Time-resolved shot-by-shot photoelectron spectroscopy of autoionizing ${\mathrm{Xe}}^{+}$ 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 ∼100 fs and ∼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.

Figure 1

Relevant energy levels of Xe, ${\mathrm{Xe}}^{+}$ (green), and ${\mathrm{Xe}}^{2+}$ (blue) and schematics of EUV-pump (40.8 eV) and NIR-probe (1.56 eV) photoelectron spectroscopy of indirect double ionization of Xe. Among the ${}^{3}P$ manifolds of ${\mathrm{Xe}}^{2+}$, the NIR-probe process is indicated only for the ${}^{3}P_2$ level.

Figure 2

(a) A schematic of the present experimental setup for time-resolved single-shot photoelectron spectroscopy using a magnetic bottle electron spectrometer. Timing monitor system provides information on relative delays between EUV-FEL and NIR laser pulses on the shot-by-shot basis, allowing us to improve the time resolution influenced by timing jitter. (b) Photoelectron spectra of He with [red (gray)] and without (black) NIR laser pulses. He $1s$ photoelectron peak and its sideband (SB) are indicated as sticks. (c) Integrated intensities of He $1s$ photoelectron sideband in (b) as a function of pump-probe delay without using a timing monitor system. Full width at half maximum (FWHM) corresponds to the intrinsic time resolution under the influence of the timing jitter. (d) Same as (c), but using the timing monitor system at $\mathrm{\Delta }t=0\mathrm{ps}$. Single-shot photoelectron spectra ($9\times {10}^3$) are sorted in the order of delay and averaged in each time bin (10 fs) before the SB integration.

Figure 3

(a) Photoelectron spectra of Xe with EUV-FEL pump ($4\times {10}^{13}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$) and NIR probe ($8\times {10}^{12}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$) pulses at pump-probe delay of $\mathrm{\Delta }t=0\mathrm{ps}$ [red (gray)] and $\mathrm{\Delta }t=-5\mathrm{ps}$ (black). Sticks α, β, and γ are for electrons formed by autoionization indicated by Fig. 1. (b) The difference spectrum between them is displayed in the upper part. Stick spectra are calculated by using Eq. (1) with ${\delta }_l=0$. Inset: NIR intensity dependence of the net photoelectron signals (0.3–1.6 eV) at $\mathrm{\Delta }t=0\mathrm{ps}$.

Figure 4

(a) Pump-probe photoelectron spectra of Xe as a function of time delay between 0 and 1000 ps. Intensities of FEL and NIR pulses are the same as those in Fig. 3. (b) Time evolution of photoelectron peaks at 0.45 eV (blue), 0.71 eV (light blue), 0.88 eV (green), and 1.02 eV (orange). Solid curves represent single exponential functions (lifetime ${\tau }_1$) fitted to the experimental data.

Figure 5

Integrated intensities (circles) of photoelectron peak at (a) 0.71 eV, (b) 0.88 eV, and (c) 1.02 eV, as a function of time delay between EUV-FEL ($1.8\times {10}^{14}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$) pump and NIR ($4\times {10}^{12}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$) probe pulses. Pump-probe time delays were postcorrected using a timing monitor system. The time zero was determined by the maximum position of the transient signals of the Xe $5p$ photoelectron sideband (gray curve). Curves in (a,b) are obtained by fitting the integrated intensity data to simulated results based on coupled-rate equations for a four-level system depicted in the inset of (c). Dotted curve represents time dependence of a Rydberg state without decay while solid curve [red (gray)] depicts time evolution of an autoionizing state with a lifetime (${\tau }_2$). Solid curve (black) is the sum of these contributions.