Using the high resolution laser photoelectron attachment method, we demonstrate that the cross section for ${\mathrm{F}}^{-}$ formation due to electron capture by ${\mathrm{F}}_2(X{}^1\Sigma _g{}^{+})$ molecules at very low energies exhibits $p$-wave threshold behavior. This finding confirms the theoretical expectation that low-energy attachment to ${\mathrm{F}}_2$ proceeds through the $\mathrm{F}_2{}^{-}({}^2\Sigma _u{}^{+})$ $p$-wave shape resonance in contrast with previous experimental claims for $s$-wave threshold behavior.

• Received 25 September 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.253202