Visualizing molecular transformations in real-time requires a structural retrieval method with Ångström spatial and femtosecond temporal atomic resolution. Imaging of hydrogen-containing molecules additionally requires an imaging method sensitive to the atomic positions of hydrogen nuclei, with most methods possessing relatively low sensitivity to hydrogen scattering. Laser-induced electron diffraction (LIED) is a table-top technique that can image ultrafast structural changes of gas-phase polyatomic molecules with sub-Ångström and femtosecond spatiotemporal resolution together with relatively high sensitivity to hydrogen scattering. Here, we image the umbrella motion of an isolated ammonia molecule (NH3) following its strong-field ionization. Upon ionization of a neutral ammonia molecule, the ammonia cation (NH3+) undergoes an ultrafast geometrical transformation from a pyramidal (φHNH = 107 °) to planar (φHNH = 120 °) structure in approximately 8 femtoseconds. Using LIED, we retrieve a near-planar (φHNH = 117 ± 5 °) field-dressed NH3+ molecular structure 7.8 - 9.8 femtoseconds after ionization. Our measured field-dressed NH3+ structure is in excellent agreement with our calculated equilibrium field-dressed structure using quantum chemical ab initio calculations.
B. Belsa et al., "Laser-Induced Electron Diffraction of the Ultrafast Umbrella Motion in Ammonia," Structural Dynamics, vol. 8, no. 1, article no. 14301, American Institute of Physics (AIP), Jan 2021.
The definitive version is available at https://doi.org/10.1063/4.0000046
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01 Jan 2021