Anomalous neutron Compton scattering from molecular hydrogen

Abstract

Application of neutron Compton scattering, which operates in the attosecond time scale, to (a) the equimolar ${\mathrm{H}}_{2}\ensuremath{-}{\mathrm{D}}_{2}$ mixture and (b) the mixed-isotope system HD (liquids, both at 20 K), reveals a strong anomalous shortfall (about 30%) of the ratio $R={\ensuremath{\sigma}}_{H}∕{\ensuremath{\sigma}}_{D}$ of H and D cross sections. This striking effect is similar to that observed in liquid ${\mathrm{H}}_{2}\mathrm{O}\ensuremath{-}{\mathrm{D}}_{2}\mathrm{O}$ mixtures [C. A. Chatzidimitriou-Dreismann et al., Phys. Rev. Lett. 79, 2839 (1997)]. Crucially, the shortfall of $R$ is equal in both samples (a) and (b). This result demonstrates that quantum exchange correlations of identical nuclei play no significant role in this effect, thus refuting corresponding theoretical models claiming its interpretation. In contrast, our findings are consistent with alternative theoretical models, in which attosecond dynamics of electronic degrees of freedom (via violation of the Born-Oppenheimer approximation) is considered to participate significantly in the dynamics of an elementary neutron-proton (-deuteron) scattering process. Possible implications for attosecond chemical dynamics, e.g., the onset of bond breaking, are mentioned.