Experimental and Theoretical Studies of Bound and Quasi-bound States of Ce−

The negative ion of cerium is investigated experimentally with tunable infrared laser photodetachment spectroscopy and theoretically with relativistic configuration interaction in the continuum formalism. The relative cross section for neutral atom production is measured with a crossed laserion beam apparatus over the photon energy range 0.54 – 0.75 eV. A rich resonance spectrum is revealed near-threshold with at least twelve peaks observed due to transitions from bound states of Ce− to either bound or quasi-bound excited states of the negative ion. Theoretical calculations of the photodetachment cross sections enable identification of the transitions responsible for the measured peaks. Two of the peaks are due to electric dipole allowed bound-bound transitions in Ce−, making cerium only the second atomic negative ion that has been demonstrated to support multiple bound states of opposite parity. In addition, the combined experimental data and theoretical analysis determines the electron affinity of cerium to be 0.628(10) eV and the fine structure splitting of the ground state of Ce− (H7/2 – H9/2) to be 0.09775(4) eV.