High ionization states observed in soft x-ray spectra from plasmas of second row transition elements produced by femtosecond laser pulses

The soft x-ray emission spectra of femtosecond-laser-produced plasmas from 2nd row transition elements from yttrium (Z = 39) to palladium (Z = 46), with the exception of technetium (Z = 43), were measured in the 1-5 nm region. Plasmas were produced by shining pulses from a titanium-sapphire laser with 65 fs pulse duration and an energy per pulse of 4.5 mJ focused to an intensity of 3 x 10(15) W cm(-2) onto bulk targets. While the emission spectra from yttrium to molybdenum (Z = 42) contain only the unresolved transition arrays (UTA) already observed in nanosecond and picosecond laser-target interactions described in our previous paper (Lokasani et al 2015 J. Phys. B: At. Mol. Opt. Phys. 48 245009), transitions from higher ionization states are clearly demonstrated in the spectra emitted from ruthenium (Z = 44), rhodium (Z = 45) and palladium targets heated by the femtosecond laser. These UTAs are interpreted by comparing the experimental spectra to calculated results, obtained using the Cowan suite of codes, the flexible atomic code as well as previous predictions of isoelectronic trends. The 3d-4f transitions arrays emitted from Ru XXI-XXIII, Rh XXI-XXIII and Pd XXI-XXIII ions are clearly seen in the observed spectra. To our knowledge, such high ionization states are demonstrated for the first time at such moderate laser energies.