Attosecond electron-spin dynamics in Xe 4d photoionization

Abstract

The photoionization of xenon atoms in the 70-100 eV range reveals several fascinating physical phenomena such as a giant resonance induced by the dynamic rearrangement of the electron cloud after photon absorption, an anomalous branching ratio between intermediate Xe⁺ states separated by the spin-orbit interaction and multiple Auger decay processes. These phenomena have been studied in the past, using in particular synchrotron radiation, but without access to real-time dynamics. Here, we study the dynamics of Xe 4d photoionization on its natural time scale combining attosecond interferometry and coincidence spectroscopy. A time-frequency analysis of the involved transitions allows us to identify two interfering ionization mechanisms: the broad giant dipole resonance with a fast decay time less than 50 as, and a narrow resonance at threshold induced by spin-flip transitions, with much longer decay times of several hundred as. Our results provide insight into the complex electron-spin dynamics of photo-induced phenomena.

Fig. 1. Excitation scheme.

a Schematic illustration of Xe 4d photoionization (violet) and Auger decay processes (green) after absorption of XUV radiation. b Xe energy diagram showing the Xe⁺ intermediate and Xe²⁺ final states involved.

Fig. 2. Two-electron coincidence map.

The coincidence map using a XUV field only and b XUV+IR fields. The number of measured two-electron pairs is indicated by the color code. The spots with constant slow-electron energy and variable fast-electron energy correspond to photoelectrons created by absorption of consecutive harmonics (labeled as H57-H61 in (a) as an example) and a given Auger electron (e.g., 4d⁻¹(²D_3/2) → 5s⁻²(¹S₀)). Photoelectrons corresponding to absorption or emission of an additional IR photon (sidebands, labeled as S58 and S60 as an example) appear in b. The projection on the fast electron energy axis c and d is the sum of the signal with slow electron energy from 10 eV to 10.4 eV, i.e., the photoelectrons in coincidence with 4d⁻¹(²D_3/2) → 5s⁻²(¹S₀) Auger electrons. The projection on the slow electron energy axis e shows the sum of the signal for the different Auger processes indicated on the right, with (red) and without (blue) IR field. A RABBIT energy scheme is indicated at the top of d.

Fig. 3. Atomic time delays and branching ratio.

Absolute atomic time delays for a 4d_3/2 and b 4d_5/2; c Difference between these delays. The experimental data are given in red dots. The black lines indicate the results of two-photon RRPA calculations. The estimation of the error bars is the standard error of the weighted mean, detailed in Methods section. In d, the calculated branching ratio of the 4d_5/2 over 4d_3/2 cross sections is shown in black. The experimental data in blue dots is from ref. .

Fig. 4. Transition matrix elements.

a, d Modulus, b, e phase and c, f time delay of the transition matrix element D_i(E) as a function of photon energy for the coupled channels (a–c) 4d_3/2 → ϵf_5/2 (blue), 4d_5/2 → ϵf_5/2 (red), 4d_5/2 → ϵf_7/2 (brown) and eigenchannels (d–f) ¹P (black), ³P (magenta), ³D (orange), extracted from ref. . We exclude the Coulomb phase-shift in b, e and c, f. A π phase shift has been added to the phase of 4d_5/2 → ϵf_5/2 for better comparison. The time delay is the energy derivative of the phase.

Fig. 5. Wigner representation and effective potentials.

a Wigner representation W(E, t) for the 4d_5/2 → ϵf_5/2 channel. The amplitude is indicated by the color code on the right. b Illustration of one-electron potentials for the ¹S → ³P, ³D transitions (red) and ¹S → ¹P (black). Dashed lines suggest possible electron trajectories in the two cases.