Laser and microwave spectroscopy of even-parity Rydberg states of neutral ytterbium and multichannel-quantum-defect-theory analysis

Measurements of high-lying even-parity 6sns0.16em0ex¹S₀ and 6snd0.16em0ex1,3D₂ levels of neutral ¹⁷⁴Yb are presented in this paper. Spectroscopy is performed with a two-step laser excitation from the ground state 4f¹⁴6s²0.16em0ex¹S₀, and the Rydberg levels are detected by using the field ionization method. Additional two-photon microwave spectroscopy is used to improve the relative energy accuracy where possible. The spectroscopic measurements are complemented by a multichannel-quantum-defect-theory (MQDT) analysis for the $J=0$ and the two-coupled $J=2$ even-parity series. We compare our results with the previous analysis of Aymar, D\'ebarre, and Robaux [J. Phys. B: At. Mol. Phys. 13, 1089 (1980)] and analyze the observed differences. From the MQDT models, a revised value for the first ionization limit I₆ₛ=50443.07041(25)4pt0excm^-1 is proposed for ¹⁷⁴Yb.