Infrared Optical Functions of Water Retrieved Using Attenuated Total Reflection Spectroscopy

Abstract

Comprehensive modelling of non-polarized infrared attenuated total reflection spectrum based on Fresnel equations and wavenumber-dependent dielectric function models of isotropic materials is shown to be a suitable and easy methodology to retrieve optical functions. The scheme is completely general and can be used even for strong dispersion and absorption resonances. Attenuated total reflection spectra in liquid water, measured in the spectral region 100 - 4400 cm-1 with diamond and germanium as internal reflection elements, were used to illustrate and test the method. The refractive index of water computed from the dispersion analysis is critically compared with literature data.

Overcoming the Contact Problem in Quantitative Attenuated Total Reflection Spectroscopy Analysis of Flat Samples

“…This equivalent arrangement consists of three isotropic and homogeneous media, with parallel interfaces: semi-infinite medium 0 (diamond, which constitutes the IRE), medium 1 (vacuum gap), and semi-infinite medium 2 (sample). The procedure here adopted to model the poor contact ATR spectra, based on Fresnel equations and dielectric function models, is similar to the method for perfect contact conditions discussed in detail by Vieira 3 ), except that now an additional layer is introduced to describe the effect of a gap between the IRE and the sample.…”

“…12. Notice that the parameter ε ∞ (high-frequency dielectric constant) was not allowed to vary in the fitting procedure, because it is not possible to extract it from ATR measurements 3 ; ε ∞ was assigned a value of 2.434, the square of the refractive index of polystyrene at about 6000 cm –1 taken from Zhang et al 13 The gap d was taken as zero in the case of perfect contact conditions and as a fitting parameter in the case of noncontact spectra. In the latter case, the fitted value of the vacuum gap is d = 32 nm.…”

“…For more quantitative analysis, it is possible to determine the optical functions of the materials by simulating the spectrum using Fresnel's equations and an appropriate dispersion model. 3 However, if perfect contact between the sample and IRE is not ensured the results are not accurate.…”