Muonium spectroscopy in ZnSe: Metastability and conversion

High-precision spectroscopic information is obtained on the muonium states in ZnSe by high-field transverse μSR measurements. At low temperatures, two muonium states MuI and MuII are observed with isotropic hyperfine parameters of AI=3283.63±0.510.3em0exMHz and AII=3454.26±0.020.3em0exMHz (74% and 77% of the vacuum value, respectively). State I is thermally unstable and converts to state II at approximately 400.3em0exK. State II is stable up to 3000.3em0exK, at least. We assign MuII to the cation interstitial tetrahedral site and discuss the possibility that MuI may correspond either to muonium at the same site but in the unrelaxed lattice or to the anion interstitial tetrahedral site. The temperature dependence of the hyperfine interaction was fitted with a local vibrational model giving an oscillator energy of approximately 80.3em0exmeV. The amplitudes and the depolarization rates are measured over the entire temperature range and are discussed in the text.