Infrared reflection-absorption spectroscopy using the photoelastic modulator

Infrared reflection-absorption spectroscopy (IRRAS) is perhaps the most important IR technique for studying and monitoring chemical species adsorbed on a metal surface. Because only a small number of molecules are adsorbed on the surface, the IR absorption signal is small. In order to improve the experimental sensitivity and spectral signal-to-noise ratio (S/N), polarization modulation using a photoelastic modulator (PEM) has been developed for IRRAS, This article describes, on the ''how-to'' level, double-modulation IRRAS instruments and discusses different demodulation approaches for obtaining the IRRAS signal. Such an instrument can be based on either a dispersive or Fourier transform-infrared (FT-IR) spectrometer.