POLARIZATION MODULATION FOURIER-TRANSFORM INFRARED REFLECTANCE MEASUREMENTS OF THIN-FILMS AND MONOLAYERS AT METAL-SURFACES UTILIZING REAL-TIME SAMPLING ELECTRONICS

The technique of polarization modulation Fourier transform infrared (PM-FTIR) spectroscopy is applied to the reflectance spectra of thin polymer films and spontaneously organized monolayers adsorbed onto gold, silver, and chromium surfaces. The differential PM-FTIR reflectance spectra are obtained by the photoelastic modulation of the FTIR beam polarization and a novel real-time sampling methodology that generated the average and differential FTIR interferograms from measurements of the infrared signal during each modulation cycle. In comparison with conventional electronics that utilize a lock-in amplifier, the real-time electronics permit the operation of the FTIR spectrometer at normal mirror velocities. The use of polarization-independent optics after the metal surface ensures that the true surface infrared differential reflectance spectrum is obtained. The theoretical wavelength dependence of the PM-FTIR spectrum is rederived for the case of the real-time sampling measurement and compared to the experimental data. Spectra of a 15-nm film of polylimide on chromium, a spontaneously organized monolayer of octadecanethiol on gold, and a spontaneously organized monolayer of arachidic acid on silver are shown to demonstrate the applicability of the method to different metals, samples, and spectral regions.