External reflection Fourier transform infrared spectroscopy: theory and experimental problems

In the reflection mode of spectroscopic measurements, IR radiation impinges on the surface of a sample and the intensity of the specularly reflected light is recorded. This is an easy to use, non-contact and non-destructive sampling technique suitable for samples with flat and smooth surfaces. The spectra obtained via external reflection depend on a number of factors and differ considerably from standard transmission spectra. These differences are explained and experimental factors that affect the nature of external reflection spectra are discussed. Keywordr: Infrared spectrometry; External reflection External specular reflection spectroscopy is an IR spectroscopic sampling technique in which the intensity of the radiation reflected from the sample is measured versus the wavelength at the desired angle of incidence and polarization. This technique has been in use from the beginning of the century [l], and it has recently been widely applied to thin films on metal and semiconductor substrates [2-51. External reflection spectroscopy is ideally suited for samples such as thin films on metal substrates, adsorbed molecules on catalyst surfaces, thin layers of surfactants on semiconductors and adsorbed species on liquid surfaces. ’ Presented at the AustrianHungarian Conference on Recent Developments in Infrared and Raman Spectroscopy, Veszprth, Hungary, April 1820, 1990. The majority of the papers presented at this symposium have been published in Vibrational Spectroscopy, Vol. 1, No. 2 (1990). The description of the reflection of light at a single interface between two optical media is given by the Fresnel equations: r,; = n, cos 8 ~52; n: sin28