2.2 Ultrafast Molecular Dynamics

Project coordinators: A. Rouzée , K. Amini

T4: Ultrafast electron diffraction of chemical dynamics

Fig.: (a) THz electron streaking deflectogram showing a sub-100 fs (FWHM) 90 keV electron pulse measurement. (b) Measured ΔI/I_0 signal as a function of pump-probe delay and momentum transfer in 800-nm photoexcitation of polycrystalline aluminium thin film. (c) Mean-squared-displacement (MSD) plots of selected diffraction peaks shown in panel (c) with residuals shown below. (d) Measured ΔI/I_0 signal of trans-to-cis isomerization in trans-azobenzene photoexcited to its S2 state by a 300 nm optical pump pulse

We use ultrafast electron diffraction (UED) to resolve structural changes in gas-phase molecules following photoexcitation with femtosecond temporal resolution and picometre spatial sensitivity. Elastic scattering of keV-MeV electrons provides direct access to transient nuclear geometries, enabling the visualization of bond rearrangements, vibrational wavepacket motion, and early-time structural signatures of nonadiabatic dynamics. In parallel, the detection of inelastically scattered electrons yields complementary information on excitation-induced electronic changes, offering additional insight into the coupling between electronic and nuclear motion. At MBI, we have a state-of-the-art, sub-100 fs 90 keV electron source operating at 40-100 kHz with direct electron detection capable of measuring both solid-state and gas-phase electron scattering. Moreover, by combining these UED measurements with complementary time-resolved spectroscopies such as transient absorption, our goal is to obtain a more complete description of photochemical reaction pathways in isolated molecules.

People involved:

Kasra Amini, Joanne Woodhouse, Athira Mini Santhosh, Sudhir Kumar, Jyothsna Varghese, Simone Stahl

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