This PMIRRAS technique exploits the fact that when infrared
light is reflected by a conducting surface, the amplitude of
the s- and p-polarized electric field vectors near the surface
differ strongly. The s-polarized electric field amplitude is
approximately zero within a distance of the order of one
wavelength of the surface, whereas the p-polarized electric
field amplitude is larger in this region.
Therefore, if a
conducting substrate bears a film whose thickness is much less
than the wavelength of the infrared light, then reflected
p-polarized light is modified by the absorption spectrum of
the film, whereas reflected s-polarized light will contain no
signal from the film. The p-polarized reflected light spectrum
can thus be treated as the signal with the s-polarized
spectrum being used as the background to be
subtracted.
PM-IRRAS instruments effectively achive this subtraction by
rapidly modulating the polarization and using a lock-in
amplifier to extract the signal. IRRAS spectra with a very low
signal- to-noise ratio can thus be obtained.