X-Ray Microprobe of Orbital Alignment in Strong-Field Ionized Atoms

We have developed a synchrotron-based, time-resolved x-ray microprobe to investigate optical strong-field processes at intermediate intensities (${10}^{14}–{10}^{15}\mathrm{W}/{\mathrm{cm}}^2$). This quantum-state specific probe has enabled the direct observation of orbital alignment in the residual ion produced by strong-field ionization of krypton atoms via resonant, polarized x-ray absorption. We found strong alignment to persist for a period long compared to the spin-orbit coupling time scale (6.2 fs). The observed degree of alignment can be explained by models that incorporate spin-orbit coupling. The methodology is applicable to a wide range of problems.

Figure 1

Microfocused x rays probe atoms in the focus of a high-intensity laser.

Figure 2

Near $K$ edge spectra of krypton. Top: fluorescence excitation near $K$ edge spectrum of neutral krypton. Bottom: same for krypton ionized by a circularly polarized optical laser field. The neutral background has been subtracted. Dashed (dotted) line is the ab initio ${\mathrm{Kr}}^{1+}$ (${\mathrm{Kr}}^{2+}$) spectrum convolved with the experimental bandwidth. Solid line is the sum of the ${\mathrm{Kr}}^{1+}$ and ${\mathrm{Kr}}^{2+}$ components. The ${\mathrm{Kr}}^{1+}:{\mathrm{Kr}}^{2+}$ ratio is $\sim 3$ within the localized viewing region.

Figure 3

Evidence of orbital alignment of strong-field ionized krypton. The laser-on/laser-off ratio of x-ray fluorescence yield at the $1s\to 4p$ resonance as a function of ${\theta }_{X-L}$, the angle between laser and x-ray polarization directions.