Chapter Two - Structural Dynamics in Isolated Molecules

Abstract

Abstract The direct observation of chemical transformations (i.e., the motions of the atoms and functional groups within the molecules just while they undergo chemical reactions) has been a longstanding goal. Time-resolved spectroscopy has made considerable advances, especially in the context of small molecules with few atoms. But for large molecules, the density of vibrational states in the vicinity of transition states is often so great that spectra cannot be resolved or interpreted. First, a structural method (i.e., a diffraction method) provides a fundamentally more promising path because the number of atomic coordinates is much smaller than the number of vibrational states potentially involved in the chemical transformation. Second, unlike a spectroscopy, there is no uncertainty that would limit the simultaneous measurement of atomic positions and the time resolution of the observation. The chapter presents the development of time-resolved electron diffraction, the determination of the time-zero position, the method of the reference frames and the synchronization of the structures, the influence of the Coulomb interaction in the electron bunch on the determined structural parameters, the molecular quantum state tomography, the coherent nuclear dynamics of spatially oriented molecules in the laser field, and the dynamics of spectrally invisible structures. It also includes the results of the work of several famous international research groups.