Series Limit Absorption in Sodium Vapor

Abstract

Transmission curves of sodium vapor for wave-lengths near the principal series limit, 2550 to 2150A.—Reliable ultraviolet photometric measurements were secured by using a steady rotating disk cadmium spark as source and by taking on a single plate, in addition to three absorption spectra, exposures for eight different intensities varied in steps from 2 to 100 by means of oscillating screens, each exposure being for the same time. (The exposure time was reduced to 16 sec. by using an absorption furnace with a column of vapor only 10 cm long.) Measurements of a single plate thus gave the characteristic curves for that plate which were needed to determine from the absorption spectra the actual transmission as a function of wave-length. As the wave-length decreases, the transmission rises to a maximum at 2465A, then decreases regularly to a sharp minimum at the series limit 2414A, then rises to a value at 2200 about equal to that at 2465. The series limit absorption thus decreases rapidly with decreasing wave-length. The eighteen curves obtained for different vapor densities are very similar, all being convex upward. The maximum discontinuity observed is from 34 per cent transmission at 2465A to 20 per cent at 2414A, for vapor of medium density; for greater or less densities of saturated vapor the discontinuity is smaller. Due to the presence of band absorption even beyond the limit, the precise variation of atomic absorption with wave-length cannot as yet be determined from these results.