Experimental investigations of the ion velocity distribution function (IVDF) are of great importance to various kinds of application: plasma nanotechnology, surface treatment, nanoelectronics, etching processes et al. In this paper, we propose a new probe method for diagnostics of anisotropic IVDF. The possibilities of the method have been demonstrated in arbitrary electric field plasma under conditions when an ion acquires a velocity on its mean free path comparable with the average thermal velocity of atoms. The energy and angular dependency of seven IVDF Legendre components for He+ in He and Ar+ in Ar have been measured and polar diagrams of the ion motion have been plotted.

In order to verify the reliability and accuracy of the method the analytic solution of the kinetic Boltzmann equation for ions in plasma of their own gas has been found. Conditions under which resonant charge exchange is the dominant process and the ambipolar field is arbitrary have been considered.

For the ambipolar field the dependence of resonant charge cross-section on the relative velocity has been taken into account. It is shown that the form of the IVDF is significantly different from the Maxwellian distribution and defined by two parameters.

The results of theoretical and experimental data taking into account the instrumental function of the probe method are in good agreement. Calculations of the drift velocity of Hg+ ions in Hg, He+ in He, Ar+ in Ar, and mobility of N-2(+) in N-2 are well matched with known experimental data in wide range of electric field values.