a reaction, but unlike other ultrafast spectroscopies, the subsequent laser pulses normally used to probe the progress of the reaction are replaced with ultrashort pulses of electrons. Time-resolved diffraction patterns are then recorded at fixed time delays relative to the zero-of-time. This directly reflects the changing internuclear distances in the species under study. For these UED studies, there exist a number of significant experimental challenges, including: (i) the problem of independently determining the temporal overlap (zero-of-time) of the pump and probe pulses in situ for clocking the change; (ii) the problem of low electron flux required to minimize space-charge induced temporal broadening of electron pulses; and (iii) the problem of low scattering and sensitivity caused by the absence of long-range order present in solids, and the low density of molecules in gases. These challenges to the realization of UED have been surmounted over the last decade, and UED now approaches the combined spatial and temporal resolution necessary for tracking all nuclear coordinates during the making and breaking of chemical bonds, thereby permitting the direct observation of molecular structural dynamics in real time. In addition, the diffraction-difference method—which employs the subtraction of a reference diffraction signal from the signals recorded over the course of the reaction—can be used to select the contributions resulting only from changes in structure in the species under study, thereby enhancing the sensitivity of UED to chemical change. Contributions only from the products can be also isolated by adding the appropriately scaled parent diffraction signal at negative time to the difference curves, thus canceling out the parent contribution in each curve. The first reaction studied by UED was the dissociation of CH2I2 into CH2I and I with fs laser pulses. The second-generation apparatus allowed us to see the amplitude change in the scattering intensity on the picosecond time scale resulting from dissociation. Since this work, UED experiments have successfully investigated the course of several prototypical chemical reactions. For example, the molecular structures and branching ratios of the final products were determined in the dissociation of Fe(CO)5 upon two-photon excitation at 310 nm. A simple intermediate, CF2, was generated by fragmentation of CF2I2 and its molecular structure was precisely determined and compared with other experiments and theoretical calculations. Furthermore, the molecular structure of the transient Fe(CO)4 species was elucidated and compared with available theoretical predictions, permitting identification of the specific electronic energy state of the intermediate and the primary reaction pathway. UED was applied to another organometallic compound, namely (C5H5)Co(CO)2 and the molecular structures of the intermediate and final products were observed. A preliminary analysis showed that either C5H5 and other species can be selectively generated depending on the excitation wavelength. Further analysis will elucidate their molecular structures. The elimination of iodine from 1,2-diiodotetrafluoroethane (C2F4I2) was also studied with the second-generation apparatus, providing early results which suggested that the molecular structure of the C2F4I radical intermediate is not bridged in nature, but instead is "classical," resembling the structure of the parent species. The need for greater sensitivity and resolution, as well as the desire to study more complex reactions, led to the development of our third-generation UED apparatus. This new machine, with vast improvements in pulsed electron flux, repetition rate, detection sensitivity, and experimental stability, permits the direct imaging of complex chemical reactions with unprecedented spatial and temporal resolution. The spatial and temporal resolution of UED approached ~0.01 Å and ~1 ps, respectively, and we were sensitive to ~1% changes in the mole fractions of the various chemical species over the course of the reaction. In its first application, the non-concerted elimination of iodine from a haloethane (C2F4I2) was re-visited and the molecular structure of the transient (C2F4I) radical was determined for the first time. Two prototypical cyclic hydrocarbons, 1,3,5-cycloheptatriene (CHT) and 1,3-cyclohexadiene (CHD), were also studied with temporal and spatial resolution of ~5 ps and ~0.04 Å respectively. At high internal energies of ~4 eV, these molecules displayed markedly different behavior. For CHT, wherein excitation resulted in the reformation of the parent, the observed diffraction change was explained with an equipartitioned model of hot structures—indicating rapid energy redistribution (within a few picoseconds). For CHD, photo-induced ring opening was shown to result in hot but highly non-equilibrium structures even up to 400 ps, with energy trapped in large-amplitude motions comprised of torsion and asymmetric stretching. These studies promise a new direction of research for studying transient structural changes both in equilibrium and non-equilibrium complex systems. The results presented here provide the new limit of improved detection sensitivity, versatility, and resolution of UED, as well as the potential for its diverse applications. The extension to even more complex systems, a process that has already begun in our laboratory, represents our next challenge in UED.