Infrared reflection absorption spectroscopy with angle selection

Abstract

Performing Infrared Reflection Absorption Spectroscopy (IRAS) experiments on bulk oxide single crystals presents a challenge because the highest sensitivity to adsorbates on these surfaces coincides with regions of low infrared reflectivity. Unlike on metallic substrates, where the surface selection rules are valid, there is no enhancement of the IR signals originating from changing dipole moments normal to the surface. In addition, the band inversion occurring for measurements with p-polarised light on non-metallic substrates results in positive and negative oriented peaks, which, if a wide incidence angle range is used, can significantly reduce the signals.The IRAS system presented in this thesis has been developed to optimise measurement parameters for non-metallic substrates, thereby maximising the signal-to-noise ratio. It utilises the commercial FTIR spectrometer Bruker VERTEX 80v to conduct IRAS measurements under ultra-high vacuum (UHV) conditions in a custom-built UHV surface chemistry chamber. The optimisation is accomplished by a solid design framework focusing on maximising IR light throughput, selecting optimal incidence angle ranges and high mechanical stability. This achieves a maximum signal-to-noise ratio and high long-term stability. To realise a high-throughput optics, the optical design incorporates sample illumination and light collection from the sample using elliptical mirrors with high numerical aperture, placed in UHV. Additionally, two adjustable aperture plates are implemented to enhance the signal-to-noise ratio, enabling the selection of the incidence angle range. This feature is essential for adequately handling the band inversion. The optimum incidence angle range selection is determined based on the calculation of the normalised reflectivity difference delta(R/R0). This thesis expands the ideal picture of delta(R/R0) by examining the influence of incidence angle spread, depolarisation,and illumination of adsorbate-free sample areas, all decreasing the achievable signal intensities, with the depolarisation having the most significant impact.Furthermore, the long-term stability of the IRAS system is ensured by connectingall components to a central flange. In addition to performance-enhancing features, the IRAS system includes a sample-position finder. A cooling system for the optics in high vacuum eliminates the need to disconnect the IRAS during a bakeout and realign the optics after the bakeout, thus enhancing user-friendliness.To evaluate the performance of the system, 1ML CO on reduced TiO2(110) wasstudied by utilising various angle rages leading to an signal-to-noise ratio (SNR) of about 70 and a peak height of 1.83×10−3 in the delta(R/R0) spectrum. These results were obtained at a resolution of 4 cm−1 and 1000 scans within 5 minutes total measurement time for one IR spectrum. Additionally, the IRAS spectra of one monolayer D2O adsorbed on a hydroxylated TiO2(110) surface show the s-polarised performance of the IRAS system.In this case, a SNR of about 65 was achieved with a R/R0 peak height of 1.4 × 10−4, 4000 scans and a resolution of 4 cm−1 in 20 minutes.