Optical Emission From Al Target Irradiated By Flash

The following text touches on some peculiarities in optical emission spectroscopy results from experiments on the free-electron laser FLASH [1, 2]. Aluminum targets were irradiated with 13.5 nm similar to 25 fs pulses at intensities of 10(13) and 10(16) W/cm(2) (20 and 1 mu m foci). Surprisingly, only neutral atom lines for the case with wider focus and traces of ion lines in the tighter focus case were observed with the optical emission spectroscopy (200-600 nm range), [2]. The motivating idea behind this work is the suggestion in [1] by Zastrau that the optical spectrometer sees only emissions from a cold expanding lower-density (< 10(22) cm(3)) plasma plume. In this contribution the notion of UV range screening is analyzed in detail.