Relativistic Magnetic Dipole Emission: Lifetime of the1s2sS13State of Heliumlike Argon

The lifetime of the $1s2s{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ state of the heliumlike atom Ar XVII has been measured by observing the decay in flight of the metastable component of a fast foil-excited beam. The decay occurs predominantly by relativistic magnetic-dipole emission, a process first discussed by Breit and Teller. The result, $\tau \left(2\mathrm{}{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\right)=(172\mathrm{}\pm 30)\times {10}^{-9\mathrm{}}$ sec, is compared with a recent calculation by Drake of the $M1\mathrm{}$ transition probability.