Tracking Ultrafast Atomic Dynamics
Inner-shell vacancies in xenon typically decay through Auger decay, an ultrafast process where a valence electron fills a core-level vacancy while ejecting a secondary electron. While previous investigations relied on spectral line widths or attosecond streaking, the research team extended photoelectron wave-packet interferometry to core holes created by photoionization.
The experiments were conducted at beamline BL1U of the UVSOR-III facility, utilizing two tandem undulators separated by a phase-shifter magnet. Operating at a photon energy of 93.4 electron volts, the setup emitted phase-coherent light pulse pairs. By tuning the phase-shifter magnet, scientists controlled the delay between pulses with attosecond precision.
Interference Fringes and Quantum Beats
When the light pulses ionized xenon 4d electrons, they produced interfering photoelectron wave packets. A hemispherical electron analyzer recorded the spectra as a function of the attosecond-scale delay. Reference measurements were taken in helium gas to account for baseline contrast loss, as helium lacks a comparable core-hole decay process.
Both gases displayed interference fringes spaced 40 attoseconds apart, matching the optical cycle of the incident light. While helium's fringe contrast decayed slowly due to instrumental factors, the xenon fringe contrast diminished rapidly over a few femtoseconds, capturing the core hole's short lifetime. Additionally, the xenon data revealed temporal oscillations with a 2-femtosecond period, pointing to a quantum beat from spin-orbit splitting.
Theoretical models incorporating literature core-hole lifetimes of approximately 6 femtoseconds successfully matched both the rapid contrast decay and the beat structure. The authors note that this interferometric approach could eventually be expanded to examine core-hole dynamics in molecules and condensed matter.
