Observation of large differences in the diffraction of normal- and para-H-2 from LiF(001)

Large differences have been observed in the diffraction intensities of norm al-hydrogen and pure para-hydrogen scattered from the surface of a LiF(001) single crystal. The observed differences are shown to result from a strong coupling between the quadrupole moment of H-2 and the surface electrostati c field, which, in a zeroth order approximation, is averaged out in the cas e of para-hydrogen. Analogous effects are expected in scattering of all hom onuclear diatomic molecules from ionic solids.