Optical methods for studying hertzian resonances.

During my first year of studies at the École Normale Supérieure in Paris, our teacher, Eugène Bloch, introduced us to quantum physics, which at that time was little taught in France. Like he, I was of Alsatian extraction and knew German. He strongly advised me to read Sommerfeld’s admirable book Atombau und Spektrallinien . In the course of this reading, I became particularly interested in the application of the principle of conservation of angular momentum during interactions between electromagnetic radiation and atoms, an application which had led A. Rubinowicz to the interpretation of the selection rules for the azimuthal quantum number and polarization in the Zeeman effect. In the hypothesis of light quanta, this principle attributed to the photons a momentum + K or -6 according to whether the light was polarized circularly to the right (σ ) or to the left (σ), natural light being a mixture of the two kinds of photons. In 1931, W. Hanle and R. Bär independently discovered an interesting characteristic of Raman spectra. The study of the polarization of Raman lines at right angles to the incident beam made it possible to classify the Raman lines of a molecule into two categories: « depolarized » lines with a depolarization factor of 6/7 and « polarized » lines, whose polarization was generally appreciable. Placzek’s theory had attributed the former to periodic molecular motions which modify the symmetry elements the molecule possesses at rest, among which are included rotational Raman lines, and the latter to totally symmetric vibrations which maintain the symmetry elements of the molecule at rest. Hanle and Bär illuminated the medium with circularly polarized incident light and observed that, under these conditions, the Raman lines scattered longitudinally had the same circular polarization as the incident light in the case of totally symmetric vibrations, but that the direction of circular polarization was reversed for lines not totally symmetrical. In a note, I pointed out that for rotational lines this curious result was an immediate consequence of the principle of conservation of angular momentum applied to light scattering. At about the same time, Jean Cabannes explained the Hanle and Bär result by the classical polarizability theory, but these publications had been preceded