An experimental investigation of the radiative lifetime of the metastable $4s^{2}4p^4({}^{3}P)4d$ ${}^{4}D_{7/2}$ level in Kr II shows an unusual situation regarding the importance of an $M2$ depopulation channel. While the first order $M1$ and $E2$ channels are expected to contribute in a dominant way to the decay, the experimental result, obtained using a laser probing technique on a stored ion beam, $\tau =0.57\pm 0.03\mathrm{s}$, is far too short to be due to these channels according to our relativistic multiconfiguration Dirac-Fock calculation. Only if second order contributions to the decay branches (including essentially the $M2$ contribution) are taken into account in the calculations could the unexpected short lifetime be explained.

• Received 29 January 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.063003