Terahertz Spectroscopy and Quantum Mechanical Simulations of Crystalline Copper-Containing Historical Pigments

Terahertz
spectroscopy, a noninvasive and nondestructive analytical technique
used in art conservation and restoration, can provide compelling data
concerning the composition and condition of culturally valuable and
historical objects. Terahertz spectral databases of modern and ancient
artists’ pigments exist but lack explanations for the origins
of the unique spectral features. Solid-state density functional theory
simulations can provide insight into the molecular and intermolecular
forces that dominate the observed absorption features as well as reveal
deviations from simple harmonic vibrational behavior that can complicate
these spectra. The characteristic terahertz spectra of solid azurite,
malachite, and verdigris are presented here, along with simulations
of their crystalline structures and sub-3.0 THz lattice vibrations.
The powerful combination of theory and experiment enables unambiguous
spectral assignment of these complex materials and highlights the
challenges that anharmonic peak broadening in organic-containing materials
may present in the construction of reference pigment databases.