Time-resolved shot-by-shot photoelectron spectroscopy of autoionizing Xe+ states by EUV–free-electron-laser and near-IR laser pulses

Ultrafast dynamics of highly excited Rydberg states of ${\mathrm{Xe}}^{+}$ is investigated by time-resolved shot-by-shot photoelectron spectroscopy with EUV--free-electron-laser pump (40.8 eV) and near-infrared probe (1.56 eV) pulses. Sorting the single-shot spectra in the order of relative delay between the two pulses allows for investigating ultrafast decays of intermediate states participating in the double ionization of Xe. Observed photoelectron spectra show a biexponential decay within \ensuremath{\sim}100 fs and \ensuremath{\sim}300 ps in addition to constant signals lasting longer than 1 ns. These ultrafast decays reflect the lifetimes of autoionizing Rydberg states converging to electronically excited states of ${\mathrm{Xe}}^{2+}$. The results demonstrate that time-resolved shot-by-shot EUV--free-electron-laser photoelectron spectroscopywith a synchronized optical laser provides a powerful tool for investigating ultrafast electron emission processes.