Nuclear-resonance photon scattering study of N2O multilayers adsorbed on Grafoil at 12 K

The nuclear-resonance photon scattering technique was used for studying the out-of-plane orientation of NO molecules physisorbed on Grafoil at 12 K an d coverages n = 0.77 to 3.7 monolayers. By monitoring the resonantly scatte red intensity ratios from the 6324 keV level in N-15 with the planes of the Grafoil adsorbing surface set parallel and perpendicular to the direction of the incident photon beam, the average out-of-plane tilt of N2O relative to the graphite planes was obtained. The behavior of the system reveals a s triking similarity to that of the N-2+Grafoil system. Specifically, the til t angles of the N2O molecules with respect to the graphite surface are prac tically the same despite its more involved character. It is shown that the occurrence of a pure four-sublattice pinwheel ordering can be ruled out. Th e possibility of explaining the measured intensity ratios at 12 K, by assum ing partial dewetting of the N2O molecules (where the molecules in the firs t layer lay flat on the graphite surface while the overlayers consist of hi ghly disordered bulk) is discussed in some detail.