Determining optimum irradiation conditions for the analysis of vermilion by Raman spectroscopy
Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas (IESL-FORTH), P.O. Box 1385, GR 711 10, Heraklion, Crete, Greece
2 Department of Chemistry, University of Crete, GR 710 03, Heraklion, Crete, Greece
3 Department of Materials, University of Oxford, Oxford, UK
Accepted: 15 November 2021
Published online: 30 November 2021
A thorough investigation has been conducted on the interaction of laser light with vermilion (α-HgS, mercury II sulfide), a common red pigment used by artists, during its analysis by Raman microscopy. The main goal of the study has been to establish quantitatively an appropriate regime of irradiation that on the one hand yields reliable analysis results while on the other ensures that no damage is caused during exposure of the pigment to the laser beam. The pigment was used neat, in powder form, and key irradiation parameters were examined including irradiance (laser beam power per surface unit area on the sample) focusing conditions (different objective lenses), time of pigment exposure to the laser beam and excitation laser wavelength. The main part of the study was carried out by use of a mobile Raman spectrometer equipped with a continuous wave (cw) laser source emitting at λexc = 785 nm. The intensity of characteristic vibrational bands of vermilion in the Raman spectra was monitored as a function of irradiance and a distinct threshold behavior was observed as regards laser-induced damage (LID) of the pigment. While below threshold the spectral band intensity exhibited a linear dependence on laser beam power or irradiance, right above the threshold, defined as laser-induced damage threshold (LIDT), spectra deteriorated with their intensity decreasing by over an order of magnitude and no new spectral features were observed. This decrease in intensity was observed even under very brief exposure of the pigment to the laser beam, less than 1 s, and was accompanied by a visible darkening of the pigment. In the case of illumination well above the LIDT even formation of a crater was observed. The abrupt appearance of pigment damage indicates that the main process underlying pigment alteration is most likely transformation of the red α phase of HgS to the thermodynamically more stable (under ambient atmosphere) β phase, known to be black in color, and to amorphous HgS. The LIDT values were dependent on the laser-irradiated surface area and in fact the higher magnification objective lens, namely the tightest beam focusing, produced a higher threshold value. In general, similar observations were made when a different NIR laser source was used, λexc = 1064 nm, while excitation at λexc = 473 nm and λexc = 532 nm, showed substantial LID effects, even at low irradiance values, attributed to the strong absorption of vermilion in the visible. Overall, this study establishes a general protocol for assessing laser irradiation effects on heritage materials and furthermore provides a methodological approach for maintaining excitation conditions within safe limits during Raman analysis of heritage objects.
© The Author(s), under exclusive licence to Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature 2021