Terahertz Spectroscopic Analysis of the Minium Pigment: Evidence of Low-Temperature Phonon Splitting.

Terahertz spectroscopy has been increasingly utilized as an effective nondestructive tool for the diagnosis, analysis, and restoration of artworks. In particular, in the case of artist's pigments, the terahertz probe reveals the vibrational modes that are unique to a given pigment species under study, motivating the ongoing efforts to establish a comprehensive terahertz spectral database of representative pigments. Standard archived spectra are typically acquired at room temperature and susceptible to spectral broadening, which often renders pigment identification difficult, if not impossible. In this paper, we report the frequencies of the vibrational modes of minium (PbO, red lead) by performing terahertz time-domain spectroscopy at room temperature and also at low temperatures. Clear absorption peaks appear at 54.9, 62.1, 71.3, and 83.9 cm at room temperature and blue-shift as the temperature decreases. In addition, new absorption peaks of 59.8 and 66.4 cm are observed below 150 K, which signify a structural phase transition occurring at 170 K in minium. Our results are expected to enhance our understanding of the vibrational activity of minium and suggest a future direction for how to improve and refine the existing terahertz spectral databases for pigment analysis.