On the Bowen fluorescence mechanism in the helium-oxygen plasmas

The dependence of the radiation intensity from the 2p3d 3P ...doubly ionized oxygen ( O III) levels 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 O III line radiation has been found in the primary O IIIBowen cascade. We have found that in plasmas with electron temperatures of about 50 000 K and electron densities higher than 10 22 m -3, the 312.163 nm O III spectral line also shows a fluorescence tendency caused by the Bowen mechanism. On the basis of the established dependence of the fluorescence efficiency 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) O III 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 O III 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.