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 orderto 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-controlledmeasurements based on a thermoelectric sensor providing the feedback signal for suitably modulatingthe output power of the laser source and then limiting undesired heating effects within the irradiatedvolume. The experimentation was carried out on cinnabar, lead white and indigo pigments frequentlyused during the past centuries, which presents pronounced photothermal instability. The results achievedin a set of instrumental and analytical tests using different measurement control parameters alloweddemonstrating the effectiveness and reliability of the present approach for preventing thermal alterationseffects during Raman spectroscopy and speeding the measurements, as well as for monitoring spectralvariations associated with the crystals anharmonicity over large temperature ranges. These features alongwith the portability of the novel device can make in situ Raman characterisation of valuable paintedsurfaces including photosensitive materials very safe and efficient.