The electron capture and ionization processes in slow collisions of positro
ns with hydrogen atoms are considered within the advanced adiabatic approac
h to atomic collisions. The mass asymmetry of the (p, e(-), e(+)) collision
system is properly taken into account. The calculated positronium formatio
n and ionization cross sections compare favourably with the available exper
imental data in the adiabatic energy region. It is shown that the potential
energy curve of the 2p sigma molecular state supports a quasi-bound level
of the three-particle (p, e(+), e(-)) system with an energy of -0.3 eV and
decay width of 0.15 eV, approximately.