The momentum transfer cross section for electron-argon collisions in the range 0-4 eV has bel(n derived from an analysis of recent measurements of DTIIl as a function of EIN at 294 K (Milloy and Crompton 1977a) and Was a function of EIN at 90 and 293 K (Robertson 1977). Modified effective range theory was used in the fitting procedure at low energies. An investigation of the range of validity of this theory indicated that the scattering length and effective range were uniquely determined ,and hence the cross section could be accurately extrapolated to zero energy.It is concluded that for 8 ,;;; O· 1 e V the error in !he cross section is less than ± 6 % and in the range 0·4 ';;;8 (eV) ,;;; 4-0 the error is less than ± 8 %. In the range 0·1 < 8 (eV) < 0·4 the presence of the minimum makes it difficult to determine the errors in the cross section but it is estimated that they are less than -20 %, + 12 %. It is demonstrated that no other reported cross sections are compatible with the experimental results used in the present derivation.

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The scattering of electrons from CO molecules has been studied over the energy range from 1 to 4 eV by analysing drift velocity data for pure CO and CO-inert gas mixtures at 294 K. The validity of using the so-called 'two term approximation' for the velocity distribution function in the solution of the Boltzmann equation to analyse drift velocity data for the pure gas (and thus also for the gas mixtures) has been established. The momentum transfer cross section for CO has been determined in the energy range 1-4 eV, and the measurements of the vibrational cross sections by Ehrhardt et al. (1968) have been renormalized. By using a solution of the Boltzmann equation which avoids the two term approximation, these cross sections have been shown to be consistent with previous measurement.s of the transport parameter D 1.1 fl in pure CO.

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Monte Carlo techniques have been used to study the validity of the two-term spherical harmonics expansion for the distribution function for electrons moving through a gas under the influence of a constant electric field and undergoing elastic collisions with the gas particles. The validity of the expansion was studied by comparing simulated values of the electron drift velocity, lateral diffusion coefficient and mean energy with the values predicted by the conventional theory. From the results of the simulations and from general considerations it is· argued that, if the momentum transfer cross section is related to the electron energy by a power-law dependence, then the two-term approximation is equally valid at all EIN. It is shown that the presence of a minimum in the cross section can render the two-term approximation invalid. However, the conditions under which the approximation is invalid do not correspond to any known electron-atom combination and it is concluded that, if only elastic scattering occurs, the two-term approximation is valid for electron motion in helium, neon and argon.