Positronlum formation by positron impact on hydrogen atoms

The sensitivity of s and p wave positron-hydrogen atom elastic-phase shifts, to the presence or absence of various multipole components of the adiabatic polarisation potential is discussed. It is concluded that higher multipoles than the quadrupole component are unimportant and support is given to the view of Drachman (1965) that in s-wave scattering the monopole component of the adiabatic potential should be suppressed. Then the cross-section for positronium formation by positron impact on hydrogen is calculated in the two-state approximation, taking account of the polarisation of the hydrogen and positronium atoms in each channel. It is found that the polarisation forces dominate the cross-section near the threshold for positronium formation and evidence is presented for the existence of a positronium-proton bound state that gives rise to a resonance in elastic e(^+) –H scattering just below the formation threshold.