Laser-excitation of the 1S-2S transition in muonium.

We present evidence for the excitation of the two-photon Doppler- free 1S-2S transition in muonium. Thermal muonium atoms were produced in vacuum by a 27.5 MeV/c pulsed muon beam incident on a powdered SiO sub 2 target. Transitions were induced by the light from a frequency- doubled pulsed dye laser and were detected by photoionization of the 2S state. The F=1->1 transition frequency is within 300MHz of the QED prediction.