Quantum State-Resolved Probing of Strong-Field-Ionized Xenon Atoms Using Femtosecond High-Order Harmonic Transient Absorption Spectroscopy

Femtosecond high-order harmonic transient absorption spectroscopy is used to resolve the complete $|j,m⟩$ quantum state distribution of Xe⁺ produced by optical strong-field ionization of Xe atoms at 800 nm. Probing at the Xe N4/5 edge yields a population distribution ρj,|m| of ${{ρ}}3/2,1/2{:}{{ρ}}1/2,1/2{:}{{ρ}}3/2,3/2=75±{}6 {:}12±{}3 {:}13±{}6%$. The result is compared to a tunnel ionization calculation with the inclusion of spin-orbit coupling, revealing nonadiabatic ionization behavior. The sub-50-fs time resolution paves the way for tabletop extreme ultraviolet absorption probing of ultrafast dynamics.