From Current Literature.

On the Influence of a Magnetic Field on the Polarization of Resonance Radiation.

R. W. Wood and A. Ellett. On the influence of magnetic fields on the polarisation of resonance radiation. Proceedings of the Royal Society A. 103, 396, 1923.

The phenomenon of polarization of the fluorescence of vapors was discovered by Wood as early as 1911. Polarization was found in the fluorescence of vapors of iodine, sodium, potassium, lithium, and others; it was not observed only in the ultraviolet resonance radiation of mercury vapor. In recent years, however, in this case as well polarization was found by Lord Rayleigh. At the end of 1922 Wood, changing the conditions of the experiment, finally obtained strongly polarized resonance radiation of mercury vapor. After the publication of this report (Phil. Mag. December 1922), Wood noticed, however, that the phenomenon is extremely capricious and now appears, now disappears, despite apparently identical experimental conditions. Further investigation showed that the cause of this instability of the results is the different orientation of the magnetic vector of the exciting light beam with respect to the magnetic field of the earth. In the case when the magnetic vector of the light was parallel to the terrestrial field, the polarization almost completely disappeared; it reached 90% when the terrestrial field was compensated by the weak field of a solenoid. The optical investigation was carried out with the aid of a quartz double-refracting prism by photographic means. The dependence of the effect on the field strength, in the case when the latter was parallel to the magnetic vector of the polarized exciting light, is shown in the following table:

It is clear from the table that the whole effect is played out almost completely within the limits of one gauss, and a further increase of the field strength should remain practically without influence.

Having established this fact, the authors proceeded to study the influence of the magnetic field on the polarization of the resonance radiation of sodium vapor. In this case the polarization, even when excited by polarized light, is very small, about 5%; the terrestrial field, as the experiments showed, has no appreciable influence. To destroy the polarization in the case of parallelism of the magnetic field to the magnetic vector of the exciting light (with observation in the direction of the magnetic vector), a field of about 100 gauss was required.

Changing, however, the direction of the magnetic field, the authors discovered a new phenomenon—a sharp increase in the degree of polarization of the resonance light.

In the experiments with sodium, the light source was a discharge tube of the same type as the tube used by Wood for exciting the hydrogen spectrum with a large number of lines of the Balmer series.^1 Metallic sodium was introduced from a side tube into a space with a pressure of about 1 mm, flushed with hydrogen. The part of the tube used—

^1 Cf. Uspekhi F. N.

appearing as the exciting source, was heated by a Bunsen burner. The light from a small section of the discharge tube passed through a diaphragm, a lens, a large Nicol, and a second lens, by means of which it was concentrated at the center of a glass sphere containing a small quantity of metallic sodium. The sphere was heated to approximately 180° by a current of hot air. The polarization was studied with the aid of a quartz wedge, a double-refracting prism, and a Babinet compensator.

Of special interest are the results obtained in the case when the magnetic field was perpendicular to the magnetic vector of the exciting polarized light. Let the electric vector of the latter be vertical, and let the observation be carried out along the magnetic vector, i.e. perpendicular to the direction of propagation of the exciting beam. Different results are obtained depending on what angle the external magnetic field makes with the electric vector of the exciting light \(H\). If, for example, the angle is 90°, then we shall denote the field by \(H_{90}\); if the angle is zero, by \(H_0\), and so on. In the absence of a field, as we have said, the degree of polarization is \(p = 5\%\); at \(H_{90}\) (100 gauss) \(p = 30\%\); at \(H_0\) equally strong polarization is obtained; at \(H_{45}\), \(p = 0\).

Let now the electric vector of the exciting light be horizontal, and let observation be carried out in its direction. In the absence of a field no polarization is observed. If the lines of force of the external magnetic field are perpendicular, in this case, to the line of observation (and to the electric vector), then for any position of the external field relative to the magnetic vector of the exciting polarized light, strong polarization is observed (field 100 gauss), approximately unchanged in magnitude, but at the same time the electric vector of the polarized resonance light rotates together with the field, lagging in phase by 90°. Thus, when the external field is vertical, the electric vector of the polarized resonance light is horizontal.

After the indicated rotation had been discovered for the horizontally positioned electric vector of the exciting light, the authors were able to establish a similar rotation also for the vertical position of the electric vector of the exciting light.

In the present case, however, the matter is complicated by the fact that, as we saw above, the degree of polarization changes when the external magnetic field is rotated, becoming zero at \(H_{45}\) and acquiring maximum values at \(H_0\) and \(H_{90}\).

The authors were unable, for sodium, to obtain polarization greater than 25–30% under any conditions. In connection with this there arose the supposition that, in mercury vapor, the corresponding effects of strengthening and weakening of polarization and of rotation of the electric vector of the resonance light could be masked by the very strong normal polarization of mercury. The authors again returned to experiments with mercury and, with the horizontal position of the exciting electric vector, found the appearance of polarization and a change in the intensity of the resonance light in the case when the magnetic lines of force of the external field were perpendicular to the line of observation, i.e. to the electric vector of the exciting light. Just as in the above-described experiments with mercury, these experiments were performed with an external field of strength 1 gauss, i.e. of the order of magnitude of the Earth’s field. It was found that in this case too the electric vector of the resonance light rotates together with the external field, just as in the experiments with sodium.

In connection with the formal explanation of the remarkable phenomena set forth, proposed by C. Darwin (Ch. Darwin), the authors also carried out observations of the longitudinal effect, measuring the polarization of the resonance light in the direction of propagation of the exciting beam. These experiments are extremely difficult, but nevertheless success was achieved in this case as well. The results of the experiments are reported briefly in an addendum to the article. The following experiment is extremely important. In the case when the external field was parallel to the magnetic vector of the exciting

light, and the electric vector of the latter vertical, then, as we saw in the transverse observation, all traces of polarization were absent. In longitudinal observation, in the same case, strong polarization was found. This exhausts the experimental data reported in the paper being reviewed.

The article further reports the following formal interpretation of the phenomenon, proposed by Ch. Darwin. It is assumed that under the action of an external magnetic field the radiating circular, plane orbits of the electrons of the corresponding atoms are set perpendicular to the field. Let us examine from this point of view the first experiment, when the directions of observation, of the magnetic vector of the exciting light, and of the external field are parallel. According to the hypothesis, the circular plane orbits will be set perpendicular to the field, and each of them, under the influence of the vertical exciting vector, will send to the observer (transverse observation) a wave polarized circularly. Right- and left-handed rotations are then equally probable; as a result the observer, in agreement with experiment, will observe no polarization. Conversely, in longitudinal observation in the same case the observer will see the same oriented orbits “in profile” and will receive from them (both from right- and left-rotating ones) linearly polarized light, again in agreement with experiment.

When the electric exciting vector is vertical and the external magnetic field is rotated in a vertical plane perpendicular to the horizontal line of observation, coinciding with the direction of the magnetic exciting vector, then, on the basis of the hypothesis just stated, in agreement with experiment, the following results are obtained. For \(H_{90}\) the orbits are perpendicular to the external field and parallel to the electric exciting vector; the observer sees them vertically “in profile” and consequently obtains vertical, linear vibrations; for \(H_0\) horizontal linear vibrations are observed analogously. For \(H_{45}\) the orbits are visible to the observer “in profile” at an angle of \(45^\circ\) to the horizon, but the vertical electric vector excites in them horizontal and vertical vibrations of equal amplitude, i.e. for the observer the light appears unpolarized.

If the electric exciting vector is horizontal and observation is made in its direction, while the external field is rotated in a vertical plane, then, as is not difficult to see, the orbits of the atoms, according to the hypothesis, will all the time be parallel to the electric vector and perpendicular to the field; the observer will see the light linearly polarized all the time to the same degree, but the electric vector of the resonance light will rotate as the field rotates, in agreement with experiment.

In connection with Darwin’s theory, the experiments of Wood and Ellett must be compared with the experiments of Stern and Gerlach \(^{1}\), published last year. The discreteness of the orientation of atoms in a magnetic field is revealed in the new experiments with extraordinary clarity. As is known, the named discreteness follows from the postulates of Bohr’s theory. Of course, many aspects of the phenomenon are still not explained by the theory set forth. In particular, it still remains to take into account the quantitative side and the connection of polarization in a magnetic field with the normal polarization of fluorescence.

S. Vavilov.

Ionization and Excitation Potentials of Nitrogen.

E. Brandt, Über die Ionisierungs- und Anregungs-Spannung des Stickstoffs. Zsch. f. Ph. 39, VIII. 32, 1922.

Brandt’s work was carried out by the usual method. The accelerating potentials were brought up to 100 V. Corrections for the contact potential difference and so forth were not calculated, but were introduced, as is usually done, by means of measurements on a standard gas, which

\(^{1}\) Cf. p. 301.