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The dependence of the radiation intensity from the 2p3d 3 P o;1;0 doubly ionized oxygen (Oiii )l evels on the He/O density ratio has been investigated in optically thin laboratory plasmas. A clear contribution of the astrophysically important Bowen mechanism to the most intensive 313.279 nm Oiii line radiation has been found in the primary Oiii Bowen cascade. We have found that in plasmas with electron temperatures of about 50 000 K and electron densities higher than 10 22 m 3 ,t he 312.163 nm Oiii spectral line also shows a fluorescence tendency caused by the Bowen mechanism. On the basis of the estab- lished dependence of the fluorescence eciency on the He/O density ratio we recommend the I(313.279 nm)/I(311.567 nm) and I(312.163 nm)/I(311.567 nm) Oiii line intensity ratios as a measure of the presence of the helium/oxygen density ratio in astrophysical plasmas. The line intensity ratio related to the 344.405 nm and 342.863 nm Oiii lines (which also belong to the primary cascade in the Bowen fluorescence mechanism and originate in the same energy level) has also been monitored in pure oxygen and helium-oxygen plasmas. We have found a good agreement with the results of previous astrophysical observations and recently published theoretical predictions. We have also found that the I(344.405 nm)/I(342.863 nm) line intensity ratio does not depends on the helium presence in plasmas and thus, it represents a convenient value in plasma spectroscopy.

On the Bowen fluorescence mechanism in the helium–oxygen plasmas