Optical interaction potentials from differential cross section measurements at thermal energies. III. He(21S)+Na

For pt.II see ibid., vol.10, no.4, p.695 (1977). Differential cross sections have been measured for He(2 1 S)+Na at five different kinetic energies between 52 and 207 meV. For the lowest kinetic energy a backward glory is observed. An optical potential is obtained from a simultaneous fit to the data at all energies. A modified Morse potential with a well depth of epsilon =300 meV and a minimum distance of r m =6.3 au gives the best fit. The well depth is in good agreement with data from Penning electron spectroscopy, but the total ionisation cross section is larger than that determined earlier. The metal beam is generated in a recirculating supersonic oven.