Exposing the role of electron correlation in strong-field double ionization: X-ray transient absorption of orbital alignment in Xe⁺ and Xe²⁺

Orbital alignment measurements and theory are used to examine the role of electron correlation during atomic strong-field double ionization (795 nm, (1-5) × 10¹⁴ W cm^-2). High-order harmonic, transient absorption spectroscopy is used to measure the angular distributions of singly and doubly tunnel-ionized xenon atomic states via 4d core to 5p valence shell transitions between 55 and 60 eV. The experimental M_J alignment distributions are compared to results of a rate-equation model based on sequential ionization, previously developed for coherent electron motion, and now applied to account for the alignment prepared by tunneling ionization. The hole generated in the ²P_3/2 state of Xe⁺ is measured to be entirely composed of |M_J| = 1/2, in agreement with theory. The result is a higher degree of alignment than previously reported. Because the model neglects effects of electron-ion recollision, the theory predicts a high degree of alignment in both spin-parallel (triplet) and antiparallel (singlet) terms of Xe²⁺. However, the alignment generated with linearly polarized light is observed to be spin-state dependent. The measured alignments for triplet spin states (³P₂ has |M_J| = [0: 1: 2] of [27±6: 45±11: 29±0] and ³P₁ has |M_J| = [0: 1] of [56±2: 44±2]) are in good agreement with the expectations of theory, which are [33: 53: 14] and [66: 33], respectively. The results validate the rate equation model for sequential tunnel ionization. However, the alignment extracted for a singlet state is greatly diminished: ¹D₂ is measured to be [18±1: 39±2: 43 ± 2] compared to theoretical expectation of [60: 39: 1] for |M_J| = [0: 1: 2]. The poor agreement with the sequential ionization model suggests that the alignment of ¹D₂ is strongly influenced by the high propensity for the liberated first electron to return to and recollide with its parent atomic orbital. Therefore, although the influence of electron recollision appears minor in the triplet states and suggests sequential ionization, electron correlation between the ionic core and the first ionized electron cannot be ignored in the singlet state. Singlet states are likely to be generated through nonsequential double ionization over the intensity range where the experiments are performed.