MEASUREMENTS OF POSITRON-MOLECULE DIFFERENTIAL SCATTERING CROSS-SECTIONS

Relative differential cross-sections (DCS's) have been measured for th e quasi-elastic scattering (elastic scattering plus rotational and vib rational excitations) of positrons from CH4, N-2, O-2, CO, CO2, N2O, C 2H2, and SF6 molecules in a crossed-beam experiment. The energy range extends from below the positronium (Ps) formation threshold of each mo lecule to well above the Ps formation threshold and the angular range extends from 30 degrees to 135 degrees. For CH4, N-2, CO, O-2, and SF6 the DCS exhibits structure (a clear minimum and maximum) at low energ ies that tends to wash out as the energy is increased above the Ps for mation threshold. In contrast, for N2O, CO2, and C2H2 we find no prono unced structure below the Ps formation threshold. These comparisons su ggest that the degree of spherical symmetry of the target may be corre lated with the presence of the structure. (C) 1998 Elsevier Science B. V. All rights reserved.