Detection of oxygen atoms in a high pressure surfatron microwave discharge via two-photon four-wave mixing

Ground state (P-3(J)) oxygen atoms are detected in a high pressure surface wave discharge driven at 2.45 GHz via two-photon resonant degenerate four-wave mixing spectroscopy (TP-DFWM) in the forward folded BOX configuration. The nonlinear optical signal provides a direct measure of the relative oxygen atom concentration with high spatial resolution inside the discharge up to pressures of 1 bar and without distortions due to linear absorption or saturation effects. The axial distribution of oxygen atom concentration is observed to depend sensitively on total pressure and gas flux. For total gas pressures up to 600 mbar both in case of a mixture of 10% O-2/He and 10% O-2/Ar the concentration of oxygen atoms increases linearly with pressure. At higher pressures an increase with larger slope is observed for 10% O-2/Ar, while the concentration remains constant for 10% O-2/He. This is interpreted by an increase of the three-body recombination rate in O-2/He mixtures.