Abstract
The 1s-2s energy splitting of muonium is measured by Doppler-free two-photon spectroscopy. Increased accuracy is expected compared to previous experiments and spectroscopy of this transition will improve the muon mass value. The 1s-2s transition is detected by the photoionization of the 2s state by a third photon from the same laser field. The slow muon set free in the ionization process is accelerated to 2 keV and guided through a momentum and energy selective path onto a microchannel plate particle detector (MCP). Background due to scattered photons and other ionized particles can be reduced by requiring that the MCP count falls into a 100 nsec wide window centered at the expected time of flight for muons.