The Excitation of the D Lines by the Green Sodium Band

Abstract

IN a recent paper (Phys. Rev., May 1928) Prof. Wood and the present writer have discussed the conditions under which it is possible to excite the D line fluorescence in sodium vapour by light which is free from wave-lengths absorbed by the atom. A band in the green at 5200 A., 50 A. in width, was found to produce a maximum D line fluorescence when a foreign gas at a few millimetres pressure was mixed with the fluorescing vapour. The presence of a foreign gas seemed essential for the production of the D lines in this way, and the most obvious explanation seemed to be that the excited molecule collided with a foreign gas molecule and dissociated into one normal and one excited atom. But the dissociation potential as calculated by Pringsheim (Zeit. f. Phys., 44, 651; 1927) and Loomis (Phys. Rev., 31, 323; 1928) from the analysis of the band spectra is much too high for this process to occur, as has been pointed out, and the alternative explanation was offered that the D lines were emitted when an atom was raised to the 2P levels on collision with an excited molecule. The molecule would then be left with but 0.3 volt energy, which would be distributed as part vibrational and part kinetic. The presence of a foreign gas would prevent rapid diffusion of the vapour to the cooler parts of the resonance tube, and allow the atomic and molecular densities to increase, thus increasing the probability of collision.