The absorption of light by thallium vapour on the short wave-length side of the series limit

Measurements of the absorption of light in thallium vapour from the series limit at 2030 to 1450 angstrom are described. The absorption cross-section at the series limit is 4$\cdot $5 $\pm $ 0$\cdot $8 $\times $ 10$^{-18}$ cm$^{2}$ and the f-number for the continuum over the range of the experiment is 0$\cdot $025 $\pm $ 0$\cdot $005. Auto-ionization lines are observed at 2007, 1610 and 1490 angstrom in agreement with those reported by Beutler & Demeter (1934b). The moderately sharp line at 2007 angstrom ($^{2}$P$_{\frac{1}{2}}^{0}\,\rightarrow \,^{4}$P$_{\frac{3}{2}}$) has an f-number of 0$\cdot $005 $\pm $ 0$\cdot $001 and a lifetime of 18 $\pm $ 5 $\times $ 10$^{-15}$ s, while the broad line at 1610 angstrom ($^{2}$P$_{\frac{1}{2}}^{0}\,\rightarrow \,^{2}$D$_{\frac{3}{2}}$) has an f-number of 0$\cdot $52 $\pm $ 0$\cdot $09 and a lifetime of 2$\cdot $2 $\pm $ 0$\cdot $1 $\times $ 10$^{-15}$ s. The 1490 angstrom line ($^{2}$P$_{\frac{1}{2}}^{0}\,\rightarrow \,^{2}$P$_{\frac{1}{2}}$) is sharp and the f-number is estimated to be of the same order as that of the line 2007 angstrom. These results are discussed in relation to the auto-ionization selection rules and previous experimental and theoretical work is reviewed.