Resonance Radiation of Sodium Vapor

(K. W. Wood and Fred Mohler. Resonance Radiation of Sodium Vapor excited by one of the D-lines).

(Physical Review, XI, p. 70 (II), 1918).

In 1905 Wood showed that if a sodium flame is used to illuminate a sphere containing vapors of pure sodium, these latter begin to emit yellow light. Wood called this radiation resonance radiation. In its spectral composition the resonance radiation proved to be identical with the exciting radiation, i.e., its spectrum consisted of two lines—$D_1$ and $D_2$. The question arises: what will the picture be if sodium vapor is illuminated by only one spectral line, $D_1$ or $D_2$; will the resonance spectrum likewise consist of one line, or of two? The answer to this question makes it possible to look more deeply into the very mechanism of radiation. Namely, it makes it possible to decide whether the centers that give the $D_1$ and $D_2$ lines are connected with one another, or whether they are independent of one another.

The paper under review represents a second attempt in this direction. The first attempt¹ did not give definite results. To separate the $D$ lines the authors used a special polarization method, described earlier by Wood². Most of the experiments were carried out with the $D_2$ line, since it is approximately twice as bright as the $D_1$ line. The result was that the resonance spectrum consists of both lines, $D_1$ and $D_2$, under two conditions: 1) when traces of hydrogen are present in the sphere containing the sodium vapor, and 2) at sufficiently high temperatures (270–350°). The authors assert that precisely under these conditions it becomes possible for energy to be transferred from the centers giving the $D_2$ line to the centers giving $D_1$; this transfer, in their opinion, takes place by means of molecular collisions. An analogous transfer of energy is observed in resonance in iodine vapor. When helium is mixed with the vapor of pure iodine, the very character of the spectrum changes: the spectrum consisting of a system of doublets becomes pale, and a band spectrum, previously scarcely perceptible, appears brightly. Here there is a transfer of energy from the systems giving doublets to the systems giving the band spectrum.

E. Shpolsky.

1. R. W. Wood and Dunoyer. Phil. Mag. (6), 27, 1018, 1914. ↩

2. Wood. Phil. Mag. (6), 27, 524, 1918. ↩