Nonlinear optical microscopies (NLOM) are emerging techniques for the diagnosis of cultural heritage providing morphological, structural and compositional information on a large variety of artistic materials and objects. For paintings, it has been recently reported that valuable information about composition, layer thickness and state of conservation can be obtained by NLOM [1-3]. Although NLOM is a non-invasive technique, ensuring a correct analytical protocol requires the determination of the laser power thresholds that allow measurements under safe conditions, an aspect especially important when studying sensitive materials such as paintings. In this work, we propose a new methodology to define the range of average powers of the near infrared, femtosecond pulsed laser used for NLOM in the modality of multi-photon excitation fluorescence (MPEF), based on the observation of changes induced in the intensity of axial profile of the emitted MPEF signal upon repetitive irradiation, which were considered as the signature of possible photochemical alteration. The study was carried out on a series of modern acrylic paint samples and allowed for the determination of safe average laser power boundaries for the non-invasive determination of thickness of paint layers. For each irradiation condition, MPEF images were also acquired by raster scanning the sample surface. MPEF measurements were complemented with acquisition of Raman spectra before and after repetitive irradiation of the sample. Besides, final off-line verification was also performed through optical microscopy (OM) imaging. FIGURE Combined analysis on Permanent Green Light acrylic paint: a) MPEF z-profiles acquired by irradiating the samples with an average laser power of 6 mW, b) relative fluorescence intensity images and c) optical microscope images acquired before (t=0) and after (t=5) the five z-scans (Measure 1-5). The irradiated area is highlighted by the dotted square. d) Raman spectra measured in the purposely damaged area (black line), in a non-irradiated area (red line) and in the irradiated area after five scans. Notes and References 1 Oujja M., Psilodimitrakopoulos S., Carrasco E., Sanz M., Philippidis A., Selimis A., Pouli P., Filippidis G., Castillejo M. Phys. Chem. Chem. Phys. 2017, 19, 22836-22843 2 Liang H., Mari M., Cheung C.S., Kogou S., Johnson P., Filippidis G. Opt. Express 2017, 25, 19640¿19653 3 Dal Fovo A., Oujja M., Sanz M., Martínez-Hernández A., Cañamares M.V., Castillejo M., Fontana R. Spectrochim. Acta A 2019, 208, 262-270

3rd. USTS 2019 Meeting, Madrid, November 6th to 8th, 2019