Novel mechanism for creating long-lived metastable atomic negative ions

A novel mechanism is proposed for creating long-lived metastable atomic negative ions in complex atoms, such as the lanthanides. It exploits the orbital collapse of the 5d orbital in Gd (Z$=$64) into the 4f orbital of Tb (Z$=$65). In the region of collapse the properties of the 5d and 4f orbitals are quite sensitive to the changes in the effective potential. Consequently the collapse phenomenon impacts the core-polarization interaction significantly in the relevant atom, namely Tb inducing a new excited Tb\textasciimacron anion. The mechanism is demonstrated in the lanthanide atoms Tb and Dy through the appearance of long-lived Tb\textasciimacron and Dy\textasciimacron anions in the Regge pole calculated electron elastic total cross sections. Ground and long-lived metastable negative ion formation occurs at the second Ramsauer -Townsend minima.