Investigation of the molecular motion of self-assembled fatty acid films

We have investigated self-assembled stearic acid films adsorbed on an Al2O3 film by means of electron paramagnetic resonance (EPR) as well as near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The stearic acid molecules are orientated in the film as shown by the NEXAFS data. Doping of the film with spin-labels in order to make the films accessible to EPR spectroscopy distorts the structure only slightly as long as the concentration of the spin-labels is low. The temperature-dependent EPR spectra reveal a distinct dependence of the onset of the rotational motion of the spin-label with variation of the location of the spin-label along the chain. The analysis of the EPR line shape allows to gain insight into the internal motion of the molecular chain.