Branching ratio for the M1 decay of the 2 (2)S1/2 state in one-electron krypton.

We report a measurement of the lifetime of the 2 [sup 2][ital S][sub 1/2] state in one-electron krypton and a direct determination of the branching ratio for the [ital M]1 decay of this state. The branching-ratio measurement provides a sensitive test of the relativistic corrections to the decay rate. The lifetime measurement yields (36.8[plus minus]1.4) ps and the branching ratio is found to be 0.356(15), both in good agreement with the theoretical values of 37.008 ps and 0.3643, respectively. From these measurements we also deduce the corresponding magnetic-dipole decay rate [ital A][sub [ital M]1]=9.68(55)[times]10[sup 9]s[sup [minus]1] and the two-photon decay rate [ital A][sub 2[ital E]1] =1.750(78)[times]10[sup 10] s[sup [minus]1].