Trapping of positronium in a size-dependent spherical square well potential

The real trapping potential energy U(r) of positronium in vacancy-type holes and self-sustaining bubbles in liquids is replaced by a spherical square well potential of finite energy depth U-m and the scaling procedure proposed by Yu et al. is applied. Available data for the or tho-positronium lifetime in molecular crystals and in liquids are reexamined and a unique relation is found between the potential energy depth U-m and the calculated radius R of the trapping site. The general form of a surface-enhanced real potential energy U(r) which is compatible with the behaviour of U-m is proposed. (C) 2000 Elsevier Science Ltd. All rights reserved.