VERY HIGH-RESOLUTION STUDY OF HIGH RYDBERG LEVELS OF THE CONFIGURATIONS 4F146SND OF YB-I

High Rydberg levels of ytterbium were studied in a beam of metastable atoms. Levels of the series 4f**1**4 6snd were populated from the metastable 4f**1**4 6s 6p **3P//0 level by means of the U. V. light of a frequency-doubled single-mode dye laser and detected using the field ionization technique. The critical ionization fields F//c of the levels 4f**1**4 6snd **3D//1 (24 less than equivalent to n less than equivalent to 57) were measured and found to obey the saddle point law (F//c equals (16 n***4)** minus **1). The hyperfine structures of the odd isotopes were investigated. They are very peculiar due to the fact that the hyperfine interaction is of the same order of magnitude as (or even for the highest n values, much more important than) the spin-orbit and electrostatic interactions. A theoretical account of the observed structures using the Slater-Condon parametric method is given. The relative influence of the excited (nd) and of the non-excited (6s) optical electron could thus be studied. Furthermore the influence of a valence level perturbing only the odd isotopes components for n equals 26 was demonstrated.