Speaker
Description
Molecules exposed to intense femtosecond laser fields display a complex interplay of multiphoton ionization, resonances, and tunneling dynamics. Although above-threshold ionization (ATI) has been extensively characterized in atoms, its resonance-enhanced counterpart in polyatomic molecules has remained elusive. Here we investigate the strong-field ionization of ammonia (NH$_3$) by 800 nm femtosecond pulses coupled with photoelectron velocity map imaging, exploiting its distinctive vibrational fingerprint in the umbrella bending mode. We uncover a weak Stark-induced Freeman resonance whose ATI replicas are shown to dominate over the parent channel. A strong resonance-enhanced above-threshold ionization (REATI) phenomenon is thus demonstrated here in a polyatomic molecule. Moreover, our results connect non-resonant and resonant multiphoton ionization, Freeman resonances, ATI, and REATI, into a unified roadmap, establishing ammonia as a benchmark system for exploring vibrationally resolved strong-field ionization. Beyond unveiling a distinct regime of molecular ionization, our findings highlight how the vibrational structure governs strong-field phenomena in complex molecules.