Continuous Absorption of Light in Sodium Vapour

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 Yates 1 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 pressure 2 . 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.