Characteristics of laser produced plasmas of hafnium and tantalum in the 1–7 nm region

Soft X-ray (SXR) spectra from hafnium and tantalum laser produced plasmas were recorded in the 1–7 nm region using two Nd:YAG lasers with pulse lengths of 170 ps and 10 ns, respectively, operating at a range of power densities. The maximum focused peak power density was 2.  3 × 1014 W cm-2 for 170 ps pulses and 1.  8 × 1012 W cm-2 for 10 ns pulses, respectively. Two intense quasicontinuous intensity bands resulting from n = 4 - n = 4 and n = 4 - n = 5 unresolved transition arrays (UTAs) dominate both sets of experimental spectra. Comparison with calculations performed with the Cowan suite of atomic structure codes as well as consideration of previous experimental and theoretical results aided identification of the most prominent features in the spectra. For the 10 ns spectrum, the highest ion stage that could be identified from the n = 4 - n = 5 arrays were lower than silver-like Hf25+ and Ta26+ (which has a 4d104f ground configuration) indicating that the plasma temperature attained was too low to produce ions with an outermost 4d subshell, while for the 170 ps plasmas the presence of significantly higher stages was deduced and lines due to 4d–5p transitions were clearly evident. Furthermore, we show an enhancement of emission from tantalum using dual laser irradiation, and the effect of pre-pulse durations and delay times between two pulses are demonstrated