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Mapping Atomic Motions with Ultrabright Electrons: The Chemists’ Gedanken Experiment Enters the Lab Frame


Mapping Atomic Motions with Ultrabright Electrons: The Chemists’ Gedanken Experiment Enters the Lab Frame

R. J. Dwayne Miller1,2,3

1Max Planck Institute for the Structure and Dynamics of Matter/Hamburg

2The Hamburg Centre for Ultrafast Imaging

3Departments of Chemistry and Physics, University of Toronto

Electron sources have achieved sufficient brightness to literally light up atomic motions during transition state processes to directly view the unifying conceptual basis of chemistry. 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). ). Even more dramatic reduction in complexity has been observed for the material, Me4P[Pt(dmit)2]2, which exhibits a photo-induced metal to metal centre charge transfer process for unit cells on par with proteins. This study represents the first all atom resolved structural dynamics with sub-Å and 100 fs timescale resolution. We are tuned to see correlations. At this resolution, without any detailed analysis, the large-amplitude modes can be identified by eye from the molecular movie. The structural transition clearly involves a dimer expansion and a librational mode that stabilizes the charge transfer. 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 the new ability to see the far from equilibrium nuclear motions driving chemistry, it will ultimately be possible to characterize reaction mechanisms in terms of reaction modes, or reaction power spectra, to give a dynamical structure basis for understanding chemistry.