Oxygen metastable molecule densities in inductively-coupled plasmas in pure O$_{2}$ measured by VUV absorption

Oxygen molecules possess two metastable states (the a$^{1}$$\Delta$$_{g}$ at 0.98 eV and the b$^{1}$$\Sigma$$^{+}$ 1.64 eV). Models have suggested that both can play an important role in the neutral and charged particle kinetics plasmas in pure O$_{2}$ at low pressures. We have used the DESIRS vacuum ultraviolet beamline and Fourier-Transform spectrometer at Synchrotron Soleil to record the absorption spectra of inductively-coupled plasmas (ICP) in O$_{2}$, as a function of gas pressure (5-50 mTorr) and injected RF power. This allowed determination of the absolute line densities of O$_{2}$ in the X, a and b states (using a new calculation of the line strength in the case of b$^{1}$$\Sigma$$^{+}$). At high RF power the a$^{1}$$\Delta$$_{g}$ state density saturates at 10-15 \% of that of the ground state, whereas the b$^{1}$$\Sigma$$^{+}$ state density remains at less than 1\%. The implications of these measurements on the plasma kinetics will be discussed.