X-Ray Yields from μ -Mesonic Atoms

The interesting suggestion has been made that the rapid drop in the yield of mesonic $K$ x rays in the light elements may be associated with the capture of $\ensuremath{\mu}$ mesons into the metastable $2s$ state. The mechanisms for making transitions from the $2s$ to the $1s$ state and from various $p$ states into the $2s$ state have been investigated in detail for Li, Be, and B. It is found that the paradoxical reduction of $K$ x rays remains unexplained. (1) Stark mixing of the mesonic $2s$ and $2p$ states by the electric fields of the atomic electrons allows "mixed" Auger-radiative transitions to the $1s$ state to compete favorably with radiationless transitions. These mixed transitions give a high-energy x ray and a relatively negligible (10-50 ev) electronic excitation and so contribute to the observed ${K}_{\ensuremath{\alpha}}$ yield. (2) Even if the above "mixed" transitions are ignored, there is no mechanism which gets a large fraction of $\ensuremath{\mu}$ mesons into the $2s$ state that at the same time does not violently contradict both theoretical estimates and observed $K$ x-ray yields from light $\ensuremath{\pi}$-mesonic atoms.