Observation of molecular layering in thin liquid films using X-ray reflectivity

We report the direct observation of internal layering in thin (similar to 4 5-90 Angstrom) liquid films of nearly spherical, nonpolar molecules, tetrak is(2-ethylhexoxy)silane, using synchrotron x-ray reflectivity. The Patterso n functions have secondary maxima indicating layer formation, and model-ind ependent fitting to the reflectivity data shows that there are three electr on density oscillations near the solid-liquid interface, with a period of s imilar to 10 Angstrom (consistent with the molecular dimensions). The oscil lation amplitude has a strong inverse dependence on the substrate surface r oughness.