Infrared reflection absorption spectroscopy setup with incidence angle selection for surfaces of non-metals

Abstract

Infrared Reflection Absorption Spectroscopy (IRAS) on dielectric single crystals is challenging because the optimal incidence angles for light-adsorbate interaction coincide with regions of low IR reflectivity. Here, we introduce an optimized IRAS setup that maximizes the signal-to-noise ratio for non-metals. This is achieved by maximizing light throughput and by selecting optimal incidence angles that directly impact the peak heights in the spectra. The setup uses a commercial Fourier transform infrared spectrometer and is usable in ultra-high vacuum (UHV). Specifically, the optical design features sample illumination and collection mirrors with a high numerical aperture inside the UHV system and adjustable apertures to select the incidence angle range on the sample. This is important for p-polarized measurements on dielectrics because the peaks in the spectra reverse the direction at the Brewster angle (band inversion). The system components are connected precisely via a single flange, ensuring long-term stability. We studied the signal-to-noise ratio (SNR) variation in p-polarized IRAS spectra for one monolayer of CO on TiO₂(110) as a function of incidence angle range, where a maximum SNR of 70 was achieved at 4 cm⁻¹ resolution in a measurement time of 5 min. The capabilities for s polarization are demonstrated by measuring one monolayer D₂O adsorbed on a TiO₂(110) surface, where a SNR of 65 was achieved at a peak height ΔR/R₀ of 1.4 × 10⁻⁴ in 20 min.