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Abstract

Positronium in the 2(3)S 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(3)S positronium is produced, in the absence of an electric field, via spontaneous radiative decay from the 3(3)P 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(3)S positronium atoms with velocity tuned between 7 x 10(4) ms(-1) and 10 x 10(4) ms(-1) were thus produced. Depending on the selected velocity, a 2(3)S production efficiency ranging from similar to 0.8% to similar to 1.7%, with respect to the total amount of emitted positronium, was obtained. The observed results give a branching ratio for the 3(3)P-2(3)S spontaneous decay of (9.7 +/- 2.7)%. The present velocity selection technique could allow one to produce an almost monochromatic beam of similar to 1 x 10(3) 2(3)S atoms with a velocity spread of <10(4) ms(-1) and an angular divergence of similar to 50 mrad.