The continuous absorption of light in alkali-metal vapours

Abstract

The first section of this paper is an account of some experiments on the absorption of light in sodium vapour from the series limit at 2412 angstrom to about 1600 angstrom (an energy difference of 2$\cdot $6 eV). The absorption cross-section at the limit is 11$\cdot $6 $\pm $ 1$\cdot $2 $\times $ 10$^{-20}$ cm$^{2}$. The cross-section decreases giving a minimum of 1$\cdot $3 $\pm $ 0$\cdot $6 $\times $ 10$^{-20}$ cm$^{2}$ at 1900 angstrom and then increases to 1600 angstrom. A theoretical calculation by Seaton based on the dipole-length formula gives good agreement with the experiments at the series limit and also correctly predicts the wave-length for the minimum, but it predicts a significantly lower absorption at the minimum. The experiments described in the first section of the paper conclude a series on the absorption of light in the alkali metals. The second section consists of a general discussion of the results of these experiments and of their relation to theoretical calculations. There is good agreement between theory and experiment except in regard to the magnitude of the absorption at the minimum.