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

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Abstract. At the Paul Scherrer Institute (PSI, Villigen, Switzerland) a new high-intensity muon beam line with momentum p < 40 MeV/c is currently being commissioned. The beam line is especially designed to serve the needs of the low-energy, polarized positive muon source (LE-" + ) and LE-" SR spectrometer at PSI. The beam line replaces the existing "E4 muon decay channel. A large acceptance is accomplished by installing two solenoidal magnetic lenses close to the muon production target E that is hit by the 590-MeV PSI proton beam. The muons are then transported by standard large aperture quadrupoles and bending magnets to the experiment. Several slit systems and an electrostatic separator allow the control of beam shape, momentum spread, and to reduce the background due to beam positrons or electrons. Particle intensities of up to 3.5 Â 10 8 " + /s and 10 7 " j /s are expected at 28 MeV/c beam momentum and 1.8 mA proton beam current. This will translate into a LE-" + rate of 7,000/s being available at the LE-"SR spectrometer, thus achieving " + fluxes, that are comparable to standard "SR facilities.Keywords: low-energy muons, muon beam.At present the Paul Scherrer Institute (PSI, Villigen, Switzerland) is operating the most powerful proton accelerator at medium energies (590 MeV) which is used for the generation of high-intensity, low-momentum muon beams at 100% duty cycle. These particles are used in stopped experiments which cover a large spectrum of physical problems, from fundamental particle physics experiments to condensed and soft matter investigations [1]. For the condensed and soft matter applications a large research program for muon-spin-rotation ("SR) [2] has been established over the last 20 years. A new important extension of the " SR program has been achieved by the recent development of a low-energy " + beam (LE-" + , FLEM_) with "SR spectrometer (LE-"SR) [3][4][5][6]. Whereas standard muon beams are produced with energies of order MeV the LE-" + beam has a tunable energy between 0.5 and 30 keV, thus allowing the controlled implantation at depths on the nanometer scale below the surface of a sample. The beam is generated by the moderation of a surface muon beam (momentum p < 29.8 MeV) in an appropriate solid rare gas film. Up to now LE-"SR has been intensity limited. In order to use the full capacity of this new technique an increase of