Experimental and MCHF isotope shifts of strongly perturbed levels in Ca I and Sr I

Theoretical and experimental isotope shifts for Ca I and Sr I levels are presented. The isotope shifts of the levels 4s6s ¹S₀, 4p^{2 1}S₀ and 4p^{2 1}D₂ of calcium have been measured between the isotopes 40, 42, 43, 44 and 48 in two-photon transitions from the Ca ground state using two lasers with different wavelengths. Results of ab initio mass and field shift calculations are presented for these levels as well as for a number of other strongly perturbed low-lying levels in calcium and strontium using the multiconfiguration Hartree-Fock method. The 'gradient' and the alternative 'Slater' forms of the specific mass shift operator have been tested; the latter is found to be unreliable with the MCHF expansions which are limited to valence correlation effects.