Abstract
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 () to planar () structure in approximately 8 femtoseconds. Using LIED, we retrieve a near-planar () field-dressed NH3+ molecular structure 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.
Highlights
Many important processes in nature rely on the motion of hydrogen atoms, such as the influence of proton dynamics on the biological function of proteins.1,2 The motion of the hydrogen atom, which is the lightest element in the periodic table, occurs on the few-femtosecond timescale and represents the fastest possible nuclear motion in molecules
6 krcosðhrÞ and k? 1⁄4 krsinðhrÞ: Fourier transform (FT)-Laser-induced electron diffraction (LIED) is based on the measurement of backscattered electrons, yields kr 1⁄4 kresc À ArðtrÞ, where ArðtrÞ is calculated for a detected momentum, kresc, employing the classical recollision model which is valid under our quasi-static field conditions
The procedure for retrieving structural information is based on the Fourier transform (FT) variant of LIED, called FT-LIED,20,23 which is known as the fixed-angle broadband laser-driven electron scattering (FABLES)25 method
Summary
Many important processes in nature rely on the motion of hydrogen atoms, such as the influence of proton dynamics on the biological function of proteins. The motion of the hydrogen atom, which is the lightest element in the periodic table, occurs on the few-femtosecond (few-fs; 1 fs 1⁄4 10À15 s) timescale and represents the fastest possible nuclear motion in molecules. The time-resolved analogues of x-ray and electron diffraction, such as ultrafast x-ray diffraction (UXD) and ultrafast electron diffraction (UED), have provided a wealth of dynamical information in molecules that contain atoms much heavier than hydrogen As a result, their scattering signal in such molecules is very large, and their respective dynamics occur on the hundreds-offemtosecond timescale. The authors experimentally studied the same dynamics but could only indirectly provide partial evidence of the umbrella motion through high-harmonic spectroscopy (HHS) These HHS measurements were, performed at different wavelengths in the nearinfrared (NIR) up to 1.8 lm, reaching a temporal range of up to 3.8 fs after ionization to be investigated.
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