The spinning photon and its scattering by molecules

Abstract

The observed fact that a molecule in scattering light may change its state of rotation, itself conveys the suggestion that the photon has an intrinsic spin, the alternative explanation of a conversion of the linear momentum of light into the angular momentum of the molecule during the collision being prima facie highly improbable. The suggestion gains strength when we notice how simply the conception of the spinning photon explains both the selection rules in Raman scattering and the accompanying phenomenon of the reversal of circular polarisation. Nevertheless, to the critically minded, such considerations may fail to carry complete conviction. The reality of photon spin is, however, established1 by quantitative studies of (1) the intensity of the lines in the rotational Raman spectra of gases, (2) the depolarisation of the Rayleigh scattering when separated from the rotational scattering. The Kramers-Heisenberg theory of dispersion as developed by Manneback a fails to give correct results on both of these points, while a modified theory based on the concept of the spinning photon meets with complete success.