Spatiotemporal evolution of soft X-ray absorption in an ablated particle cloud created by a 100-fs laser pulse on an aluminum target

This study develops a soft X-ray absorption spectroscopy system for photo-excited material dynamics with one-dimensional spatial resolution. The system consists of a femtosecond-laser-produced plasma X-ray source and a critical illumination system using a KB microscope and has a temporal resolution of approximately 30 ps and a spatial resolution of at least better than 25 μm. Snapshots of expanding aluminum plasma as spatially resolved absorption spectra are successfully obtained. The captured images show the blue shift of the Al L_II,III absorption edge caused by ionization in the ablated plasma. From the amount of soft X-ray absorption, the mean particle density near the target surface 5 ns after the laser pulse irradiation is estimated to be of the order of 10¹⁹ cm^-3.