Figure 1

Schematic of the time-resolved x-ray absorption setup. The first laser pulse (optical pump) heats the 1000 Å thick aluminum sample that is deposited on a specific grid. The second laser pulse is focused on the x-ray conversion target that is set 7 mm behind the sample, and generates a bright, broadband, isotropic, and ultrafast x-ray source. X rays are spectrally dispersed by two different Bragg crystals (not represented) on a CCD camera: the upper one directly measures the x-ray source spectrum (x-ray reference), while the lower one measures the transmitted spectrum through the heated aluminum sample (x-ray probe). The solid angles of respective collected x-ray emission are represented. The delay between laser pulses is varied in order to obtain time-resolved XANES with respect to the heating.Reuse & Permissions

Figure 2

XANES spectra measured as a function of the time delay between the x-ray probe and the optical pump. The origin is set when the maxima of optical and x-ray pulses coincide. The spectrum registered at the delay $-7.5\mathrm{ps}$ exhibits the same features as for unheated Al sample: a sharp $K$ edge at 1.559 keV, followed by XANES modulations. These modulations vanish promptly after heating, revealing a fast loss of atomic short-range order (therefore an ultrafast solid-liquid transition). At delay 2.5 ps, the $K$-edge slope is broadened due to a high electron temperature $T_e$. At longer delays, a transition from $K$ edge to $1s\mathrm{\text{−}}3p$ atomic absorption line can be observed, revealing the progressive relocalization of valence electrons in atomic orbitals during the liquid-vapor transition.Reuse & Permissions

Figure 3

Aluminum density and temperature temporal profiles as calculated with a two-temperature hydrodynamic code. The 1000 Å thick aluminum is heated with $6\mathrm{J}/{\mathrm{cm}}^2$ laser pulse at 60° incidence in $p$ polarization. Top: the density $N_i$, the electron temperature $T_e$ and the ion temperature $T_i$ are plotted as a function of time (solid lines). For comparison with experimental data of Fig. 2, the calculated values corresponding to the delays of XANES measurements are also indicated by symbols ($\mathrm{\text{circle}}=N_i$, $\mathrm{\text{square}}=T_e$, $\mathrm{\text{triangle}}=T_i$). Bottom: the measured x-ray temporal profile is reported (cross) for x-ray probe optical pump $\mathrm{\text{delay}}=0\mathrm{ps}$, and compared to the temporal response of the streak camera (line). After deconvolution, the x-ray source duration is $3.15\pm 0.25\mathrm{ps}\mathrm{rms}$.Reuse & Permissions

Figure 4

Schematic of the aluminum solid-liquid-vapor transition dynamics at the atomic level. Top: density of states (DOS) and localization of electrons with corresponding x-ray absorption spectra features as measured from the $1s$ level (vertical arrows). The potential is presented as a function of the distance $r$ from the aluminum nucleus (right side of each figure). The occupation of electron DOS (left side of each figure) is indicated in black. Bottom: Simple two-dimensional representation of the spatial distribution of ions and electron density in each phase. Solid Al: in the solid case, the conduction band is partially occupied by valence electrons up to the Fermi energy $E_F$, leading to a sharp x-ray absorption $K$ edge. Liquid Al: the laser deposits energy in the valence electrons and induces a thermal broadening of the band occupation, leading to the $K$-edge broadening. The energy is transferred from electrons to the lattice, breaking the crystalline order. Atomic Al: due to high thermal pressure, hydrodynamic expansion occurs driving the transition to atomic vapor. In isolated atoms, the electrons are localized on the atomic orbitals. As the $3p$ orbital is partially unoccupied, the $1s\mathrm{\text{−}}3p$ line is observed in the x-ray absorption spectrum.Reuse & Permissions