Experimental and theoretical isotope shifts of strontium levels subject to very strong configuration mixing

The isotope shifts of the levels 5p 2 1 D 2 , 1 S 0 and 5sns 1 S 0 (n=7, 8) of strontium have been measured between the isotopes 84, 86, 87 and 88 in two-photon transitions from the ground state using two lasers with different wavelengths. These levels occur below the beginning of the Rydberg series, and are known to exhibit very strong configuration mixing. Mass and field isotope shifts are calculated ab initio by means of the MCHF method for these levels and 5s4d 1 D 2 , 5s6s 1 S 0 . The qualitative agreement between theory and experiment confirms the necessity of taking into account the mixing (up to 50%) with both the 5p 2 and 4d 2 perturbers in the wavefunction expansions.