Out-of-plane orientation of multilayer N₂ films adsorbed on Grafoil at 20 K

A study of the out-of-plane tilt angle of adsorbed ¹⁵N₂ molecules on Grafoil using the nuclear resonance photon scattering (NRPS) technique is presented. Molecular coverages between n = 0.6 and 5.0 commensurate monolayers (CML) at 20 K were studied. The tilts were determined by measuring the anisotropy ratio of resonantly scattered intensities from the 6324 keV level in ¹⁵N with the photon beam parallel and perpendicular to the Grafoil planes. The results at n≤1 CML are in good agreement with calculations based on molecular dynamic simulations (MDS). At n>1 CML, it was possible to deduce only the average tilt angle of N₂ molecules over all layers. However, by relying on the reported structural properties of N₂ multilayers obtained by n-diffraction (ND), the tilt of N₂ molecules in the various layers was deduced and compared with theoretical expectations. The measured anisotropy ratios seem to rule out the occurrence of a pinwheel (PW) structure at n<2.7 CML predicted by MDS and Monte Carlo (MC) calculations.