POLARIZED EXTERNAL INFRARED REFLECTION ABSORPTION SPECTROMETRY AT THE AIR-WATER-INTERFACE - COMPARISON OF EXPERIMENTAL AND THEORETICAL RESULTS FOR DIFFERENT ANGLES OF INCIDENCE

Hexadecan-1-ol monolayers were investigated at the air/water interface by external infrared reflection-absorption spectrometry. It is shown that the molecules are oriented vertically, packed in a hexagonal subcell and prefer an all-trans conformation. The symmetrical and anti-symmetrical methylene stretching vibrations are investigated for p- and s-polarization and different angles of incidence. For s-polarization the reflection-absorption decreases with increasing angle of incidence, whereas for p-polarization the band converts its sign from negative to positive near the Brewster angle. An optical model is derived which describes the reflection-absorption for p- and s-polarization at different angles of incidence for an anisotropic, inhomogeneous surface layer on an isotropic low-absorbing substrate. Comparison of the curves calculated by this model with experimental results shows good agreement for s-polarization, whereas for p-polarization a discrepancy between experimental and theoretical data is observed in the range 40-70-degrees. Possible reasons for this deviation are discussed. In particular, evidence is presented that for low-absorbing substrates variations of the components of the real and imaginary parts of the refractive indices are closely related to strong changes of the reflection-absorption of p-polarized radiation near the Brewster angle.