Infrared photodetachment of Ce - : Threshold spectroscopy and resonance structure C. W. Walter, * N. D. Gibson, † C. M. Janczak, K. A. Starr, A. P. Snedden, and R. L. Field III Department of Physics and Astronomy, Denison University, Granville, Ohio 43023, USA P. Andersson Department of Physics, Göteborg University, SE-412 96 Göteborg, Sweden Received 19 July 2007; revised manuscript received 19 September 2007; published 7 November 2007; publisher error corrected 9 November 2007 The negative ion of cerium is investigated using tunable laser photodetachment threshold spectroscopy. The relative cross section for photodetachment from Ce - is measured over the photon energy range 0.61– 0.75 eV using a crossed laser-beam–ion-beam technique. The spectrum of neutral atom production reveals a photode- tachment threshold at 0.65 eV, which is interpreted as the threshold for the Ce - 4 f 5d 2 6s 24 H 7/2  to Ce 4 f 5d6s 21 G 4  ground-state to ground-state transition yielding the electron affinity of Ce to be 0.653 eV. At least five narrow peaks are observed in the cross section over the range 0.62– 0.70 eV due to negative ion resonances, and their energies and widths are measured. The results are compared to other recent experimental and theoretical studies of Ce - . DOI: 10.1103/PhysRevA.76.052702 PACS numbers: 32.80.Gc, 32.10.Hq I. INTRODUCTION Investigations of the dynamics of negative ions provide valuable insight into the fundamental problem of many-body motion, which is critical for a detailed understanding of the electronic structure of atoms and molecules. Negative ions provide challenging problems and critical test cases for atomic theory because the added electron is bound to a neu- tral core; thus, there is no long-range Coulomb binding force. Therefore, the influences of such effects as electron-electron correlation and core polarization are greatly enhanced in negative ions relative to neutral atoms and positive ions. Im- pressive progress has been made over the past two decades in understanding negative ions through both theoretical and ex- perimental advances; the properties of most atomic negative ions, including their binding energies and electronic struc- tures, are now well established 1–3. In addition to the prop- erties of ground-state negative ions, there is considerable in- terest in the excited states of negative ions, including both bound and unbound resonance states 4,5. Perhaps the most glaring exception to the high-precision information available for the negative ions of most elements remains in the lanthanide atoms 1,2. The lanthanides are particularly interesting and challenging because the large number of electrons and the presence of several open shells lead to strong valence-valence and core-valence correlation effects. From an experimental standpoint, studies of lan- thanide negative ions are challenging because of the diffi- culty in producing substantial stable beams of the ions 6,7, the need to use less common infrared light sources for threshold investigation due to the small binding energies  1 eV of the ions, and the possibility of overlapping sig- nals due to multiple bound excited states. These challenges have led to substantial discrepancies between experimental and theoretical determinations of the electron affinities for several of these atoms 2. Furthermore, even the ground- state configurations of their negative ions have not been firmly established, as the additional electron may enter the open 4 f ,5d, or 6p valence shells. Of the lanthanides, the negative ion of cerium has re- ceived the most theoretical and experimental attention. The ground-state configuration of neutral Ce Z =58 has primary LS character Xe 4 f 5d6s 21 G 4 o 8. Early relativistic con- figuration interaction RCI calculations 9 predicted that the ground state of the negative ion would be formed by 6p attachment to the neutral ground state. However, more recent larger-scale calculations using the techniques of RCI 10,11 and pseudopotential multireference configuration interaction MRCI12 concluded that the Ce - ground state is formed by 5d attachment, giving a ground-state configuration for the negative ion of primary character Xe 4 f 5d 2 6s 24 H 7/2 o . The binding energy of the ground state of Ce - , corresponding to the electron affinity of Ce, was calculated to be 0.428 eV by O’Malley and Beck 10 and 0.5810 eV by Cao and Dolg 12. A later calculation by O’Malley and Beck 11 with a larger basis set yielded an electron affinity of 0.511 eV, but the authors note that test calculations suggested that more binding would be obtained through inclusion of opening of the 5p subshell. Ce - has been investigated in several experiments over the past 14 years. In 1993, Garwan et al. 13 estimated the electron affinity to be 0.6 eV based on the relative yield of Ce - from a sputtering source in accelerator mass spectrom- etry experiments. They also suggested the possibility that Ce - may have multiple bound excited states. Subsequently, in 1997, Berkovits et al. 14 observed two sharp increases in a coarsely stepped Ce - photodetachment cross-section spec- trum at photon energies of 2.130 eV and 2.165 eV, which were interpreted as opening thresholds for detachment to ex- cited states of Ce. Their interpretation of the spectrum led to an electron affinity of 0.70010 eV. However, that value is questionable because it was based on the assumption of 6p attachment for the ground state of the negative ion, which is not consistent with more recent theoretical results that indi- cate 5d attachment 10–12. *walter@denison.edu † gibson@denison.edu PHYSICAL REVIEW A 76, 052702 2007 1050-2947/2007/765/0527028 ©2007 The American Physical Society 052702-1