The Rotational Raman Effect in Polyatomic Gases

An account is given of the relation of the theory of the Raman effect developed by Placzek to the theory based upon the Kramers Heisenberg dispersion formula. The equivalence of the two is shown in the general case. An outline is given of the procedure used by Prof. W.V. Houston and the writer in evaluating the integrals resulting from this theory for the intensities of the individual rotation lines for molecules of the symmetrical top class. The results of this calculation are verified experimentally in the case of ammonia gas, using accurate photometric technique. It is shown (1) that the general intensity distribution over the pure rotation band is as predicted; (2) that the effect of the nuclear spin of the hydrogen atoms must be taken into account; (3) that the relative intensity of the R-branch lines and S-branch lines corresponds to a molecule which is nearly planar, in agreement with results from the infra red spectra. The frequencies of the rotation and vibration lines are also discussed. The line at 3219 cm-1 is interpreted as due to the unsymmetrical vibration of the hydrogen nuclei, not previously observed. The rotation structure of a series of the simpler hydrocarbons, as well as carbon dioxide, has been investigated. Methane shows no pure rotation, in agreement with theory. The rotation band of acetylene has been obtained, and gives the value 23.45·10-40 gm cm2 for the moment of inertia. Ethylene shows a structure with slight irregularities which have not been fully accounted for; the moment of inertia is found to be approximately 30.1·10-40 gm cm2. An interpretation is suggested for the results with ethane which yields A 38.2·10-40 gm cm2. The resolution in the case of carbon dioxide is entirely unambiguous, and gives A 70.2·10-40 gm cm2