Time-resolved probing of aluminum plasma created by a 100-fs laser pulse using a picosecond soft X-ray pulse

We measured the spatiotemporal evolution of soft X-ray absorption in Al-plasma created by a 100-fs, 10¹⁴-W/cm² laser pulse. We observed a clear structure in the transmission spectrum, in which the L_II,III edge was blue-shifted by about 6 eV, more than 20-50 ps after the laser irradiation due to the formation of liquid droplets or small clusters. We assigned some of the structures in the transmission spectra as Al³⁺ ion absorption lines, which indicate the stable existence of Al³⁺ ions. We estimated the density and expansion velocity of a particle cloud with a scale length of ~100 gm at τ_D ~1 ns to be of the order of 10¹⁸ cm^-3 and 10⁵ m/s, respectively