LETTER TO THE EDITOR Yb − 6p1=2—low-lying shape resonance rather than a bound state

Abstract. Strong evidence against stability of the negative ion of ytterbium is presented, and the prediction is made that the lowest negative ion state Yb − 4f146s26p 2P1=2 is a narrow shape resonance in the electron–atom continuum at 20 meV. These results are obtained from the many-body theory calculations with the electron–atom correlation potential appropriately scaled to reproduce the experimental data for the isoelectronic neutral Lu and Ba−. They are in agreement with the recent thorough experimental investigation of Yb − by the Aarhus group. The aim of this letter is to show that contrary to earlier theoretical evidence, the ytterbium negative ion is most likely unstable, and the ns2np negative ion configuration represents a p1=2-wave shape resonance in the electron–atom continuum at about 20 meV. Our calculation confirms the recent experimental work by Andersen et al (1998) that has found no evidence for the existence of stable or long-lived metastable Yb−. Before 1987 the only negative ion known to be formed by a closed-shell atom was Pd − 4d105s (Hotop and Lineberger 1985). Together with neutral Pd with its unique d10 ground-state configuration, this ion looked to be an exception to the rule. This rule had to change following the discovery of the stable Ca − negative ion (Froese Fischer et al 1987