Multichannel quantum-defect-theory analysis of the observed odd-parity 5dnd autoionization Rydberg series of neutral europium

The spectra of autoionizing Rydberg series of europium atoms are analyzed by a semiempirical multichannel quantum defect theory (MQDT). Experimental data were obtained using two-step resonance ionization spectroscopy below the 5d D-9(J) ionic multiplet. The MQDT model involving five channels and three limits is used to analyze the observed four odd-parity Rydberg series. Theoretical energies are computed and compared with the experimental values. The Rydberg series are assigned on the basis of calculated admixture coefficients. The series having J=3/2 are interpreted as 5dnd (2,3/2), 5dnd (2,5/2), 5dnd (3,3/2), and 5dnd (3,5/2). Interseries perturbations and perturbations due to levels of configuration 5dnd (4,5/2) are investigated. Some of the unobserved levels are also predicted.