ONLINE MONITORING OF LASER CLEANING OF LIMESTONE BY LASER-INDUCED BREAKDOWN SPECTROSCOPY AND LASER-INDUCED FLUORESCENCE

The application of laser-induced breakdown spectroscopy (LIBS) to moni tor the laser cleaning process of polluted limestone from a historic b uilding is examined. The combination of a Q-switched Nd:YAG pulsed las er with on-line diagnostics by the LIBS technique is shown to be very useful for controlling and characterizing the cleaning process in orde r to avoid overcleaning. In addition, the coupling of this spectroscop ic technique to the cleaning process provides important information ab out the optimal experimental conditions to be selected for achieving a n adequate cleaning procedure. Furthermore, the spectroscopic study of the plasma emission can be used to determine the elemental compositio n of both the black crust and the underlying stone. The application of LIBS as a diagnostic technique to monitor and control the laser clean ing process of limestone is based on the different elemental compositi on of the black encrustations covering the stone surface and the under lying stone. On the other hand, a different experimental setup for pro bing the ablation products by laser-induced fluorescence (LIF), in ord er to achieve a signal amplification of some atomic emission lines wit h weak intensity in the LIBS spectrum, is described.