Microwave spectroscopy of positronium atoms in free space

We report measurements of the positronium (Ps) $2^{3}S_{1}\ensuremath{\rightarrow}2^{3}P_{2}$ interval in which atoms traveling free space were irradiated with microwave radiation generated using a horn antenna. Previous this transition, performed waveguides, exhibited asymmetric and shifted line shapes. In free-space we here, much smaller line-shape asymmetry was observed, but large frequency shifts that varied orientation Our observations are supported by simulations demonstrate variations field distribution can perturb measured shapes give rise to apparent without necessarily causing asymmetries; effect explain previous an discrepancy predictions from quantum electrodynamics ob- served [L. Gurung, T. J. Babij, S. D. Hogan, B. Cassidy, Phys. Rev. Lett. 125, 073002 (2020)].