Anomalous Quasielastic Electron Scattering from Single H2, D2, and HD Molecules at Large Momentum Transfer: Indications of Nuclear Spin Effects

Quasielectron electron scattering from gaseous H2, D2, a 50:50 mixture of H2 and D2, and HD is investigated with 2.25 keV impact energy and a momentum transfer variant Planck's over 2piq of 19.7 a.u. The energy transfer is less than the dissociation energy. The spectral positions of the H and D recoil peaks agree with Rutherford scattering theory. Surprisingly, in the spectrum of the 50:50 H2-D2 mixture, the integrated intensity of the H peak is 31%+/-4% lower (as compared to that of D) than predicted by Rutherford scattering, despite equal screening of nuclear charges by the electrons. In contrast, the ratio of scattering intensities from H and D in HD agrees with the predictions of Rutherford scattering. Comparison is made with neutron Compton scattering results from the same systems, but at higher energy transfers causing bond breaking. Possible theoretical explanations are outlined.