PM-IRRAS mapping of ultrathin molecular films with high spatial resolution.

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) is a very sensitive technique to characterize the degree of molecular ordering in thin films on metallic surfaces. This is the first report of the coupling of a PM-IRRAS microscope to a free electron laser (FEL), a light source of highest brilliance. Some FELs emit in the infrared region and permit the mapping of molecular properties at high lateral resolution. We studied the molecular orientation of octadecanephosphonic acid (OPA) attached to a gold surface with microstructured aluminum oxide islands on the gold. The spatial resolution achieved is 12 microm which corresponds to the diffraction limit of the infrared light used in this study. This is a substantial improvement compared to previous studies using a PM-IRRA accessory together with a commercial Fourier transform infrared spectrometer, where the lateral resolution is noise-limited rather than diffraction-limited. The spectral maps reveal that OPA is preferably attached to the aluminum oxide islands via the bidentate binding mode whereas the tridentate mode is dominating in case of OPA attached to the gold areas.