Abstract
In this Letter, the counterintuitive and largely unknown Raman activity of oxygen atoms is evaluated for its capacity to determine absolute densities in gases with significant O-density. The study involves ${\rm CO}_2$ microwave plasma to generate a self-calibrating mixture and establish accurate cross sections for the $^3{\!P_2}{\leftrightarrow ^3}{\!P_1}$ and $^3{\!P_2}{\leftrightarrow ^3}{\!P_0}$ transitions. The approach requires conservation of stoichiometry, confirmed within experimental uncertainty by a 1D fluid model. The measurements yield ${\sigma _{J = 2 \to 1}} = 5.27 \pm _{{\rm sys}:0.53}^{{\rm rand}:0.17} \times {10^{- 31}}\;{{\rm cm}^2}/{\rm sr}$ and ${\sigma _{J = 2 \to 0}} = 2.11 \pm _{{\rm sys}:0.21}^{{\rm rand}:0.06} \times {10^{- 31}}\;{{\rm cm}^2}/{\rm sr}$, and the detection limit is estimated to be $1 \times {10^{15}}\;{{\rm cm}^{- 3}}$ for systems without other scattering species.
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