Optimum condition of Fourier transform infrared multiple-angle incidence resolution spectrometry for surface analysis

A novel measurement technique of pure out-of-plane vibrational modes of thin films on a nonmetallic substrate has recently been proposed, which is named multiple-angle incidence resolution spectrometry (MAIRS). Since this technique could not be replaced by other conventional techniques, MAIRS was expected to be a promising tool for analysis of thin soft materials and surface adsorbates. Nevertheless, some experimental conditions have been found to be inappropriate for MAIRS, which yields incorrect results. In the present study, therefore, the problems in the technique have been investigated in terms of optics to improve the accomplishments of MAIRS. The problems have been found to have a strong relationship with optics in FT-IR, which is influenced by refractive index of the sample material and angle of incidence. In particular, optimization of the size matching of the detector surface and the infrared spot at the detector was a key to having MAIRS perform properly. It has been concluded that reliable MAIRS measurements require overfilling of the detector and a substrate with a high-refractive index.