Density of metastable O$_{2}$ (b$^{1}$$\Sigma$$^{+}$) molecules in an O$_{2}$ DC discharge measured by vacuum ultraviolet absorption and optical emission spectroscopy

The number density of O$_{2}$ (b$^{1}\Sigma^{+}$) molecules in the positive column of a dc discharge in pure O$_{2}$ was determined by high-resolution vacuum ultraviolet (VUV) absorption of the (4p$\pi^{1}\Sigma_u^{+}\leftarrow$ b$^{1}\Sigma^{+}$) band at 131.3nm (using the Fourier Transform Spectrometer at the DESIRS beamline at Synchrotron Soleil) and from the absolute intensity of the A-band (b$^{1}\Sigma^{+}$v=0\rightarrow^{3}$$\Sigma_g^{-}$,v=0) emission at 760nm. The 131.3nm absorption band is the only one that is well separated from O$_{2}$ X and a absorption bands. An analysis of the rotational structure of this heavily-perturbed band is presented as well as an $ab-initio$ calculation of the (previously-unknown) transition strength. The densities obtained by the two methods agree within 20\%, confirming the accuracy of the transition strength calculation. The O$_{2}$ b density increases with O$_{2}$ pressure up 2 Torr (reaching about 1 \% of the total gas density), then decreases at higher pressure. At low pressure the b density increases with discharge current, whereas the opposite trend is observed at high pressure. These observations can be explained by O$_{2}$ b quenching by O $^3$P atoms with a rate that increases with temperature.