Neutron scattering studies of the structure and dynamics of methane absorbed on MgO(100) surfaces

Abstract

Although neutrons are not normally viewed as surface-sensitive probes, elastic and high-resolution inelastic neutron scattering (INS) techniques are surprisingly effective ways to investigate the structural and dynamical properties of films adsorbed on solid surfaces. In fact, in cases where the adsorption takes place on crystalline powder samples with large surface-to-volume ratios and with highly uniform surfaces, neutron data can yield remarkably detailed pictures of gas–solid interactions. As an illustration of the power of this approach we will describe here what has been learned with neutrons about the structure and rotational dynamics of methane films adsorbed on MgO(1 0 0) surfaces. Since the sensitivity of such investigations is largely determined by the quality of the adsorbates employed we will also briefly describe how the MgO powders used in these studies were prepared and characterized. Elastic neutron diffraction data show that at low temperatures solid methane films on MgO surfaces exhibit a layer-by-layer growth pattern essentially identical to the growth of the (1 0 0) face of the bulk solid. It is not possible, however, to determine from the diffraction data how the molecules are oriented relative to the MgO surface plane. Interestingly, questions relating to the orientational order of the molecules can be addressed by studying low-temperature rotational tunneling in monolayer methane films with high-resolution INS. Unlike the simple, two-line tunneling spectrum observed in bulk methane, the monolayer film produces a more structured spectrum which, when analyzed, is found to be only consistent with a dipod-down configuration of the molecules (C 2v axis normal to surface plane). We have been able to follow the evolution of the tunneling spectrum as the film builds progressively, layer-by-layer, from one-to-six layers. Strong indication of a crossover from 2D–3D behavior are seen.