Real-time observation of disintegration processes within argon clusters ionized by a hard-x-ray pulse of moderate fluence

We present a time-resolved study of disintegration within atomic clusters ionized by a hard-x-ray pulse of moderate fluence. It was performed with electron and ion spectroscopy, and complemented by theoretical simulations. The expanding clusters were probed with a near-infrared (NIR) laser pulse over a range of pump-probe delays from −4 to 10 ps. In addition to an increasing number of singly charged atomic ions, originating from the ionization of Rydberg atoms formed through electron-ion recombination, we observe a decrease of oligomer yields. The latter is due to the interaction of oligomers with the NIR probe pulse, leading to their dissociation. At time delays between −1 and 2 ps, efficient absorption of the NIR laser energy occurs, even though the NIR intensity is too low to trigger tuneling ionization of Ar atoms. Our observations are similar to earlier observations of the fragmentation behavior of clusters excited by soft-x-ray pulses. This indicates that the relaxation dynamics of x-ray-excited nano-objects are universal over a wide range of excitation photon energies.

Figure 1

Experimental scheme for the XFEL-pump–NIR-probe measurement at the SPring-8 Angstrom Compact Free Electron Laser facility. The XFEL and NIR laser were focused into the interaction region, and crossed with the cluster beam from the cluster source through two skimmers. The relative intensity per XFEL pulse was measured by a p-intrinsic-n photodiode located downstream of the interaction point. The produced ions were accelerated by a time-of-flight spectrometer, and detected by a detector consisting of a delay-line anode and two microchannel plates (MCPs). The emitted electrons were recorded by a velocity-map-imaging spectrometer, which consists of two MCPs, a phosphor screen, and a CCD camera.

Figure 2

Time-of-flight spectrum from argon clusters with an average size of $\langle N\rangle \sim 1000$ atoms after irradiation with XFEL pulses (5.5 keV) at an average peak fluence of 4.1 $\mu \mathrm{J}/\mu {\mathrm{m}}^2$ (black line). Using an additional weak NIR pulse (800 nm) with an intensity of $5.0\times {10}^{12}$ ${\mathrm{W}/\mathrm{cm}}^2$ as a probe pulse, we observe that the ${\mathrm{Ar}}^{1+}$ ion contribution is enhanced at a time delay of 10 ps, whereas the ${\mathrm{Ar}}_3^{1+}$ ion contributions are reduced (red line).

Figure 3

(a) Kinetic-energy spectra of ${\mathrm{Ar}}^{1+}$ as a function of the time delay between XFEL pump and NIR probe pulses. The intensity of the NIR pulse is $5.0\times {10}^{12}$ ${\mathrm{W}/\mathrm{cm}}^2$. (b) Kinetic-energy spectra obtained with the XFEL pulse only and with the additional NIR pulse at time delays of 0.1 and 10 ps. The inset of (a) shows the yields of ${\mathrm{Ar}}^{1+}$ with the kinetic energies of 0–6 and 6–50 eV, respectively, as a function of time delay.

Figure 4

Electron and ion spectroscopy data obtained at an XFEL peak fluence of 4.1 $\mu \mathrm{J}/\mu {\mathrm{m}}^2$ and an NIR intensity of $5.0\times {10}^{12}$ ${\mathrm{W}/\mathrm{cm}}^2$, with kinetic-energy spectra of (a) electrons and (b) ${\mathrm{Ar}}^{1+}$ ions and with (c) yields of electron and ${\mathrm{Ar}}^{1+}$ ion distributions as a function of the time delay between XFEL pump and NIR probe pulses.

Figure 5

Theoretical simulations with xmdyn code for electron and ion spectroscopy data obtained at the XFEL peak fluence of 4.1 $\mu \mathrm{J}/\mu {\mathrm{m}}^2$ and the NIR intensity of $5.0\times {10}^{12}$ ${\mathrm{W}/\mathrm{cm}}^2$, with kinetic energy spectra of (a) electrons and (b) ${\mathrm{Ar}}^{1+}$ ions and with (c) yields of electron and ${\mathrm{Ar}}^{1+}$ ion distributions as a function of the time delay between XFEL pump and NIR probe pulses. The simulation data take into consideration the focal volume integration.