Time Resolved K$_{\alpha}$ Measurements in laser-solid interaction at relativistic laser intensities*

We present the time-resolved K$_{\alpha}$ emission measurement in short pulse laser-solid interactions using a picosecond time-resolved x-ray spectrometer for laser intensities at 10$^{17}$, 10$^{18}$ and 10$^{19}$ W/cm$^{2}$. Our measurements indicate that most of the K$_\alpha$ radiation is generated several ps after the laser pulse is over. This suggests that the electrons responsible for the underlying process originate in an ionization cascade initiated by the hot electrons. This can be explaned by a simple model based on collisional coupling, plasma expansion and M-shell ionization that can reproduce the characteristics of the K$_\alpha$ history.