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

Measurements of high-lying even-parity 6 s n s ¹S₀ and 6 s n d ^{1 ,3}D₂ levels of neutral ¹⁷⁴Yb are presented in this paper. Spectroscopy is performed with a two-step laser excitation from the ground state 4 f¹⁴6 s²¹S₀ , 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, Débarre, and Robaux [J. Phys. B: At. Mol. Phys. 13, 1089 (1980), 10.1088/0022-3700/13/6/016] and analyze the observed differences. From the MQDT models, a revised value for the first ionization limit I_{6 s}=50443.07041 (25 ) cm^-1 is proposed for ¹⁷⁴Yb.