Three-step laser spectroscopy and multichannel quantum defect analysis of odd-parity Rydberg states of neutral ytterbium

High-lying odd-parity levels of neutral ytterbium are investigated by the method of selective three-step laser spectroscopy. Excitation from the ground state 4f 14 6s 2 is performed by means of three dye lasers and the Rydberg levels are detected by using the field ionisation method. Three different levels are used as starting levels of the transitions to Rydberg levels. Thus Rydberg levels pertaining to 6snp J=0-2 and 6snf J=2, 3 series as well as some 4f 13 5d 2 6s levels are observed. The new experimental data are analysed by means of the multichannel quantum defect theory. The theoretical analysis consists of a J by J parametrisation of the observed energy spectra. The perturbations of the Rydberg series 6snp J=1, 6snp 3 P 0 and 6snf J=3 by levels of the 4f 13 5d 2 6s configuration are clearly interpreted. The limitations of the MQDT models derived solely from energy data are discussed. A revised value of the first ionisation limit is proposed.