Two experimental setups allow Cassidy et al. to measure and compare the excitation spectra of “free” and confined positronium. (Top plot) The excitation spectrum of positronium that escapes the sample surface is measured using laser light aimed parallel to the sample surface (top right schematic). By rotating the sample 45 degrees (bottom right schematic), it is possible to probe Ps that lies within the silica pores as well as the Ps outside (bottom plot). The spectrum has two components: one shifted to longer wavelength (free Ps, in vacuum) and the other shifted to shorter wavelength (cavity Ps, inside the pores).