First observation of resonant positronium scattering from carbon dioxide

Carbon-dioxide was chosen as a target for Ps projectiles because the electron scattering probability displays a significant structure at a velocity, and of a width, accessible with the present Ps beam. An analysis similar to that of Brawley et al suggests that the structure corresponds to the well-known ²Π_u shape-resonance which occurs for electrons at an incident velocity of 0.5 a.u. (1a.u ~2000 km/s). Whilst the present work strengthens the finding of Brawley et al regarding the correspondence in the overall scattering probabilities of Ps and equivelocity electron projectiles, the basic physical mechanism responsible for the phenomenon by which Ps scatters like a quasi-free electron remains uncertain. Further investigations are eagerly anticipated.