Mobility of gaseous lons in weak electric fields

Abstract

Kihara's extension of the Chapman-Enskog theory of transport phenomena is used to obtain the second order and third order approximations to the mobility of gaseous ions in a weak electric field as a function of temperature and field strength. In this method it is assumed that there is no charge exchange between ions and molecules, there is no clustering, and that quantum effects can be neglected. The mobility is expressed as a series in ascending powers of the square of the field strength with coefficients which are complicated functions of the temperature, the mass ratio of the ions and molecules, and of the force law between the ions and molecules. The collision integrals which enter into the coefficients have been evaluated by numerical integration for a force law which takes into account the charge-induced dipole, charge-induced quadrupole, and the London dispersion forces, and an inverse twelfth power repulsion potential. In the potential energy function three disposable parameters specify the depth and position of the minimum, and the relative magnitudes of various terms. The results of the present calculation are then used to analyze published experimental data on mobility to obtain the disposable parameters which determine the ion-molecule force law. The agreement between theory and experiment is good except for those cases in which clustering is to be expected.