Electron energy dependence of the energy and angular distributions of O− from dissociative ion pair formation in O2

The energy and angular distributions of O− produced from O2 by dissociative ion pair formation have been measured at selected incident electron energies from threshold to 100 eV. The kinetic energy distributions show well-defined maxima near 2.0 and 3.3 eV with appearance potentials, respectively, at 20.0 and 23.0 eV. The angular distributions are peaked in the forward and backward directions relative to electron beam and exhibit nondipolar structure. The shape of the distributions depends on electron energy, becoming more isotropic with increasing energy. An attempt has been made to explain the behavior of the angular distributions in terms of a superposition of final states using a multipole expansion of the differential cross section which includes the effects of higher-order partial waves.