Ultrafast structural dynamics with electrons.

CORTHEY, G.; XIAN, R.; HAYES, S. A.; MILLER, R. J. D.

Buenos Aires

Congreso; Frontiers in Physical Sciences; 2016

Watching atoms move during a chemical reaction has always been the dream experiment in chemistry and it is now becoming feasible even for complex systems by the development of different experimental approaches. While optical probes can give useful information about the electronic dynamics during a chemical reaction, they can only sense the atomic structure in an indirect way, as radiation with a wavelength on the order of the atomic distances is needed. Since the changes in the atomic position fall in the sub-angstrom length scale, either X-rays or electrons should be used to probe the structural dynamics. And the source has to comprise pulses with time duration on the order of 100 fs, the time domain for chemical reactions [1]. X-ray sources fulfilling these requirements are the free electron lasers (FELs), with kilometer-size facilities and costs on the order of 109 USD. An alternative to that is the femtosecond electron diffraction (FED) technique, which can be implemented in tabletop setups, reducing the costs by four orders of magnitude with similar performance for some systems and without the radiation-induced damage issue that accompanies X-rays. Due to the millionfold increase in scattering cross-sections of electrons compared to X-ray photons [1], FED is suitable for sub- micrometer-thin samplesHere we will discuss the experimental details of this method and present example studies of the ultrafast dynamics of chemical reactions [2] and phase transitions [3] using FED, combined with transient absorption spectroscopy to follow the changes in the electronic structure.[1] Miller, R. J. D. Femtosecond Crystallography with Ultrabright Electrons and X-Rays: Capturing Chemistry in Action. Science 2014, 343 (6175), 1108?1116.[2] Xian, R.; et al. Coherent ultrafast lattice-directed reaction dynamics. Submitted 2016.[3] Ishikawa, T.; et al. Direct Observation of Collective Modes Coupled to Molecular Orbital?driven Charge Transfer. Science 2015, 350 (6267), 1501?1505.