THz spectroscopy for art conservation…and even more

THz time-domain spectroscopy (THz–TDS) is a suitable approach to study the state of preservation of cultural heritage made of organic materials in a non-destructive way. This technique highlights, in particular, the low–energy vibrational properties of these materials and the presence of the H-bonds between molecules [1]. We focus in particular on the study of modern artificially aged and ancient paper samples in order to determine in the THz spectra the distinctive features as a possible fingerprint of their state of aging and/or degradation. As water plays a significant role in the THz range, the behavior of THz spectra as a function of the hydration of samples was also investigated allowing discrimination between the spectral features induced by the simple presence of water and those due by the degradation of cellulose polymers in the paper. However, the use of the THz–TDS technique to establish the state of preservation of paper sheets poses significant challenges, in particular when measurements on low refractive index and thin samples, such as ancient documents, must be carried out [2]. We have successfully solved this problem by employing an efficient computational procedure to remove from the experimental signals the spurious interference effects generated by the Fabry–Pérot resonances in single freely standing paper sheets. The THz absorption spectra were then explained in terms of absorption peaks of the cellulose crystalline phase superimposed to a background contribution due to a disordered H-bonds network with a clear dependence from the degradation of samples [3]. DFT simulations confirmed that the spectral profile is composed of several peaks associated with long-range cellulose crystal phononic modes. A comparison between theoretical and experimental results also showed the important role of water in cellulose polymers dynamics at THz frequencies. An experimental and theoretical DFT-based study of alpha-lactose was also performed. This compound possesses a chemical structure similar to those of cellulose but the lack of disordered regions allows a precise measurement of vibrational THz peaks. Furthermore, the knowledge of hydrated and non-hydrated crystal structures allowed us to study the effect of water molecules on their vibrational properties. Due to the very importance of water in determining the THz behavior of organic material, we also focus our attention on the measurement and analysis of the dielectric response of pure water and chloride solutions in the range 0.2-1.6 THz. We were able to satisfactory represent the permittivity of solutions as the weighted sum of two single-parameter Debye relaxations describing clusters of correlated water molecules coexisting with poorly or uncorrelated single dipoles. Finally, since cultural heritage materials are in general complex and disordered systems, we have investigated the propagation of THz waves in disordered media with a refractive index varying on the scale of the wavelength. Results showed evidence of a resonant transport supported by finite-size scatterers that strongly affects the velocity of propagation of light and breaks the diffusion approximation.