Relativistic many-body calculations of magnetic dipole transitions in Be-like ions

Reduced matrix elements and transition rates are calculated for all magnetic dipole (M1) transitions within 2l2l0 con gurations and for some 2l3l0 2l2l0 transitions in Be-like ions with nuclear charges ranging from Z = 4 to 100. Many-body perturbation theory (MBPT), including the Breit interaction, is used to evaluate retarded M1 matrix elements. The calculations start with a (1s) Dirac-Fock potential and include all possible n = 2 con gurations, leading to 4 odd-parity and 6 even-parity states, and some n = 3 con gurations. First-order perturbation theory is used to obtain intermediate coupling coeÆcients. Second-order MBPT is used to determine the matrix elements, which are evaluated for all 11 M1 transitions within 2l2l0 con gurations and for 35 M1 transitions between 2l3l0 and 2l2l0 states. The transition energies used in the calculation of oscillator strengths and transition rates are obtained from second-order MBPT. The importance of negative-energy contributions to M1 transition amplitudes is discussed.