Following the Birth of a Nanoplasma Produced by an Ultrashort Hard-X-Ray Laser in Xenon Clusters

X-ray free-electron lasers (XFELs) made available a new regime of x-ray intensities, revolutionizing the ultrafast structure determination and laying the foundations of the novel field of nonlinear x-ray optics. Although earlier studies revealed nanoplasma formation when an XFEL pulse interacts with any nanometer-scale matter, the formation process itself has never been decrypted and its timescale was unknown. Here we show that time-resolved ion yield measurements combined with a near-infrared laser probe reveal a surprisingly ultrafast population ($\sim 12\mathrm{fs}$), followed by a slower depopulation ($\sim 250\mathrm{fs}$) of highly excited states of atomic fragments generated in the process of XFEL-induced nanoplasma formation. Inelastic scattering of Auger electrons and interatomic Coulombic decay are suggested as the mechanisms populating and depopulating, respectively, these excited states. The observed response occurs within the typical x-ray pulse durations and affects x-ray scattering, thus providing key information on the foundations of x-ray imaging with XFELs.

Popular Summary

Extreme forms of light with novel properties—such as ultraintense and ultrashort pulses of very high photon energy—have recently been made using high-power free-electron lasers. These forms of light have made it possible to study aspects of radiation-matter interactions, and they have also opened up new fascinating opportunities for structural investigations of nanosized matter. One example of the radiation-matter interactions is the creation of a nanometer-sized plasma (nanoplasma) via ultraintense hard x rays. However, the timescale of the formation of a nanoplasma has not been investigated so far. Here, we present the “birth” of a nanoplasma created by a hard-x-ray free-electron laser pulse from the SACLA free-electron laser in Japan. We study how this pulse interacts with a target of approximately 5000 condensed xenon (Xe) atoms (a “Xe cluster”). Such clusters are ideal for studying radiation-matter interactions because the number of contained atoms can be controlled, and the energy dispersion cannot be caused by a surrounding medium.

Ultraintense laser light strips electrons away from atoms contained within a cluster—we show that roughly 30,000 electrons were stripped away from 57% of atoms within the Xe cluster. These electrons remain trapped by the deep Coulomb potential of the multiply-charged cluster. By observing and modeling these excited states and atomic fragments on femtosecond timescales, we show that the population and depopulation of excited states occur over roughly 12- and 250-fs timescales, respectively. Electron-ion recombination and electron impact ionization are often considered to be primarily responsible for charge migration and energy transfer of highly ionized clusters. However, we show that the formation and decay of excited atoms also play a role in charge migration and energy transfer. Our results hinge on a dramatic improvement in temporal resolution, a boost made possible by correcting the uncertainty between the arrival times of the hard-x-ray pulse ionizing the Xe clusters and the near-infrared pulse probing the nanoplasma.

Our findings are important for studies of single-shot imaging of nanosized matter using extreme light pulses.

Figure 1

Ionization of ${\mathrm{Xe}}^{n+}$ states by the NIR laser in isolated atoms. (a) ${\mathrm{Xe}}^{13+}$ yields as a function of time delay of the NIR-probe pulses (800 nm) with respect to the XFEL-pump pulses (5.5 keV). The experimental results are given by white circles. The red solid curve shows the results of the fit with a product of two exponential functions with increasing and decreasing time constants of ${\tau }_p$ and ${\tau }_d$, respectively. The function was convoluted with a Gaussian function accounting for the instrumental function with a width $\sigma$. ${\tau }_p\simeq 10\mathrm{fs}$ and ${\tau }_d\simeq 40\mathrm{fs}$ are the time constants for the population and depopulation of ${\mathrm{Xe}}^{n+}$ $(n\le 12)$ states, respectively. The zero time delay was obtained by the fit with an accuracy of $\pm 20\mathrm{fs}$. The fitting parameter $\sigma \simeq 20\mathrm{fs}$ (rms) or $\sim 50\mathrm{fs}$ (FWHM) gives the time resolution. All data points in the time delay range from $-800$ to $+1500\mathrm{fs}$ have been used for the fit. (b) Schematic diagram for the ionization of the ${\mathrm{Xe}}^{n+}$ $(n\le m)$ states which decay into ${\mathrm{Xe}}^{m+}$ $(m<13)$ states without NIR probe. The orange, purple, yellow, and red arrows show the photoionization of the Xe atoms and Xe ions by x ray, Auger cascades of core-ionized Xe ions, fluorescence, and the photoionization of transient ${\mathrm{Xe}}^{n+}$ states by the NIR laser, respectively.

Figure 2

Ultrafast dynamics of XFEL-induced nanoplasma. (a) ${\mathrm{Xe}}^{+}$ and ${\mathrm{Xe}}^{2+}$ yields as a function of the time delay of the NIR-probe pulse (800 nm) with respect to the XFEL-pump pulse (5.5 keV). The experimental ${\mathrm{Xe}}^{+}$ and ${\mathrm{Xe}}^{2+}$ yields are given by circles and crosses, respectively. The solid curves show results of the theoretical modeling of the process of NIR laser heating up of XFEL-induced nanoplasma, leading to the gradual increase of the ${\mathrm{Xe}}^{+}$ and ${\mathrm{Xe}}^{2+}$ yields, as well as the process of formation of excited atoms ${\mathrm{Xe}}^{*}$ that can pairwise undergo interatomic Coulombic decay (ICD), or alternatively be directly ionized by the delayed NIR-probe pulse, resulting in a sharp jump and a slower depletion in the ${\mathrm{Xe}}^{+}$ yield. The highly excited states are formed by the inelastic scattering of the Auger electrons during the early stages of nanoplasma formation. (b) Schematic diagrams for the inelastic scattering of Auger electrons and the ICD.

Figure 3

Schematic picture of the XFEL-pump–NIR-probe experiment. The incident XFEL pulse impinges the grating [34], which generates multiple beams. A main part of the XFEL beam is focused on the interaction point by Kirkpatrick-Baez (KB) mirrors. The XFEL pulse [5.5 keV, $<10\mathrm{fs}$ (FWHM), $30\mu \mathrm{J}/\mu {\mathrm{m}}^2$] and the NIR laser pulse [800 nm, 32 fs (FWHM), $15\mathrm{nJ}/\mu {\mathrm{m}}^2$] illuminate the Xe clusters ($⟨N⟩=5000$ atoms) directed to the interaction point through a skimmer. The ion time-of-flight (TOF) spectrometer accelerates the produced ions, and the position sensitive detector, constructed with microchannel plates (MCPs) and a delay-line anode (DLA), detects them. The different ion charges are clearly separated in the time-of-flight spectra. The other parts of the XFEL and NIR laser illuminate single crystals from gallium arsenide (GaAs) serving as targets for the arrival timing monitor [34]. The CCD camera records the spatial modulation of the optical transmittance of the NIR laser in order to determine the accurate time delay of the NIR-probe pulse with respect to the XFEL-pump pulse. M1 and M2 are mirrors for the XFEL beam, M3–M5 are mirrors for the NIR laser, and SM is a split mirror for the NIR laser.