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 2 l 2 l ' configurations and for some 2 l 3 l ' - 2 l 2 l ' 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) 2 Dirac-Fock potential and include all possible n = 2 configurations, leading to 4 odd-parity and 6 even-parity states, and some n = 3 configurations. First-order perturbation theory is used to obtain intermediate coupling coefficients. Second-order MBPT is used to determine the matrix elements, which are evaluated for all 11 M1 transitions within 2 l 2 l ' configurations and for 35 M1 transitions between 2 l 3 l ' and 2 l 2 l ' 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.