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 ${\mathrm{Xe}}^{+}$ produced by optical strong-field ionization of Xe atoms at 800 nm. Probing at the Xe $N_{4/5}$ edge yields a population distribution ${\rho }_{j,|m|}$ of ${\rho }_{3/2,1/2}:{\rho }_{1/2,1/2}:{\rho }_{3/2,3/2}=75\pm 6\mathrm{\text{:}}12\pm 3\mathrm{\text{:}}13\pm 6\%$. 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.

Figure 1

Schematic illustration of the experimental setup. The light and dark lines correspond to the 800 nm pump and EUV-probe beam paths, respectively.

Figure 2

Transient absorption spectrum of ${\mathrm{Xe}}^{+}$ acquired at a time delay of $+500\mathrm{fs}$, obtained from the average of 3 data sets. $\Delta \mathrm{OD}$ denotes the change in optical density (absorbance). The ${}^{2}P_{3/2}\to {}^{2}D_{5/2}$ and ${}^{2}P_{1/2}\to {}^{2}D_{3/2}$ fine-structure transitions are located at 55.4 and 56.1 eV, respectively. The fit to a sum of two Gaussian curves is depicted by the solid line. The inset shows the harmonic spectra obtained at $-500\mathrm{fs}$ and $+500\mathrm{fs}$ pump-probe time delay.

Figure 3

Time evolution of the ${}^{2}P_{3/2}\to {}^{2}D_{5/2}$ transition for parallel and perpendicular relative polarizations between the optical pump and EUV-probe pulses, obtained from the average of 8 time scans for each relative polarization. The polarization anisotropy observed at positive time delays implies that the ${}^{2}P_{3/2}$ state of ${\mathrm{Xe}}^{+}$ produced by strong-field ionization is aligned.