Abstract
The odd-parity J=11/2 autoionizing states of europium atom below the 5d D-9(4) threshold are investigated by a two-step laser photoionization scheme via the 4f(7)5d6p F-10(9/2) intermediate state. The observed resonances are classified into two autoionizing Rydberg series (with n=15 to 55) converging to the 5d D-9(4) limit and eight members (n=13 to 20) of two autoionizing Rydberg series converging to the higher 5d D-9(5) limit. The single series converging to the 5d D-9(3) limit is not efficiently excited. The observed four Rydberg series are assigned in jj coupling as 5dnd (4,j) and 5dnd (5,j) with j=3/2 and 5/2. Apart from the 5dnd (5,5/2) series, the majority of autoionizing resonances exhibits narrow linewidths and quasisymmetric line shapes, reflecting this way the weak coupling between the discrete levels and the adjacent continua. The experimental data are analyzed through the semiempirical phase-shifted multichannel quantum defect theory. The theoretically calculated energy level positions and excitation profiles are in good agreement with the experimental ones, thus confirming the identification of the observed structures. The constructed Lu-Fano plot and computed admixture coefficients reveal weak and localized interactions among the Rydberg series members.