In this paper, we present a simple way to increase the sensitivity of polarization modulation infrared reflection–absorption spectroscopy (PM-IRRAS) for the study of ultrathin films deposited on dielectric and semiconductor substrates. The enhancement of the absorption band intensity is obtained by reducing the signal arising from the substrate. This is achieved by adding a polarizer after the sample in order to balance the polarized reflectivities of the sample. As a consequence, the contribution of the film to the PM-IRRAS signal is increased relative to that of the substrate. An enhancement factor of about 10 has been obtained for ultrathin organic films deposited on glass and spread at the air–water interface. This method has also allowed the study of the very thin native oxide layer present on silicon without the need for the reference spectrum of bare silicon.

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