The real trapping potential energy U(r) of positronium in vacancy-type hole
s and self-sustaining bubbles in liquids is replaced by a spherical square
well potential of finite energy depth U-m and the scaling procedure propose
d by Yu et al. is applied. Available data for the or tho-positronium lifeti
me in molecular crystals and in liquids are reexamined and a unique relatio
n 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 potent
ial energy U(r) which is compatible with the behaviour of U-m is proposed.
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