Absolute elastic differential electron scattering cross sections in the intermediate energy region. I. H2

A new method for measuring absolute electron-impact differential cross sections (DCS) for elastic scattering in gases is described. The technique uses a well-defined crossed-beam scattering geometry in which the relative target densities of an unknown and a secondary standard gas are accurately determined by a calibrated flowmeter and a pressure gauge. The method is applied to the measurement of elastic scattering from H2 (which, in the present work, also includes rotational excitation) relative to elastic scattering from He, the secondary standard. The relative DCS for H2 are then placed on the absolute scale by using recently measured absolute elastic DCS for He. Measurements for H2 are reported at incident electron energies of 3--75 eV, and in the angular range 20degree--135degree. Differential and integral cross sections at each energy are compared with previous measurements and recent theoretical calculations