Large Contributions of Negative Energy States to Forbidden Magnetic-Dipole Transition Amplitudes in Alkali-Metal Atoms

The influence of negative-energy states (NES) on forbidden magnetic-dipole $ns_{1/2}-(n+1)s_{1/2}$ transitions in alkali-metal atoms is investigated. We find that the NES contributions are significant in almost all cases and for rubidium reduce the transition rate by a factor of 8. We tabulate magnetic-dipole (M_1) transition amplitudes for the alkalis. Our M_1 value for cesium, where accurate measurements are available, differs from experiment by 16%. We briefly discuss the feasibility of an experimental test of NES effects.