THERMOSTABILITY AND PHOTODESORPTION OF LANGMUIR-BLODGETT FILMS

We report on thermodesorption and photodesorption studies of cadmium and magnesium arachidate Langmuir-Blodgett films using a newly developed optical reflection technique and Nomarsky light microscopy. The mechanism of thermodesorption can be described as a process where only those parts of a monolayer desorb that are exposed to air in the normal direction. However, molecules in the inner monolayers can also be removed by photo-desorption. This is understood within a model of defect-mediated desorption where the action of light is twofold: it creates defects and provides energy to overcome binding. The model explains the sigmoidal desorption kinetics and the independence of this kinetics on film thickness and is supported by the observation of grain structure during the desorption process. The latter indicates that crater-like defects are generated throughout the entire depth of the film.