Comment on "Cavity induced shift and narrowing of the positronium Lyman-α transition".

We report experiments in which the line shape of the Lyman-alpha (1S-2P) transition was measured for positronium (Ps) atoms both inside and outside a porous silica target. The energy interval ΔE for confined atoms was observed to be larger than that of free Ps by 1.26±0.06  meV. A configuration interaction calculation yields results that are consistent with our ∼5  nm sample, and suggests that ΔE decreases dramatically for larger cavity diameters. The linewidth of the transition, (0.066±0.004)  nm (FWHM), is about half of what one would expect for free Ps at room temperature due to the Dicke line narrowing effect of confinement. Such measurements can be used to determine void sizes in porous films and Ps dynamics therein, and elimination of the Doppler spread of atoms in a porous film could be useful for the efficient excitation of a Ps gas.