Experimental and theoretical study of core-excited 3pnd Rydberg series of Mg

We present an experimental and theoretical study of the structure and autoionization dynamics of the ${(3{p}_{{j}_{\text{c}}}n{d}_{j})}_{J=1,3}$ core-excited Rydberg states of Mg. Spectra were recorded experimentally for principal quantum numbers $n$ in the range from 30 to 100 using the isolated-core-excitation technique. Large-scale configuration-interaction calculations combined with the exterior-complex-scaling method were also carried out, which do not rely on the assumptions used in the usual $R$-matrix multichannel-quantum-defect-theory treatment of core-excited Rydberg states. Agreement between theory and experiment is excellent over the entire range of principal quantum numbers studied and allows us to elucidate in detail the structure of the core-excitation spectra. The dominant autoionization mechanisms are identified, and in particular the very fast spin-orbit autoionization of some ${(3{p}_{3/2}n{d}_{j})}_{J}$ states above the $3{p}_{1/2}$ ionization threshold. We discuss the influence of the principal and total-angular-momentum quantum numbers $n$ and $j$ of the Rydberg electron and the total-angular-momentum quantum number $J$ of the atom on the autoionization dynamics. We also identify previously unobserved resonances attributed to members of the ${(3{p}_{{j}_{\text{c}}}n{g}_{j})}_{J}$ series.