Strong-field ionization of Xe probed by femtosecond high-order harmonic absorption spectroscopy

Recent experiments on strong-field ionization of atoms and ions have led to conflicting conclusions regarding the existence of orbital alignment in the ionized species. Using table-top, femtosecond high-order harmonic absorption spectroscopy, we have measured the alignment of Xe$^{+}$ formed via strong-field ionization. High-order harmonics generated by focusing an intense 800 nm pulse into a Ne-filled capillary are spatially overlapped with an optical pump pulse in a sample gas cell before they are spectrally dispersed in an extreme ultraviolet spectrometer. Probing the transition from the 4$d$ core level to the $^{2}P_{3/2}$ state of Xe$^{+}$ at 55.4 eV yields a polarization anisotropy of 0.12 $\pm $ 0.01, in good agreement with the theoretical value of 0.1. This result suggests that strong-field ionization exclusively populates the $m_{J}=\pm $1/2 sub-levels in the Xe$^{+} \quad ^{2}P_{3/2}$ state.