Dissociative electron attachment to pulsed supersonic O${}_2$ jet : Violation of ${\Sigma }^{+}\rightleftharpoons {\Sigma }^{-}$ selection rule and dependence on carrier gas proportion

Abstract

The formation of $O^{-}$ and $O_2^{-}$ ions via dissociative electron attachment to a pulsed supersonic jet of $O_2$ molecules containing weakly bound small van der Waals clusters seeded in a beam of argon is reported. The energy dependence of the $O^{-}$ and $O_2^{-}$ yield exhibits three peaks near 7, 11 and 16 eV incident electron energies. The 7 eV peak arises from the ${}^2{\Pi }_u$ state of $O_2^{-}$ whereas, the 11 and 16 eV peaks are ascribed to two distinct resonance states:${}^2{\Sigma }_g^{+}$ and ${}^2{\Sigma }_u^{+}$ states of $O_2^{-}$, respectively, via a violation of the ${\Sigma }^{+}\rightleftharpoons {\Sigma }^{-}$ selection rule. The dependence of the cross-section of these two new peaks at $\sim$11 and $\sim$16 eV on the proportion of the carrier gas is also investigated and an optimum proportion has been observed experimentally which gives the lowest temperature of 14.86 K and highest Mach number of 72.31 for the pulsed supersonic jet.