Polarization Modulation Infrared Reflection Adsorption Spectroscopy (PM-IRRAS) is a powerful surface specific spectroscopic method for thin films and floating monolayers. Compared to more conventional IRRAS, the method utilizes the differences in reflectivity of interfaces for p-polarized (perpendicular to surface) and s-polarized (planar to surface) light. This makes even the characterization single molecule thin layers and molecule orientation possible.
In IR spectroscopy, photons of specific wavenumbers between 102 and 104 cm-1 induce a transition of the molecule under study from a vibrational ground state to an excited state. The molecule in the excited state returns quickly to the ground state by thermal relaxation, which is seen as molecular vibrations. This means that when IR radiation is transmitted or reflected from a sample (solid, liquid or gas) radiation is absorbed and molecular vibration is induced. Examination of the transmitted/reflected light reveals how much energy was absorbed at each wavelength. The amount and frequency that is absorbed can be correlated with the molecular structure of the sample, which gives each material its own spectral ‘fingerprint’. Additionally, functional groups have their characteristic frequencies where they absorb.
The technology of the KSV NIMA PM-IRRAS is based on a reflective IR measurement in combination with a polarization unit.
Fresnel equations help describe the amount of reflected (and transmitted) light when light is reflected from a surface. The incoming light can be split into a polarized component parallel to the plane of reflection and a component perpendicular to this plane (see figure below). By knowing that light with different polarizations interacts differently with the interface it is reflected from, it is possible to use polarization modulation to reduce the noise of reflective FTIR-measurements and to compensate for the water vapor absorption bands.
KSV NIMA PM-IRRAS uses a photo-elastic modulator to modulate the polarization of the light and the intensity modulation of the spectrometer (see figure below). The modulations that allow signals to be separated and collected by the detector have different frequencies.
The measured signal at the detector consists of a high frequency component based on the difference (Δ) in light intensities between the p- and s-polarized light in the photo-elastic modulator, and a low frequency component based on the sum (Σ) of these signals from the FTIR unit. The normalized differential reflectivity spectrum S is calculated from the collected difference (ΔR) and sum spectra (ΣR) of the detected intensities of the p- and s- polarized light.
The PM-IRRAS method allows determination of the molecular orientation of the functional groups and the whole molecule. In floating monolayers, the PM-IRRAS has a strong incident angle dependency and this can be used to determine the orientation. Also, the relative peak ratios of functional groups of known orientation can be used to determine the tilt of the molecules compared to the surface. An effect called ‘surface image selection rule’ that is present in PM-IRRAS performed on good conductors leads to enhancement of perpendicular and cancellation of planar dipoles in the spectra. This can be utilized to establish the molecular orientation by comparing peak intensities, or by calculating from a theoretical spectrum.
Changes in the PM-IRRAS signal intensity and position can be used to infer:
- Molecular absorption/desorption behavior and kinetics
- Molecular packing and orientation
- Phase transitions
- Different surface reactions in a thin film
- Griffits, P and Haseth, J., ‘Chemical Analysis 83: Fourier Transformation Infrared Spectrometry’ (1983), Wiley
- V. Zamlynny, I. Zawisza, J. Lipkowski, ‘Langmuir’ 19 (2003) 133
- Kosters, P., ‘FT-IR Spectroscopy of thin biological layers’ (2000), University of Twente