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 plane taking properly into account non-normal incidence are reported. Spectra were recorded for the incidence angles alpha = 8°, 16°, 40° and 60°. For the first two angles the spectra were also analyzed under the assumption of normal incidence and the results were compared to those which resulted from the exact treatment. Accordingly, oscillator parameters of IR active modes obtained under the assumption of normal incidence show no 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 dielectric background tensor which show averaged standard deviations of about 2 % at 8° and about 9 % at 16°. As a consequence, if the resulting dielectric tensor functions are employed to calculate reflectance in the whole frequency range at higher angles of incidence (e.g. 40°) the errors are increased by about 100 % for 8° and by 900 % for 16°. In contrast, the data obtained by the four dispersion analyses under proper consideration of the non-zero angle of incidence are consistent among themselves and allow to reliably generate reflection spectra for other angles of incidence. Since the increase of computational effort due to the exact treatment is small we strongly suggest and encourage its use instead the simplified treatment.