Light-pulse pairs reveal the femtosecond decay of an inner-shell hole in xenon atoms

Researchers have observed the decay of an inner-shell vacancy in xenon atoms directly in the time domain, using pairs of phase-coherent extreme ultraviolet pulses from a synchrotron. The two pulses released photoelectron wave packets that interfered with each other. The contrast of the resulting fringes decreased over a few femtoseconds as the delay between the pulses increased, and a model that used literature core-hole lifetimes of 5.9 and 6.3 femtoseconds reproduced this rapid decay of the fringe contrast. This shows that ordinary electron spectroscopy with coherent pulse pairs can follow core-hole dynamics, and the authors suggest it could be extended to molecules and condensed matter.

Researchers at the UVSOR Synchrotron Facility (Institute for Molecular Science, Japan) and partner institutions have observed the decay of a xenon 4d core hole, an inner-shell vacancy, directly in the time domain. They used pairs of phase-coherent extreme ultraviolet pulses from a synchrotron to make the photoelectron wave packets they released interfere. The contrast of the interference fringes decreased over a few femtoseconds as the delay between the pulses increased. A model using literature core-hole lifetimes of about 6 femtoseconds reproduced this rapid decay of the fringe contrast.

Auger decay is an ultrafast process. A valence electron fills an inner-shell vacancy, and another electron is ejected. It dominates the decay of the xenon 4d hole. Until the mid-2000s, experiments mostly inferred such dynamics from the spectral widths of photoelectron lines. Attosecond streaking later made time-domain observation possible. While attosecond streaking has been widely employed for time-domain observations of Auger processes, the research team has introduced an alternative approach based on coherent pulse pairs. The present work extends the photoelectron wave-packet interferometry to core holes created by photoionization.

At beamline BL1U of the UVSOR-III synchrotron, two undulators in tandem, separated by a phase-shifter magnet, emitted phase-coherent pulse pairs. The photon energy was 93.4 eV, with a bandwidth of about 3%. The phase-shifter magnet set the delay between pulses with attosecond resolution. The pulses ionized xenon 4d electrons, and a hemispherical analyzer recorded photoelectron spectra as a function of delay. Helium, which has no comparable decay, served as a reference. Its fringe contrast revealed the instrument's own contrast loss.

Both gases showed fringes spaced 40 attoseconds apart, matching the optical cycle of the light. In helium, the contrast decreased slowly. The authors attribute this mainly to the electron beam's energy spread and emittance, with imperfect overlap and intensity imbalance between the undulators also contributing. In xenon, the contrast decreased much faster, which the authors attribute to the femtosecond lifetime of the core hole. In addition, the xenon contrast exhibited oscillations with a period of about 2 fs. This beat originates from the coherent excitation of the spin-orbit splitting. Model calculations incorporating reported lifetimes successfully reproduced both the decay and the beat, yielding a spin-orbit splitting in excellent agreement with established values. This clearly demonstrates that the present interferometric method directly reflects the ultrafast core-hole dynamics.　

According to the authors, this is the first application of photoelectron wave-packet interference to a short-lived core-hole state. It shows that coherent pulse pairs from a synchrotron can follow core-hole dynamics with ordinary electron spectroscopy. The authors propose extending the approach to molecules and condensed matter. They suggest shorter wavelengths could be reached with ultralow-emittance synchrotrons.