Raman spectroscopy and UV pulsed laser: an excellent symbiosis?

The fundamental idea of the present work concerns photon technology performance. On the one hand, in order to improve the Raman spectral quality when pigment analysis is carried out, we use UV pulsed laser radiation (266 nm) and its non‐thermal photoablation‐induced effect. In this way, the unwanted superficial layers (old varnishes, for example, in easel paintings) can be locally removed and so a reduction of the total fluorescence in the spectrum of the analyzed zone is achieved. On the other hand, for the purpose of analyzing the quality of the process, we employ Raman spectroscopy to obtain molecular information about the previous and later compositions of the surface irradiated with UV pulses. The fluorescence reduction of the Raman spectra and its consequent spectral quality improvement are quantified by a proposed figure of merit (signal‐to‐noise ratio) which is calculated in a suitable spectral range. The UV pulses are obtained starting an Nd:YAG pulsed laser (1064 nm) in combination with non‐linear optical crystals, and the Raman system used in the analytical measurements is a Jobin Yvon Induram with optical fiber technology and an HeNe continuous laser (632.8 nm). A Haas shutter guarantees the spatial focusing of the optical pulses over a concrete point of the irradiated sample. Based on the experimental results, two conclusions can be established: first, non‐thermal photoablation is very effective at removing small areas of the superficial varnished layers; second, Raman spectroscopy and the proposed figure of merit are very useful for measuring the resulting improvement in spectral quality. Copyright © 2004 John Wiley & Sons, Ltd.