Laboratoire Collisions Agrégats Réactivité - UMR5589

Experimental data are presented for the scattering of cold electrons by SF6 and C6F6, down to energies of a few meV, with an energy resolution varying between 0.95 and 1.5 meV (full width at half maximum) in the electron beam. The measured scattering cross sections rise rapidly at low energy and represent effects of bound-state attachment and scattering in the case of SF6 and virtual state scattering in the case of C6F6. Data are combined with known attachment cross sections for SF6 to yield elastic-scattering cross sections, from which phase shifts for elastic scattering are derived. The s-wave phase shift rises as energy falls as theory requires for a potential supporting a nonadiabatic bound state and possessing a large positive s-wave scattering length. The behavior at very low energy of the s-wave phase shift is, however, anomalous, the phase shift remaining far from an odd multiple of pi at 5 meV collision energy ; a similar anomaly was found in electron scattering by CCl4. These results should stimulate further theoretical development. By contrast with SF6, C6F6 offers an example of a system with a large negative scattering length. Data are analyzed to reveal a strong virtual state effect at low collision energy, similar to CO2 but with a considerably greater cross section. The s-wave phase shift falls as energy drops and both this and the p-wave phase shift follow the precepts of effective range theory. It is proposed that the strong virtual state effect may act as a gateway to attachment through collisional or radiative stabilization of long-lived anions, with implications for both man-made and natural plasmas.

doi:10.1103/PhysRevA.69.052716