The momentum transfer collision cross sections, q my of nitrogen molecules for slow electrons and the mean fractional electron energy loss, X, in these collisions have been determined by the method of microwave interaction in gaseous discharge plasmas. The mean energy of the electron was varied from 0.04 ta0.46 ev. The cross section, q m , was found to be ~17X10~1 6 cm 2 at 0.04 ev. It decreases to ~8X10~1 6 cm 2 at 0.46 ev. The mean fractional energy loss of the electron, X, in the energy range investigated, was found to be ~5.5 in units of 2m/M. These results lend strong support to the theory of rotational excitation of N 2 by slow electrons.
The method of interaction of microwaves in gaseous discharge plasmas has been applied to a determination of the variation of the effective collision cross section for momentum transfer, q m , of helium atoms with electrons having energies from 0.04 to 0.4 ev. Under our experimental conditions with electron densities >10 10 electrons/cc, appropriate charge interactions have been considered. The mean fraction of excess energy loss of slow electrons upon collision, G, was found to be ^2.7X10 -4 , the expected 2m/M value.
The advent of the laser has made it possible to conduct a more complete study of Rayleigh scattering. In the present experiment the angular distribution of light scattered by gas molecules was measured from 45 to 135° from the direction of the incident beam in argon and xenon. The experiment was conducted both in the plane of polarization of the incident laser beam and in the plane perpendicular to it. Absolute values of the differential-scattering cross section are determined for neon, argon, xenon, oxygen, nitrogen, air, carbon dioxide, sulphur hexafluoride, and propane at NTP. The experimental results obtained with noble gases are compared with Rayleigh's theory. It is found that the ruby-laser light beam is not scattered isotropically either by argon or by xenon as predicted by the linear theory, in the plane perpendicular to the plane of polarization. In the plane of polarization the scattered intensity appears to be consistent with a cosinesquared distribution. The measured differential-scattering cross sections for noble gases at 60° are found to be approximately twice as large as the theoretical values.
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