Competition between dynamic resonance and internal conversion in strong-field molecular ionization with chirped ultrafast laser pulses

We show how the time-varying phase or frequency of a chirped strong-field ultrafast laser pulse can be used to control the ionization yield to different ionic states in a polyatomic molecule. The control is based upon a competition between multiphoton resonance and internal conversion. The multiphoton resonance is facilitated by dynamic Stark shifts, while the internal conversion (coupling between electronic states) is due to violation of the Born-Oppenheimer approximation. Our measurements are compared with and interpreted in terms of calculations of the strong-field ionization dynamics, which include multiphoton resonance, dynamic Stark shifts, as well as vibrational dynamics and internal conversion during the ionization process. The measurements and calculations illustrate how the light matter coupling can compete with the coupled motions of electrons and nuclei in strong-field laser-molecule interactions.