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 ${\mathrm{ns}}_{1/2}-(n+1\mathrm{})s_{1/2}$ 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 $\left(M_1\right)$ 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.