Differential cross sections for elastic scattering of electrons by CH4in the energy range of 3 to 20 eV

Abstract

Differential, integral and momentum transfer cross sections have been determined for the elastic scattering of electrons by CH4. With the help of a crossed electron beam, molecular beam apparatus using the relative flow technique, the ratios of the elastic differential cross sections (DCS) of CH4 to those of He were measured at electron impact energies of 3, 5, 6, 7.5, 9, 10, 15, and 20 eV for scattering angles from 30 to 140 degrees . From the ratios, the absolute DCS were determined by utilising the known DCS of He. Because pure rotational excitation could not be resolved, the elastic DCS are the sum of elastic and pure rotational excitations at room temperature. Integral and momentum transfer cross sections were calculated from the DCS. The CH4 DCS are similar in size and in angular behaviour to those of Ar at impact energies below 7.5 eV, and also agree approximately with a close-coupling calculation for electron scattering from polyatomic molecules. A broad maximum in the integral and momentum transfer cross sections is located at 7.5 eV. The angular distributions at energies up to about 7.5 eV are characteristic of a d-wave type of resonance.