Thermodesorption of ultrathin organic films studied by reflection

Optical reflection measurements are described as a new and simple technique to measure thicknesses and thickness changes of ultrathin organic overlayers on inorganic support. The prime advantages are high sensitivity in thickness measurements (0.1–1 Å), applicability under varying environmental conditions, nondestructiveness, applicability on different supports and high time resolution. These features are assessed theoretically and experimentally studying Langmuir-Blodgett films on SiO₂ and Au supports. These films, in this work prepared with the model compound arachidic acid, provide the salient feature of well-defined thicknesses in integer multiples of 26.7 Å. Application of the technique in thermodesorption experiments reveals distinguished binding states with binding energies (73–110 kJ/mol) depending on the counterion (Mg²⁺ or Cd²⁺) used in preparing the films and ratios of different states depending on thickness. The binding energy for the main component of multilayers (73 and 81 kJ/mol) with thickness larger than 100 Å is comparable to the heat of evaporation of bulk fatty acids.In addition an analytic expression between layer thicknesses and reflection is derived from the Fresnel equation.