Summary

Visualizing molecular transformations in real-time requires a structural retrieval method with Ångström spatial and femtosecond temporal resolutions. Laser-induced electron diffraction (LIED) is the first table-top technique that can image ultrafast structural changes of gas-phase polyatomic molecules with sub-Ångström and femtosecond spatiotemporal resolution. Here, we image the umbrella motion of the ammonia molecule (NH3) following its strong-field tunnel ionization. Upon ionization of a neutral ammonia molecule, it undergoes an ultrafast geometrical transformation from a pyramidal (φHNH=107º) to planar (φHNH=120º) structure. Using LIED, we retrieve a quasi-planar (φHNH=114 +/- 2º) NH3+ molecular structure 7-9 femtoseconds after ionization. Our measured NH3+ structure is in excellent agreement with the calculated equilibrium field-dressed structure using quantum chemical ab initio calculations.