Measurement of Barium Vapor Density

The density of atomic barium emitted from a small aperture in a uniformly heated stainless‐steel oven has been measured by the method of resonance fluorescence. The range of the oven temperature was 730°–1200°K. At lower densities, the resonance transition ${\text{6s}}^2{}^1{\text{S\hspace{0.17em}→\hspace{0.17em}6sp}}^{1}P$ was used for the density measurements, at higher densities the forbidden transition ${\text{6s}}^{21}{\text{S\hspace{0.17em}→\hspace{0.17em}6p5d}}^{3}P\mathrm{or}{}^{3}D$ . From the measured spatial distribution of the density above the aperture, the equilibrium vapor pressure in the oven was deduced. The latter can be expressed empirically reasonably well by the formula ${\log }_{10}\mathrm{p(}\mathrm{torr}\text{)\hspace{0.17em}=\hspace{0.17em}6.80\hspace{0.17em}−\hspace{0.17em}(8800\hspace{0.17em}/\hspace{0.17em}T)}$ with the temperature $T$ in degrees Kelvin. At temperatures higher than 1000°K the measured densities were progressively slightly lower than this formula predicts. However, the deviation could be due to certain systematic errors in the measurement which become progressively more serious with increasing density. In addition to the barium density, the measurements yielded some information on the atomic transitions employed, generally confirming previous knowledge in this matter.