We have developed a new kind of recoil ion momentum spectroscopy technique, using a precooled supersonic gas jet target, to determine state selective, scattering angle dependent cross sections for swift ion-atom collisions ( $0.25,\dots ,1\mathrm{}\mathrm{MeV}{\mathrm{He}}^{2+}$ on He), by measuring the transverse and longitudinal momentum of the recoil ion. A longitudinal momentum resolution of $\pm 0.13\mathrm{a}.\mathrm{u}.$ was achieved, about a factor of 30 better than ever obtained before, which enables a clear separation of K and L shell capture. In the transverse direction a resolution corresponding to a projectile scattering angle uncertainty of $\Delta {\vartheta }_P=\pm 1\mu \mathrm{rad}$ was obtained.

• Received 15 July 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.2200