Excitation of positronium: from the ground state to Rydberg levels

Following bombardment of a mesoporous silica sample by positrons ejected from a two-stage buffer gas trap, ortho-positronium (o-Ps) was emitted into vacuum with an efficiency of around 27%-28%. This ensemble, with a density close to 10(11) m(-3), was then irradiated by multiple solid state-based laser beams up to 50. ns after positron implantation. Tunable over a wide range (230-2200 nm), the lasers have enabled excitation of the o-Ps Lyman-alpha transition, followed by subsequent excitation and, if desired, ionisation to vacuum. Excitation from the 2P state to intermediate states in the principal quantum number, nPs, range 3 <= n(Ps) <= 18 has been achieved with efficiencies epsilon(nps)(ex) greater than or similar to 80% whilst the excitation efficiency of ground state o-Ps to the 2P state of epsilon(2)(ex) similar to 13% is currently limited by a mismatch between the Doppler broadening of the 1S-2P transition and the 225 GHz laser bandwidth at 243 nm.