Positronium in confinement

Positronium in confinement. Porous materials have numerous scientific and technological uses, including catalysis, filtering, and, it turns out, atomic physics. By confining atoms within pores, one can reduce the effects of various mechanisms that broaden spectral lines. Moreover, porous materials can be very efficient for making positronium (Ps). As incident high-energy positrons scatter off the material’s nuclei and electrons, they slow down and can bind with one of the electrons. The resulting Ps atoms subsequently diffuse into the pores, where they get trapped, further thermalize, and can have long lifetimes, upward of 50 ns. David Cassidy, Allen Mills, and colleagues at the University of California, Riverside and San Diego State University have now looked at the spectroscopic consequences of Ps confinement, in particular the effect on the 1S–2P Lyman-α transition. Working with a silica film with pores on the order of 5 nm, they compared the spectral lines of Ps atoms trapped in the film with those of Ps atoms that had escaped the film. The team found that the line is significantly shifted and that confinement indeed narrows the width. Calculations incorporating confinement’s effect on not only a Ps atom’s orbitals but also its center of mass explained the energy shift: Whereas the 1S state tends to keep away from the walls, the 2P state tends to fill cavities smaller than about 5 nm, and the wall interactions raise the state’s energy. (D. B. Cassidy et al., Phys. Rev. Lett. 1079-7114 PRLTAO 106, 023401, 2011.)