Autoionization dynamics and Feshbach resonances: Femtosecond EUV study of O2 excitation and dissociation

In this work, we present a time-domain study of the complex, multi-step, evolution of highly excited states of oxygen (O2) that result from EUV photoionization. By monitoring the dissociation of molecular oxygen ions, we show that autoionization cannot occur until the internuclear separation is 30 � or greater. As the ion and excited neutral atom separate, we directly observe the transformation of electronically bound states of the molecular ion into Feshbach resonances of the neutral oxygen atom. We achieve this by using laser high-harmonics in a femtosecond EUV-IR pump-probe scheme, combined with a triple coincidence reaction microscope measurement. Finally, we show control of the dissociation pathway through IR pulse induced ionization.