Temperature-controlled portable Raman spectroscopy of photothermally sensitive pigments

In this work, an innovative NIR Raman device (excitation wavelength at 1064 nm) was developed in order to avoid thermal stress and consequent chemical alterations of the materials analyzed. In particular, we devised and tested for the first time a sensored Raman probe allowing for temperature-controlled measurements based on a thermoelectric sensor providing the feedback signal for suitably modulating the output power of the laser source and then limiting undesired heating effects within the irradiated volume. The experimentation was carried out on cinnabar, lead white and indigo pigments frequently used during the past centuries, which presents pronounced photothermal instability. The results achieved in a set of instrumental and analytical tests using different measurement control parameters allowed demonstrating the effectiveness and reliability of the present approach for preventing thermal alterations effects during Raman spectroscopy and speeding the measurements, as well as for monitoring spectral variations associated with the crystals anharmonicity over large temperature ranges. These features along with the portability of the novel device can make in situ Raman characterisation of valuable painted surfaces including photosensitive materials very safe and efficient.