The Raman Effect in Ammonia and Some Other Gases

Abstract

The Raman rotation spectra of polyatomic molecules can be interpreted in terms of the polarizabilities of the molecule, without explicit knowledge of the electronic states. This point of view makes possible the calculation of the relative intensities of all rotation lines to be expected. The spectrum of gaseous ammonia shows the calculated intensity relations when a suitable shape is assumed for the molecule. The measurements are good enough to exclude the possibility of the case with three equal moments of inertia, and tend to support the flat pyramid inferred from infrared data. The larger moment of inertia is 2.79·${10}^{-40\mathrm{}}$ g ${\mathrm{cm}}^2$. Methane shows no pure rotation band in spite of the presence of a vibration-rotation band. Ethane shows a rotation band which has not been resolved and an apparent vibration-rotation band which has not been interpreted. Ethylene gives a pure rotation band which indicates a moment of inertia of about 30.0·${10}^{-40\mathrm{}}$ g ${\mathrm{cm}}^2$, while acetylene shows a moment of inertia of 23.52·${10}^{-40\mathrm{}}$ g ${\mathrm{cm}}^2$ with alternating intensities as in hydrogen.