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

Researchers at the UVSOR Synchrotron Facility in Japan and partner institutions have directly observed the decay of a xenon 4d core hole in the time domain using phase-coherent extreme ultraviolet pulse pairs. The photoelectron wave packets released by the pulses interfered, with the contrast of the interference fringes decreasing rapidly over a few femtoseconds as the delay between the pulses increased.

A model using established core-hole lifetimes of about 6 femtoseconds reproduced this rapid decay. Auger decay, a dominant ultrafast process, involves a valence electron filling an inner-shell vacancy and ejecting another electron. While attosecond streaking has been used to observe such dynamics, the research team introduced a new approach based on coherent pulse pairs.

At beamline BL1U of the UVSOR-III synchrotron, the phase-coherent pulse pairs were emitted with a delay set by an attosecond resolution phase-shifter magnet. The xenon core-hole decay, with a femtosecond lifetime of about 6 fs, exhibited faster fringe contrast decay compared to helium (a reference gas). Additionally, the xenon contrast showed oscillations with a period of about 2 fs, originating from the spin-orbit splitting of the core hole.

The authors successfully modeled both the decay and beat oscillations, demonstrating that the interferometric method directly reflects the ultrafast core-hole dynamics. This is the first application of photoelectron wave-packet interference to a short-lived core-hole state, showcasing the potential of coherent pulse pairs for studying such dynamics using ordinary electron spectroscopy.

The researchers propose extending this approach to molecules and condensed matter, suggesting that shorter wavelengths could be achieved with ultralow-emittance synchrotrons.