Resonance ionization spectroscopy of stable strontium isotopes and 90Sr via 5s2 1S0→5s5p 1P1→5s5d 1D2→5s11f 1F3→Sr+

Single-, double- and triple-resonance ionization in the 5s2 1S0→5s5p 1P1→5s5d 1D2 →5s11f 1F3→Sr+ system of strontium was investigated to achieve an efficient and highly selective ionization scheme for ultra-trace detection of the radioisotope 90Sr. Isotope shifts and hyperfine structure for all stable isotopes of Sr were measured in all three resonant transitions. Good agreement with previous data was found in the 5s2 1S0→5s5p 1P1 transition and precision was improved by approximately one order of magnitude. Isotope shifts, including that for 90Sr, and hyperfine structure in the two- and three-photon transitions were measured for the first time. The double- and triple-resonance schemes were characterized with respect to their isotopic selectivity for detection of 90Sr. The double-resonance process offers an optical selectivity of 2×104, while unfavorable isotope shifts in the triple-resonance process limit its selectivity to ∼100. Using double-resonance ionization combined with mass spectrometric detection, an overall detection efficiency of >10−5, a selectivity >1010, and a detection limit of 0.7 fg 90Sr in samples with a 1010 excess of stable Sr isotopes have been demonstrated.