Dynamics in ultrathin liquid films studied by simultaneous dielectric spectroscopy (DRS) and organic molecular beam deposition (OMBD)

Real-time dielectric relaxation spectroscopy for a molecular beam deposited glass forming liquids is proposed as a versatile approach for the study of the dynamic glass transition in geometric confinement. To achieve the highest sensitivity down to monomolecular organic layers in a wide frequency range (0.1-10(7) Hz) during simultaneous deposition and desorption, we have used mu m spaced interdigitated electrodes under ultrahigh vacuum conditions. Experiments using glycerol deposited on fused silica at -40 degrees C revealed a dielectric glass transition process for a layer thickness as low as 0.7 nm. While its peak position hardly changes upon thickness reduction, a clear broadening is observed that implies an increasing heterogeneous mobility scenario for the thinnest films caused by molecules being part of a reduced (at the substrate) or enhanced (free surface) mobility layer. This finding is supported by desorption experiments that reveal a strong retardation of the desorption rate for films below 1 nm.