Strong-field ionization of photoexcited ammonia with correlated electron-ion velocity map imaging*

Strong field ionization of atoms and molecules reveal multi-faceted ultrafast dynamics. Coulomb explosion following multiple ionization can reflect the instantaneous nuclear geometry, while information about the excited-state electronic structure is imprinted on the photoelectron momentum distribution. Gas-phase ammonia photoexcited at 200 nm to its 3s Rydberg undergoes dissociative motion and evolution in its electronic character, splitting into adiabatic and nonadiabatic product channels. Our recent work studied this system using time-resolved hard X-ray scattering and directly observed the evolving electronic structure following photoexcitation and evolution along the dissociative potential. Here we study the same system using strong-field ionization as a probe and capturing both the ions and electrons in coincidence using a voltage-switched velocity map imaging apparatus. The photoelectron momentum distribution exhibits above-threshold ionization (ATI) with angular distributions that vary with ATI order. The low-energy electron spectrum contains fingerprints of the neutral excited states on the pathway to ionization. The simultaneous study of ions and electrons leads to a more complete understanding of the structural and spectral evolution of photoexcited ammonia.