Deexcitation Dynamics of Muonic Atoms Revealed by High-Precision Spectroscopy of Electronic K X Rays.

We observed electronic K x rays emitted from muonic iron atoms using superconducting transition-edge sensor microcalorimeters. The energy resolution of 5.2 eV in FWHM allowed us to observe the asymmetric broad profile of the electronic characteristic K α and K β x rays together with the hypersatellite K h α x rays around 6 keV. This signature reflects the time-dependent screening of the nuclear charge by the negative muon and the L -shell electrons, accompanied by electron side feeding. Assisted by a simulation, these data clearly reveal the electronic K - and L -shell hole production and their temporal evolution on the 10–20 fs scale during the muon cascade process.