  A small difference between the energy of the para-positronium (p-Ps) and
ortho-positronium (o-Ps) states suggests the possibility of superposition of
p-Ps and o-Ps during the formation of positronium (Ps) from pre-Ps terminating
its migration in the matter in a void ('free volume'). It is shown that such a
superposition decohere in the basis of p-Ps and o-Ps and the decoherence time
is estimated. The time scale of the decoherence estimated here motivates
respective correction in decomposition of the positron annihilation lifetime
spectra. The way of the correction is sketched. The timescale of the
decoherence suggests a need of awareness when experimental data from positron
annihilation techniques are processed. More generally, the superposited state
of Ps should contribute to the evolution theory of positronium in matter.
