The first measurements of positronium (Ps) formation cross sections (upper and lower limits) for positron scattering by K (1 to 100 eV) and Na (1 to 20 eV) are reported. They indicate significant Ps formation below 20 eV and, along with recent measurements and calculations of total cross sections, provide evidence that coupling effects between Ps formation and other scattering channels may be very important for energies below 10 eV.

• Received 20 December 1993

DOI:https://doi.org/10.1103/PhysRevLett.73.236