Elastic electron scattering from hydrogen molecules at high-momentum transfer

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 H ₂, 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.