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We report the first observation of the annihilation of positronium from the2S state. Positronium (Ps) is excited with a two-photon transition from the 1Sto the 2S state where its lifetime is increased by a factor of eight comparedto the ground state due to the decrease in the overlap of the positron electronwavefunction. The yield of delayed annihilation photons detected as a functionof laser frequency is used as a new method of detecting laser-excited Ps in the2S state. This can be considered the first step towards a new high precisionmeasurement of the 1S-2S Ps line.

Observation of positronium annihilation in the 2S state: towards a new measurement of the 1S-2S transition frequency