Long-lived nonthermal electron distribution in aluminum excited by femtosecond extreme ultraviolet radiation

We report a time-resolved study of the relaxation dynamics of Al films excited by ultrashort intense free-electron laser (FEL) extreme ultraviolet pulses. The system response was measured through a pump-probe detection scheme, in which an intense FEL pulse tuned around the Al $L_{2,3}$ edge (72.5 eV) acted as the pump, while a time-delayed ultrafast pulse probed the near-infrared (NIR) reflectivity of the Al film. Remarkably, following the intense FEL excitation, the reflectivity of the film exhibited no detectable variation for hundreds of femtoseconds. Following this latency time, sizable reflectivity changes were observed. Exploiting recent theoretical calculations of the EUV-excited electron dynamics [N. Medvedev et al., Phys. Rev. Lett. 107, 165003 (2011)], the delayed NIR-reflectivity evolution is interpreted invoking the formation of very-long-living nonthermal hot electron distributions in Al after exposure to intense EUV pulses. Our data represent the first evidence in the time domain of such an intriguing behavior.

Figure 1

Top: schematic diagram of the experimental setup at EIS-TIMEX at FERMI. Bottom: calibration of the zero delay between FEL and laser pulses via the transient optical transmission through a thick ${\mathrm{Al}}_2{\mathrm{O}}_3$ crystal.

Figure 2

Left: Normalized reflectivity $R\left(\mathrm{\Delta }t\right)/R_0$ as a function of the pump-probe delay time. Red (blue) markers correspond to data measured for FEL central wavelength of 16.9 nm (18.1 nm), i.e., at photon energy above (below) the Al $L$ edge. The marker size is inversely proportional to the statistical error associated with each data point. Larger markers with darker hue correspond to more reliable data. Inset: Zoom around the zero-delay region for 16.9 nm radiation at 40 $\mu \mathrm{J}/\text{pulse}$. Right: Sketch of above- (red lines) and below-threshold (blue lines) photoexcitation schemes and short-delay nonthermal populations. Wavy lines represent photoinduced transitions; straight lines represent Auger transitions. The gray-shaded area represents the valence band. The orange (blue) areas are a pictorial representation of the high-energy excited-electron populations at very small $\mathrm{\Delta }t$.

Figure 3

Normalized reflectivity $R\left(\mathrm{\Delta }t\right)/R_0$ as a function of the pump-probe delay time for FEL central wavelength of 16.9 nm. Red (open blue) markers correspond to data measured with average FEL pulse energy of $40\mu J$ ($60\mu J$). The marker size is inversely proportional to the statistical error associated with each data point. Larger markers correspond to more-reliable data. Black markers: Time-dependent normalized optical reflectivity measured for low-photon-energy excitation experiments (Ref. [18]). Notice the different reflectivity scale between our data and those of Ref. [18].