Ultra-thin layers on metal or dielectric substrates can be characterized in reflection mode using FTIR technique. Due to the surface selection rules on metal substrates, s-polarized light cannot interact with the adsorbate molecules independent of incident angles, whereas p-polarized light reaches the maxima of absorbance at grazing incidence angle. Therefore, ultra-thin layers on metal substrates will be measured with Grazing Incidence Reflection (GIR) or Infrared Reflection Absorption Spectroscopy (IRRAS) using an incidence angle of around 80°.
Things become less obvious changing to non-metallic substrates, since both p- and s-polarized light can be absorbed by the thin layers. The intensity of the absorption bands varies depending on the incidence angle. Bands can even switch from negative to positive and vice versa by changing the incidence angle or polarization. Therefore, to fully characterize a thin layer on dielectric substrates, measurements using at least two incidence angles and with both polarizations must be carried out and also transmittance can be a valuable approach.
Ultra-thin layers show typically very weak absorption bands in IRRAS spectra, e.g. ranging down to 10-3 au (absorbance unit) for a monomolecular layer with a thickness of few nanometers or even 10-5 au on a dielectric substrate. For such weak absorbance bands highest sensitivity of the instrument is required. In the right side picture IRRAS spectra of a self-assembled monolayer on Au substrate are compared. The blue spectrum shows the result measured in a VERTEX 70 purge spectrometer. The residual water vapor absorbance masks the weak sample absorbance bands in the finger print region. After applying the automatic water vapor and CO2 compensation function in OPUS software the red spectrum will be received.