Dispersion analysis of non-normal reflection spectra from monoclinic crystals

Results of the first dispersion analyses of polarized reflection spectra from a monoclinic a-c crystal plane taking properly into account the monoclinic symmetry and non-normal incidence on the crystal surface are reported. Spectra were recorded for the incidence angles α = 8°, 16°, 40° and 60°, and for each at four different polarization angles. For the first two incidence angles the spectra were additionally analyzed under the assumption of normal incidence and the results were compared to those which resulted from the exact treatment. The comparison focused on two different aspects. The first aspect concerns the validity of the oscillator parameters. Accordingly, oscillator parameters of IR active modes obtained by dispersion analysis under the assumption of normal incidence do not show systematic errors despite the simplified treatment. This is a consequence of the fact that errors due to the incorrect assumption of normal incidence are compensated mainly by increases of the diagonal terms of the high frequency dielectric tensor. The corresponding values obtained under the assumption of normal incidence show averaged standard deviations of about 2% at 8° and about 9% at 16°. The second aspect investigated focuses on the employment of the oscillator parameters to generate the dielectric function tensor for optical calculations. Here we found that, as a direct consequence of the increased high frequency dielectric tensor, errors substantially increase if the resulting dielectric tensor functions are employed to calculate the reflectance at higher angles of incidence. Therefore, and since the increase of computational effort due to the exact treatment is small, we strongly suggest and encourage its use instead of the simplified treatment.