Elastic scattering of fast electrons by highly polar molecules

In this paper, peculiarities are considered of the angular dependence of the intensity of elastic scattering of fast electrons by dipolar LiH molecules, calculated in the first Born approximation using Ransil's wavefunction. The molecular component of the scattering intensity is determined. It is shown that the contribution of chemical bond effects to the intensity of electron scattering by molecules featuring highly polar bonds includes the part which is formally analogous to the structure dependent part of the intensity. Numerical integration of the Fourier transform of the molecular electron density was utilized to calculate the intensity of elastic electron scattering by LiH, LiF and Li2O molecules, using Hartree-Fock molecular wavefunctions. The major portion of the contribution of chemical bond effects to the intensity of electron scattering by the highly polar (ionic) LiF and Li2O molecules is made up by the 'ionic' contribution due to a redistribution of electrons between atoms making up an ionic molecule. A model is suggested of independent ions in a molecule, which correctly describes the 'ionic' contribution of chemical bond effects to the intensity of electron scattering by highly polar molecules and is suitable for practical utilization for interpreting electron-difffraction patterns. (Auth.)