Experimentally unravelling the ultrafast dynamics of thermal-energy chemical reactions
Matthew S. Robinson and Jochen Küpper Center for Free-Electron Laser Science, Germany
For decades, femtochemistry has given us glimpses into the ultrafast world of chemical dynamics [1]. With the tried and tested pump-probe technique, it has been possible to extend the approach to probes that allow us to track and interpret the photo-induced excited-state dynamics of countless molecules using transient-absorption [2], photoelectron spectroscopy [3] and diffraction [4] techniques to name but a few. However, it has now become clear that a major challenge of femtochemistry is the extension from photochemical processes to general chemical dynamics. To date, most ultrafast experiments have focused on processes in which the dynamics are initiated in highly- excited states of molecules using visible, ultraviolet, or x-ray photons, or even intense ultrashort ionising laser pulses. However, most chemical processes that are key to everyday life, from biology to materials, occur at thermal energies, i. e., at much lower energy scales than the majority of photochemical processes studied so far. Here, we propose to vastly extend the applicability of femtochemistry and the imaging of ultrafast dynamics to the study of thermal-energy chemical dynamics, in a first step for unimolecular chemical reactions, such as half- collision [5] or folding-type isomerization rearrangements [6]. The reactions will be initiated by ultrashort mid- infrared laser pulses [7], which excite vibrational wavepackets at relatively low energies in the electronic ground state. Such pulses are readily available, for instance, from optical parametric amplifiers [7]. Merging this with our approaches to prepare pure and controlled samples of well-defined complex molecular systems [8] and with state-of-the-art atomic-resolution-imaging modalities for gas-phase molecules [9], one can start to unravel the real dynamics of thermal-energy chemical reactions. Pure samples of molecular dimers, including micro-solvated systems [10], provide an ideal starting point for imaging the ultrafast thermal-energy unimolecular (hydrogen-)bond-breaking processes between the two moieties. Further experiments could concentrate on individual-conformer [11] or model-peptide [12] isomer- interconversion dynamics [6] and image the ultrafast dynamics of these prototypical folding-type unimolecular biochemical reaction processes [13]. This ground work will provide us with the basis for assigning time-resolved rationales to the generally-used statistical models of everyday chemistry as well as allowing us to derive intrinsic key reaction modes. Overall, this will enable the development of a basic picture of chemistry that explicitly includes time. References
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