Determination of the 154Sm ionization energy by high-precision laser spectroscopy

High-resolution resonance ionization mass spectrometry has been used to determine the ionization energy of 154Sm. Three-step resonant excitation with single-frequency lasers populates a series of l = 3, J = 4 Rydberg levels in the range of n = 60-160, covering the range of 30 cm-1 to 4 cm-1 below the first ionization limit. Although samarium has a complex electronic structure with eight valence electrons, series of nearly unperturbed levels could be observed. Analysis includes shifts caused by a single perturbing state, an extended Ritz term for quantum defect variation at lower n, and corrections for residual electric fields. The resulting series convergence limit has an uncertainty of 4 x 10-5 cm-1, while the final value EI (154Sm) = 45 519.307 93(43) cm-1 also accounts for the uncertainty in absolute laser frequencies coupling the Rydberg spectrum to the J = 0 ground state and other systematic errors. Precision is improved by nearly four orders of magnitude over previous values