Coherent scattering of the hydrogen dimer from a LiF crystal

Abstract

A molecular beam of hydrogen dimers is scattered from a LiF(001) crystal at 48 meV. About 5% of the dimers are found to scatter elastically and appear in well-defined coherent first-order diffraction peaks. Debye–Waller experiments are used to estimate the amount of thermal phonon interaction and, together with elastic scattering, accounts for about 20% of the scattering. The remaining 80% are believed to be fragmented and lost due to exchange of translational-internal energy within the dimer bond in an elastic collision with the surface. Para enriched hydrogen beams are used to show that para dimers, despite having a larger binding energy, are more likely to fragment during surface scattering than ortho dimers. The survival of trimer and higher clusters is at least an order of magnitude less than the dimer so that the diffractive scattering provides a filter to form well-defined pure dimer beams.