Laser-induced electron diffraction of the ultrafast umbrella motion in ammonia

TL;DR

This paper demonstrates the use of laser-induced electron diffraction to visualize the ultrafast umbrella motion in ammonia molecules, capturing structural changes with atomic resolution within a few femtoseconds.

Contribution

It introduces LIED as a sensitive method for real-time imaging of hydrogen-involving molecular transformations with femtosecond and Ångström precision.

Findings

Ultrafast pyramidal to planar transformation in NH3+ within 8 fs.

Retrieved near-planar NH3+ structure 7.8-9.8 fs after ionization.

Excellent agreement between experimental and ab initio calculated structures.

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.