Low energy elastic electron scattering from ethylene

Normalized differential cross sections for elastic electron scattering from ethylene ( C2H4) are reported. The measurements are obtained using a novel version of the relative flow method with helium as the standard gas. Here, the relative flow method is applied without any prior knowledge of the gas kinetic molecular diameters of either the standard gas whose differential cross sections for electron scattering are known or the unknown gas whose differential cross sections for electron scattering need to be determined. Removal of this restriction is made possible by using an aperture- collimating source of the target gas instead of a conventional tube source. Importantly, the present method is accurate and more rapid than past elastic electron scattering experiments using the relative flow method, which employed tube- collimating gas sources. Our present measurements are used to test the reliability of past measurements for ethylene and so to resolve a recent disagreement between theory and experiment concerning the elastic electron scattering differential cross- section data for ethylene at low energy, by providing experimental values independent of any systematic errors that might arise when estimating gas kinetic molecular diameters for large polyatomics. Our new arrangement also extends the relative flow method to be applicable to gases whose gas kinetic molecular diameters may not be available, such as gaseous biomolecules.