Dissociative electron attachment to pulsed supersonic O₂ jet : Violation of Sigma⁺ rightleftharpoons Sigma⁻ selection rule and dependence on carrier gas proportion

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.