Differential scattering of metastable argon on argon: Excitation transfer oscillations

Differential scattering measurements for a beam of metastable argon (Ar*) incident on an argon target have been made in the center-of-mass energy range 10-75 eV. In the angular range beyond the rainbow region, there is a strong series of gerade-ungerade symmetry-exchange oscillations which is the dominant structured component of the scattering. These oscillations are identified with coherent scattering on a single pair of states, the $2_g$, $2_u$ pair correlating to the ${\mathrm{Ar}}^{*}\left({}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}\right)+\mathrm{Ar}\left({}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\right)$ asymptote; an explanation for this choice is presented. Analysis of the structure in terms of a theory developed for ion-atom symmetric charge transfer yields the potential difference between the $2_g$ and $2_u$ potential curves. A time-of-flight pulsing technique has been used to identify two structureless components of the differential cross section, photons from radiative states populated by inelastic transitions, and slow metastables formed through excitation transfer to undeflected target particles.