Elastic electron scattering from hydrogen molecules at high-momentum transfer

Abstract

In high-momentum transfer electron scattering experiments the ‘elastic signal’ is separated into different components, depending on the mass of the scatterer due to the recoil effect. Here, we compare the peak positions and shapes obtained from H2, D2 and HD with theory developed for neutron scattering experiments at similar momentum transfer. The hydrogen peak width increases with increasing momentum transfer. The observed width is in line with the vibrational properties of H2. The line shape of the elastic peak is also studied for HD and D2 molecules. The H peak of HD is broader than the H peak of H2, and the D peak of D2 is broader than the D peak of HD. We also investigate elastic scattering at high-momentum transfer of gas mixtures containing hydrogen and either heavy (Xe) or light (He) noble gases. Changing the energy of the incoming beam changes for the Xe/H2 gas mixture the ratio of the Xe to H2 signal in a dramatic way, but for the He/H2 mixture the intensity ratio is constant. The energy dependence of the observed intensity ratio is in both cases accurately described by ‘standard’ differential elastic cross section calculations. Results are discussed in the context of a recent report of anomalies in electron scattering results of H2 under similar experimental conditions and anomalous neutron scattering results of H2, D2 mixtures and HD. An in-depth look at the peak shape of hydrogen reveals deviations from a simple Gaussian line shape which are interpreted to be, at least in part, a consequence of the bonding of the nucleus to a molecule.