Abstract

Electron sources have achieved sufficient brightness to literally light up atomic motions on the primary timescales of structural change that are key to understanding many body interactions in problems ranging from strongly correlated electron lattice systems to chemistry and biological functions. Two new electron gun concepts have emerged from detailed calculations of the propagation dynamics of nonrelativistic electron pulses with sufficient number density for single shot structure determination (Siwick et al. JAP 2002). The atomic perspective, that these sources have opened up, has given a direct observation of the far from equilibrium motions that lead to structural transitions (Siwick et al. Science 2003). Recent studies of formally a photoinduced charge transfer process in charge ordered organic systems has directly observed the most strongly coupled modes that stabilize the charge separated state (Gao et al Nature 2013). It was discovered that this nominally 280 dimensional problem distilled down to projections along a few principle reaction coordinates. Similar reduction in dimensionality has also been observed for ring closing reactions in organic systems (Jean-Ruel et al JPC B 2013). This phenomenon appears to be general and arises from the very strong anharmonicity of the many body potential in the barrier crossing region. The far from equilibrium motions that sample the barrier crossing region are strongly coupled, which in turn leads to more localized motions. In this respect, one of the marvels of chemistry, and biology by extension, is that despite the enormous number of possible nuclear configurations for any given construct, chemical processes reduce to a relatively small number of reaction mechanisms. We now are beginning to see the underlying physics for these generalized reaction mechanisms. The “magic of chemistry” is this enormous reduction in dimensionality in the barrier crossing region that ultimately makes chemical concepts transferrable. With a large enough basis, it may be possible to characterize reaction mechanisms in terms of reaction modes, or reaction power spectra, in analogy to the characterization of equilibrium fluctuations in terms of vibrational normal modes. Additional examples will be presented in which it has been possible to directly observe underdamped modes involved in metal ligand charge transfer processes, as well as structural changes involved in intersystem crossing, to a direct observation of Pauli explosion in alkali halides (Hada et al, Nature Comm, 2014) – the reverse of the classic “electron harpooning” reaction that helped establish transition state concepts.

© 2015 IEEE