A systematic study of photoionization of free lanthanide atoms in the 4d giant resonance region

Charge-separated photoion-yield spectroscopy using monochromatized synchrotron radiation was conducted on free Ce, Nd, Gd, Dy, and Er lanthanide atoms in the 4d giant resonance region to measure the relative partial cross-section curves σ(Mⁿ⁺) for the formation of Mⁿ⁺ (n = 1-4) ions. In addition, photoelectron spectroscopy was conducted on free Nd, Dy, and Er atoms to measure the cross-sections σ(nl) for photoionization of the nl (=4d, 5s, 5p and 4f) subshell electron. Drawing upon the obtained spectra and the results of previous photoion-yield studies on Xe, Cs, Ba, Sm, Eu, and Yb atoms, we discuss the character of photoionization processes in the 4d region. The obtained charge-state distribution shows that as the atomic number Z increases, the 4d giant resonance changes gradually, rather than suddenly, from a shape resonance to an autoionization resonance. One possible explanation for this change is that it is due to the dependence of the potential for the excited 4f electron and the corresponding 4f radial wavefunction on terms in the 4d⁹4fⁿ⁺¹ configuration. For σ(M⁺) and σ(4f), which exhibit an asymmetric Beutler-Fano autoionization profile, a curve fitting analysis was conducted to obtain the profile parameters E_r, Γ, and q. It was found that for the Gadolinium atom, which has a configuration [Xe]4f⁷5d6s² characterized by a half-filled 4f⁷ subshell plus a 5d electron, both the asymmetry parameter q and the charge-state distribution varied peculiarly with the atomic number Z.