External reflection IR spectroscopy
ERIRS
IR-ERS

The relevance of external reflection IR spectroscopy in heritage science has experienced a constant grow in the last two decades owing to analytical peculiarities that make it an extremely useful tool to answer the questions posed by the study and conservation of art-historical and archaeological materials. High versatility, sensitivity, molecular specificity, noninvasiveness and contactless are all crucial requirements that IR spectroscopy fulfills allowing for comprehensive compositional studies of artworks and cultural heritage materials (inorganic and organic materials). External reflection is a non-contact acquisition mode in which light propagates from a medium (i.e. air with n = 1) toward a sample (with n > 1). In these conditions two types of reflection take place: surface reflection and volume reflection. Surface reflection also called front-surface reflection since it corresponds to light which is not (or slightly) penetrating the analysed surface. It is ruled by the Fresnel’s law at a normal incidence depends on the real part (n, representing the light dispersion) and the imaginary part (ik, with k representing the absorption index) of the complex refractive index ñ (ñ = n + ik) as it follows: 𝑅 =[(𝑛 − 1)^2 + 𝑘^2]/[(𝑛 + 1)^2 + 𝑘^2] Where n has a typical dispersion with the wavenumbers and assumes a derivative-like profile in correspondence of the absorption bands; k is proportional to the absorption coefficient of the Lambert Beers law thus it changes along the wavenumber axis similarly to an IR absorption profile When k < 1 (most of the organic compounds, including polymeric systems) R follows mainly the behaviour of n, namely inflection points replace the IR absorption bands. When k≫1 (inorganic compounds containing oxyanions, i.e. carbonates, sulphates, phosphates), R tends to unity. The strong absorption hampers radiation to penetrate the sample in that energy range due to the strong change of n; as a result, light is mainly reflected (at these wavenumbers the sample behaves as a metallic surface) giving rise to a maximum of reflection at the maximum absorption (inverted band); this phenomenon is better known as reststrahlen effect. In volume reflection, IR penetrates the sample undergoing refraction, reflection, and scattering prior to be reflected out from the surface in all the directions (in fact it is also called diffuse reflection). Volume reflection gives rise to spectra like those recorded in transmission mode, but with variation in the band relative intensities and with band broadening mainly linked to the degree of light penetration. In external reflection mode, generally, cultural heritage materials show spectral distortions arising from the variable contributions of both volume and surface reflection, depending from the roughness and absorption properties of the sample surface which vary along the IR spectral range (generally including both the mid and near IR ranges 10000-350 cm-1).

Fourier transform infrared spectroscopy

Infrared Spectroscopy

Analysis Method

1.0