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, 10/sup 14/-W/cm/sup 2/ laser pulse. We observed a clear structure in the transmission spectrum, in which the L/sub 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 Al/sup 3+/ ion absorption lines, which indicate the stable existence of Al/sup 3+/ ions. We estimated the density and expansion velocity of a particle cloud with a scale length of /spl sim/100 gm at /spl tau//sub D/ /spl sim/1 ns to be of the order of 10/sup 18/ cm/sup -3/ and 10/sup 5/ m/s, respectively.