Large Angle Elastic Scattering of Ar+ by He

Measurement of accurate differential cross sections for systems of the type ``heavy projectile light target'' are shown to be practical, provided measurements are restricted to large center‐of‐mass scattering angles θ. As an example, cross sections for the system ${\mathrm{Ar}}^{+}-\mathrm{He}$ have been measured from θ=60–180° in the energy range 2.7–9.1 eV. Using two‐parameter repulsive potentials the results are inverted to yield a potential which reproduces the scattering at all energies. Dramatic evidence is given that potentials of the form ${\mathrm{V(r)=K/r}}^3$ are unsatisfactory for fitting the intermolecular potential in the region of 1–10 eV. The potential derived from these experiments is an average of two molecular potentials, since both the X ²Σand A ²Π states of ${\mathrm{Ar}}^{+}-\mathrm{He}$ correlate to ground state atoms. Comparisons are made with theoretical estimates of the two molecular potentials and with the $\mathrm{Ar}-{\mathrm{He}}^{+}$ and $\mathrm{Ar}-\mathrm{He}$ potentials.