Rabi oscillations in extreme ultraviolet ionization of atomic argon

Abstract

We demonstrate Rabi oscillations in nonlinear ionization of argon by an intense femtosecond extreme ultraviolet (XUV) laser field produced by high- harmonic generation. We monitor the formation of Ar2+ as a function of the time delay between the XUV pulse and an additional near-infrared (NIR) femtosecond laser pulse, and show that the population of an Ar+* intermediate resonance exhibits strong modulations both due to an NIR laser-induced Stark shift and XUV-induced Rabi cycling between the ground state of Ar+ and the Ar+* excited state. Our experiment represents a direct experimental observation of a Rabi-cycling process in the XUV regime.