Effective radius of ground- and excited-state positronium in collisions with hard walls

We determine effective collisional radii of positronium (Ps) by considering Ps states in hard-wall spherical cavities. B-spline basis sets of electron and positron states inside the cavity are used to construct the states of Ps. Accurate Ps energy eigenvalues are obtained by extrapolation with respect to the numbers of partial waves and radial states included in the bases. Comparison of the extrapolated energies with those of a point-like particle provides values of the effective radius ρnl of Ps(nl) in collisions with a hard wall. We show that, for1s, 2s, and 2p states of Ps, the effective radius decreases with the increasing Ps center-of-mass momentum and find ρ1s = 1.65 a.u., ρ2s = 7.00 a.u., and ρ2p = 5.35 a.u. in the zero-momentum limit.