Continuous Absorption of Light in Sodium Vapour

Abstract

THE continuous absorption of light in sodium vapour, corresponding to the photo-ionization of the atom, has been measured with an apparatus generally similar to that used by Ditchburn, Tunstead and Yates¹ for potassium vapour. Under the conditions of the present experiment, the absorption due to molecules is small compared with that due to atoms at the series-limit (2411 A.). After correcting for the molecular absorption, the atomic absorption coefficient at the series-limit (σ_l) is found to be 1.16 × 10⁻¹⁹ cm.². This value is subject to an error of ± 10 percent, about half of which is due to uncertainty in the vapour pressure². The variation of absorption coefficient with wave-length is shown in the accompanying graph, σ_λ/σ_l being plotted as a function of λ down to 2150 A. The accuracy of photometry decreases rapidly as the wave-length decreases beyond this point, and we can only state that the absorption lies between the two dotted curves. It appears probable that there is a minimum in the absorption curve at, or near, 2150 A.; but this cannot be stated with certainty.