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The total diffusion cross section is calculated for low-energy impact of electrons upon polar molecules in an approximation which treats the molecules as fixed rotators. The amplitude is evaluated and the differential cross section corresponding to fixed orientation of the molecule is then averaged over the Maxwell distribution. It is shown how this simple approach follows from an adiabatic approximation to the excitation amplitudes. The results show excellent agreement with available data.

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where we have made the substitution v=| (zZe 2 /E)u and are using the dimensionless constants ^=[2£/( 2 Ze 2 )>=tan(i$ 0 ), (15) p=a8(hc/e 2 )z-A Z-2 (E/hcy = 2.68X10 39 as-4 Z-2 E Me v 3 , (16) where a is in cm 3 .Hardmeier 7 has given a formula for 8 at small scattering angles and has evaluated it explicitly for large scattering angles for 0= 1, 2, 4, 6. We have evaluated 8If we evaluate this as a power series in /3, we obtain for backward scattering • -(0) = 1-0.2290-O.O3540 8 .For /3=0.2, this gives us a 4.7 percent deviation from pure Coulomb scattering in the backward direction. For the deuteron z=l and from reference 1, we have 8 8 The value of a is taken from Eqs. (13) and (21) of reference 1 by noting that for the scattering problem, we must average over T HE analysis of exchange scattering was originally made by Oppenheimer, 1 and an entirely different treatment of this problem which has become standard was given by Mott and Massey. 2 The reason for the superiority of the latter method is that the general matrix element for the exchange scattered amplitude is identified, while Oppenheimer s solution is an approximate one from the outset so that the possibility for improved estimates is automatically ruled out. HardmeierV results for small scattering angles.In the case of head on collisions, 0, we have a=0.56X10-39 cm 3 . Then, applying Eqs. (16) and (18) to some examples where the nuclear effects should be comparatively small 2 -9 and hence our treatment be valid, we find for 8-Mev deuterons scattered by 83Bi where /3=0.112 a deviation of 2.7 percent in the backward direction, and for 10-Mev deuterons scattered by 92U where 0=0.177 a deviation of 4.2 percent in the backward direction. the magnetic quantum number so that (a)to = ass-\r2aDD~ass = 0.56X10-39 cm 3 . 9 D. C. Peaslee, Phys. Rev. 74, 1001 (1948); C. J. Mullin and E. Guth, Phys. Rev. 82, 141 (1951). assumption that the usual stationary state solution from which the direct scattered amplitude is derived has the asymptotic formwhere ri and r 2 refer to the primary and hydrogenic electrons, respectively. (This labeling will prevail throughout the present paper.) The

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A detailed investigation of the theory describing the electromagnetic perturbation of a weakly ionized plasma such as the atmospheric D region demonstrates the existence of a generalized momentum transfer cross section for the drift velocity which reduces to the ordinary momentum transfer cross section for elastic processes. However, the drift velocity in the case of air is still determined only by the elastic momentum transfer cross section, since the inelastic differential cross sections are small. It is also shown how inelastic collisions of electrons determine the electron cooling law, and a new result describing the way the average electron loses energy is established. It is a generalization of the approximate description always employed in cross‐modulation experiments. Apart from the assumption of a Maxwellian velocity distribution for the electrons this new result is exact, and it is shown how the mean energy loss per collision is related to the energy‐dependent excitation cross section. In the case of nitrogen molecules the effective energy relaxation time, (Gv)−1, varies as the square root of the electron temperature. This result is applied to show how a dielectric discontinuity or electromagnetic mirror may be induced in the D layer by ground‐based high‐powered transmitters.