Figure 1.

Schematics of the experimental setup to investigate the surface and subsurface solid-density plasma dynamics with GISAXS using single femtosecond X-ray FEL pulses. (a) An ML sample consists of 5 repetitions of tantalum (Ta) and copper nitride (Cu₃N) of 4.5 and $8.5 \mathrm{n}\mathrm{m}$ thickness covered by a $200 \mathrm{n}\mathrm{m}$ aluminium capping layer. The samples were irradiated by an optical laser with a central wavelength of $800 \mathrm{n}\mathrm{m}$, intensity of $(8\pm 0.2)\times {10}^{14}$ or $(8\pm 0.2)\times {10}^{15} \mathrm{W}/\mathrm{c}{\mathrm{m}}^2$ with $40 \mathrm{f}\mathrm{s}$ pulse duration. The laser is irradiated by an incident angle of ${17}^{\circ }$ from the surface normal in p polarization. The X-ray pulses with $8.81 \mathrm{k}\mathrm{e}\mathrm{V}$ photon energy, $7 \mathrm{f}\mathrm{s}$ FWHM duration are irradiated on the sample at the grazing-incidence angle of ${\alpha }_i={0.75}^{\circ }$, i.e., slightly above the critical angle of external total reflection of the layer materials. The laser beam is defocused to obtain a ∼500 $\mathsf{\mu }$m spot diameter to cover the X-ray footprint of ∼300 $\mathsf{\mu }$m at the surface. Scattered photons are recorded by 2 modules of the MPCCD area detector placed around the specular direction. The strong specular peak at $Q_z=1.16 \mathrm{n}{\mathrm{m}}^{-1}$ is blocked. (b) In-plane signal along $Q_z$ (at $Q_y=0$) for different time delays between 0 and $14\mathrm{ps}$ after the laser intensity peak.