We report the first direct observation of excited state muon transfer in hydrogen/deuterium mixtures by direct measurement of $q_{1s}$, the probability that a $\mu p$ atom, which is initially formed in an excited state, reaches the $1s$ ground state. The dependence of $q_{1s}$ on deuterium concentration $c_d$ was measured for two different densities at cryogenic temperatures using charge coupled devices to detect the muonic x rays. First results based on the analysis of the $K_{\alpha }$ lines of the two isotopes are presented.

• Received 29 September 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.4693