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Abstract

Visualizing molecular transformations in real-time requires a structural
retrieval method with {\AA}ngstr\"om spatial and femtosecond temporal atomic
resolution. Imaging of hydrogen-containing molecules additionally requires an
imaging method that is 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-{\AA}ngstr\"om and femtosecond spatiotemporal resolution together with
relatively high sensitivity to hydrogen scattering. Here, we image the umbrella
motion of an isolated ammonia molecule (NH$_3$) following its strong field
ionization. Upon ionization of a neutral ammonia molecule, the ammonia cation
(NH$_3^+$) undergoes an ultrafast geometrical transformation from a pyramidal
($\Phi_{HNH} = 107 ^\circ$) to planar ($\Phi_{HNH}=120^\circ$) structure in
approximately 8 femtoseconds. Using LIED, we retrieve a near-planar
($\Phi_{HNH}=117 \pm 5^\circ$) field-dressed NH$_3^+$ molecular structure
$7.8-9.8$ femtoseconds after ionization. Our measured field-dressed NH$_3^+$
structure is in excellent agreement with our calculated equilibrium field
dressed structure using quantum chemical ab initio calculations.