Atomic-structure investigations of neutral einsteinium by laser resonance ionization

Excited atomic states of neutral einsteinium were investigated by resonant laser ionization using 10 pg (2×10^{10} atoms) of ^{254}Es. Ten transitions from the 5f^{11}7s^{2}^{4}I_{15/2}^{o} ground state to even-parity states were investigated in detail, via studies of lifetimes and further excitation steps into higher-lying odd-parity states below and above the first ionization potential. This led to the identification of 37 previously unknown odd-parity energy levels in the region between 37000 and 42500cm^{−1} and a number of autoionizing states, which ensure a high ion yield in resonant ionization. Rydberg states were identified and the corresponding Rydberg series converging either to the ionic ground or an excited state were analyzed to determine the first ionization potential of einsteinium to a value of E_{IP}=51364.58(14)_{stat}(50)_{sys}cm^{−1}, improving the previous result by a factor of four.