Fig. 5

Spectra of Rydberg states of positronium. a Two-colour two-photon excitation spectrum of Rydberg states of Ps with values of n from 9 up to the ionisation limit. States with values of n>17 are detected when they annihilate after ionisation at a wire grid close to the photoexcitation region (early annihilation). States with values of n<17 pass through this grid and are detected when they annihilate at the walls of the vacuum chamber (late annihilation). b When photoexcitation is carried out in the presence of an electric field of 1.9 kV/cm individual Rydberg-Stark states can be selectively prepared. From [82]