Time-domain measurement of Auger electron dynamics in xenon atoms after giant resonant photoionization

Abstract

Time-resolved measurement of Auger-Meitner (AM) decay [Nature 419, 803 (2002)] marked a milestone in the development of attosecond science. To date, the time constants for the AM decay processes obtained from the time-domain experiments were found to be consistent with the values deduced from conventional energy-domain measurements. One of the main factors limiting the temporal resolution of these studies is the unlocked carrier-envelope-phase (CEP) of the laser pulses used to probe the electronic dynamics triggered by inner-shell photoabsorption. In this work, we report time-resolved inner-shell electron spectroscopy of xenon and krypton using attosecond soft X-ray (atto-SXR) pulses centered at 130 eV in combination with CEP-stabilized few-cycle Yb laser pulses. We observed that the N$_{4,5}$OO Auger electrons from xenon exhibit a clear streaking pattern, but with an unexpected time shift of $\sim$ 1.32 fs relative to the 4$d$ photoelectrons. Furthermore, the energy-integrated yield of streaked Auger electrons from xenon exhibits a pronounced minimum at a pump-probe time delay of 4 fs. Neither of these observations can be explained by current streaking theories and both are inconsistent with lifetimes inferred from energy-domain measurements. The M$_{4,5}$NN Auger electrons from krypton partly overlap in energy with the 3$d$ inner-shell photoelectrons and do not show these anomalous features. This study offers new insights into the inner-shell electron dynamics of heavy atoms in the giant dipole resonance region, laying the groundwork for attosecond soft X-ray spectroscopy of molecular systems containing iodine or bromine atoms.

Figures (4)

• Figure 1: Inner-shell electron spectroscopy of xenon and krypton atoms ionized by tabletop attosecond soft X-ray pulses. (A) Schematic diagram of photoelectron (magenta arrow) and Auger electron (green arrow) emissions. (B) CEP-resolved HHG supercontinuum spectra measured after a 200-nm-thick Ag filter, where the filter transmission (red curve) is overlaid. (C) Very smooth Gaussian-shaped HHG spectrum at the relative CEP of 3.2 $\pi$, which is used in the ionization experiments. (D, E) Measured electron momentum distributions from xenon and krypton atoms, respectively. The distributions are Abel-inverted central-momentum slices and thus there are some artifacts along the vertical axis. (F, G) Corresponding electron energy distributions after an angular integration from 5$^\circ$ to 30$^\circ$, where 0$^\circ$ is defined as the vertical axis (i.e. polarization direction).
• Figure 2: Attosecond streaking measurements on xenon and krypton atoms. (A, C) Measured electron streaking traces with an angular integration from 5$^\circ$ to 30$^\circ$ from xenon and krypton, respectively. In (A) we observe the momentum streaking effect on both 4$d$ photoelectrons and Auger electrons with a time shift $\tau_{\rm{shift}}$. To quantify this time shift, (B) shows the extracted streaking amplitudes with a magnification for Auger electrons. (D) Schematic illustration of the streaking effect on photoelectrons (magenta curve) and Auger electrons (blue curve). The generation rate of photoelectrons is proportional to the intensity envelope of the attosecond SXR pulse, while the generation of Auger electrons is modeled as a convolution with an exponential decay function, reflecting the finite lifetime of the Auger-Meitner process. (E) Calculated streaking time shifts and streaking amplitudes of the Auger electrons as a function of the their lifetime. The dashed lines represent the classical analytical results [Eq. (1)] and the data points are the results from quantum simulations. The predicted largest time shift is approaching to 0.75 fs, i.e., a quarter of an optical cycle. However, our experimental data reveal a time shift of approximately 1.32 fs, posing a challenge to current theories.
• Figure 3: Long-range pump-probe experimental data. (A-B) Measured electron energy spectra as a function of pump-probe time delay for Xe and Kr atoms, respectively. (C-D) Energy-integrated Auger electron yields in the ranges of 29–35 eV for krypton and 29–37 eV for xenon. The orange curves represent the results after applying a low-pass filter ($<$0.5 eV). In (A) and (C) the red arrow marks the delay position of the yield minimum of Auger electrons.
• Figure 4: Related energy levels and transitions for (A) Xe penent_multielectron_2005 and (B) Kr palaudoux_multielectron_2010. The dashed boxes indicate a manifold of Rydberg satellite states.