Isolated core excitation Stark shifts of Yb Rydberg states without autoionization

Isolated core excitation (ICE) provides an efficient means of applying an AC Stark shift to bound Rydberg state of a cold two electron Rydberg atom. ICE is the laser excitation of the second valence, or core, electron, with the Rydberg electron remaining a spectator, resulting in a doubly excited Rydberg state. When applied to low $\ell$ Rydberg states the result is most often rapid autoionization, which seems to preclude the use of ICE as a nondestructive tool. However, a quantitative description of ICE using multi channel quantum defect theory (MQDT) shows that there are zeros in the ICE cross section, due to the overlap integral between the bound and doubly excited rydberg states, which were shown to exist some time ago. The same MQDT model yields an AC Stark shift of the bound Rydberg state proportional to the principal part integral over the ICE cross section. Since the zeros can be close to the peak ICE cross section, the shift can be large where the ICE cross section vanishes. Using bound $6sns$ states of cold Yb atoms in a magneto optical trap we have verified that the ICE cross section to the $6p_{1/2}ns$ states has the expected zeros and observed substantial shifts of the $6sns$ states at these zeros. More generally, we have shown that the shifts are well described by the MQDT model.