Crossed-second-order effects in the isotope shift for170–166Er and168–166Er in the configuration 4f 12 6s 2

High resolution interference and intermodulated optogalvanic saturation spectroscopy has been applied for isotope shift (IS) studies in the ground-configuration 4f ¹² 6s ² of ErI. For the isotope pairs^170–166Er and^168–166Er the results confirm a strongJ- and term dependence of the IS caused through crossed-second-order (CSO) effects. We performed a parametric analysis to evaluate the CSO-parametersz _4f for the interpretation of theJ-dependence andT(³ H),T(³ F),T(¹ G) to describe the term dependence. The following parameter values (in MHz) were determined, for^170–166Er:z _4f =40.1(0.6),T(³ F)=?153(5),T(³ H)=?220(6), andT(¹ G)=?79(4); for^168–166Er:z _4f =20.1(0.7),T(³ F)=?77(6),T(³ H)=?111(7), andT(¹ G) =?43(5). The normalization of these parameters with the corresponding nuclear parameters λ for both isotope pairs leads to almost identical parameter values indicating a dominant influence of the field shift effect in second-order for thez _4f parameter and in third-order for the parametersT(³ F),T(³ H), andT(¹ G).