Making use of hyperspherical coordinates, the feature of the low-lying spectrum of a positronium molecule in a two-dimensional quantum well is investigated. The second bound state of a positronium molecule in a two-dimensional quantum well, with orbital angular momentum L = 1, is reported.
The angular correlation of the two-photon annihilation of positrons annihilating in gases at high pressures demonstrates the two modes of decay---(1) from a bound state, positronium, and (2) from an unbound state---in all gases tested that have low-lying energy states. The increase in the narrow component in argon, plus suitable thermalizing gases, due to the application of a dc magnetic field obeys the theoretical increase due to the mixing of the 11S and 13Sm=0 states. The increase in positronium formation due to the application of a dc electric field is observed. Preliminary evidence for the existence of n=2 states of positronium is seen.