Cold electron scattering inSF6andC6F6: Bound and virtual state channels

Abstract

Experimental data are presented for the scattering of cold electrons by ${\text{SF}}_{6}$ and ${\mathrm{C}}_{6}{\mathrm{F}}_{6}$, down to energies of a few meV, with an energy resolution varying between 0.95 and $1.5\phantom{\rule{0.3em}{0ex}}\text{meV}$ (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 ${\text{SF}}_{6}$ and virtual state scattering in the case of ${\mathrm{C}}_{6}{\mathrm{F}}_{6}$. Data are combined with known attachment cross sections for ${\text{SF}}_{6}$ 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 $\ensuremath{\pi}$ at $5\phantom{\rule{0.3em}{0ex}}\text{meV}$ collision energy; a similar anomaly was found in electron scattering by ${\text{CCl}}_{4}$. These results should stimulate further theoretical development. By contrast with ${\text{SF}}_{6}$, ${\mathrm{C}}_{6}{\mathrm{F}}_{6}$ 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 ${\text{CO}}_{2}$ 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.