Efficient two-color two-step laser ionization schemes of λ∼ 250 nm and λ = 307.9 nm for heavy refractory elements-Measurements of ionization cross-sections and hyperfine spectra of tantalum and tungsten.

We demonstrated efficient two-color two-step laser ionization schemes in the combined use of λ ∼ 250 nm and λ = 307.9 nm, which are applicable to heavy refractory elements with an atomic number in the wide range of Z = 69-78. We investigated newly observed ionization schemes of tantalum and tungsten atoms in an argon-gas-cell-based laser ion source for the efficient ionization of atoms of unstable nuclei through the two-color two-step laser resonance ionization technique. We experimentally determined the ionization cross sections from the measured saturation curves by solving the rate equations for the ground, intermediate, and ionization continuum populations. Hyperfine structures of these elements were also studied to deduce the isotope-shift, pressure-shift, and pressure-broadening in the resonance spectra of the excitation transitions in the argon gas cell. The electronic factor F of the excitation transition λ = 255.2115 nm between the ground and intermediate states was deduced from the measured isotope shifts of stable W isotopes. The ionization schemes investigated here are applicable to extract any isotopes of these elements by considering the measured pressure shift and nuclear isotope shift in optimizing the wavelength λ.