Synchrotron X-ray Specular Reflectivity Measurements of Dotriacontane Films Adsorbed on a Ag(111) Surface

Alkane films adsorbed on metal surfaces are of interest as model lubricants. To investigate the structure of both solid and liquid films of intermediate-length alkanes, we have conducted a series of x-ray specular reflectivity measurements as a function of temperature on dotriacontane ($n$-C$_{32}$H$_{66}$ or C32) films vapor-deposited on a single-crystal Ag(111) surface in UHV. The initial C32 coverage was sufficient to observe coexistence of a multilayer film with preferentially oriented bulk C32 particles. After heating the samples to remove the bulk particles, we obtained specular reflectivity curves at room temperature consistent with one complete C32 layer followed by partial second and third layers of progressively smaller occupancy. In each layer, the molecules are oriented with their long axis parallel to the surface. We find no evidence of a perpendicular monolayer structure as observed for C32 films deposited from solution.$^{2}$ From heating studies, we determine the melting and desorption temperatures of the first and second C32 layers.$^{ 2}$H. Mo \textit{et al}., Chem. Phys. Lett. \textbf{377}, 99 (2003).