4d and 5p photoabsorption in laser-produced thulium plasmas

The dual-laser plasma method has been used to record photoabsorption spectra of Tm, Tm+, and Tm2+ ions. The dominant process is 4d photoexcitation, giving rise to structure in the 140-240-eV region while structure due to 5p excitation is observed in the 27-38-eV region. Due to successive removal of 6s electrons outside a 4f(13) core with increasing ionization, the 4d photoabsorption spectra of Tm, Tm+, and Tm2+ are very similar. The spectrum of neutral thulium shows autoionizing resonances, excited from both fine-structure levels of the 4f(13)6s(2) ground state, evidenced by a clear double-peaked Fano structure in the 140-240-eV range Similar features are observed and can be associated with 4d-4f transitions in Tm+ and Tm2+. Broad structure, distinctly different from the spectra of Tm, Tm+, and Tm2+, in the same 4d-4f region of the spectrum at 25 ns suggests that Tm3+, with a different number of 4f electrons, may be present in the absorbing plasma immediately after its formation. Significant broadening due to autoionization was observed in the 5p absorption region for all three ion stages. The observed transitions were identified with the aid of the COWAN suite of codes, which are based on Hartree-Fock with configuration interaction approach.