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

Abstract

Visualizing molecular transformations in real-time requires a structural retrieval method with à ngström 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-à ngström 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 ($\hat{I}{¦}_{HNH}=107^{\circ }$) to planar ($\hat{I}{¦}_{HNH}=120^{\circ }$) structure in approximately 8 femtoseconds. Using LIED, we retrieve a near-planar ($\hat{I}{¦}_{HNH}=117\pm 5^{\circ }$) 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.