Velocity-selected production of 2S3 metastable positronium

Positronium in the 2^3S metastable state exhibits a low electrical polarizability and a long lifetime (1140 ns), making it a promising candidate for interferometry experiments with a neutral matter-antimatter system. In the present work, 2^3S positronium is produced, in the absence of an electric field, via spontaneous radiative decay from the 3^3P level populated with a 205-nm UV laser pulse. Thanks to the short temporal length of the pulse, 1.5 ns full width at half maximum, different velocity populations of a positronium cloud emitted from a nanochanneled positron-positronium converter were selected by delaying the excitation pulse with respect to the production instant. 2^3S positronium atoms with velocity tuned between 7×10^4 ms^−1 and 10×10^4 ms^−1 were thus produced. Depending on the selected velocity, a 2^3S production efficiency ranging from ∼0.8% to ∼1.7%, with respect to the total amount of emitted positronium, was obtained. The observed results give a branching ratio for the 3^3P−2^3S spontaneous decay of (9.7±2.7)%. The present velocity selection technique could allow one to produce an almost monochromatic beam of ∼1×10^3 2^3S atoms with a velocity spread of <10^4 ms−1 and an angular divergence of ∼50 mrad.