Applicability of modified effective-range theory to positron-atom and positron-molecule scattering

We analyze low-energy scattering of positrons on Ar atoms and N-2 molecules using the modified effective-range theory (MERT) developed by O'Malley, [J. Math. Phys. 2, 491 (1961)]. We use the formulation of MERT based on exact solutions of the Schrodinger equation with polarization potential rather than low-energy expansions of phase shifts into momentum series. We show that MERT describes the experimental data well, provided that effective-range expansion is performed both for s- and p-wave scattering, which dominate in the considered regime of positron energies (0.4-2 eV). We estimate the values of the s-wave scattering length and the effective range for e(+)-Ar and e(+)-N-2 collisions.