Efficient removal of foxing from a medieval Ptolemaic map using a molecular fluorine laser at 157 nm
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In this communication, we have used a molecular fluorine laser at 157 nm to remove foxing marks, for the first time, from a medieval Ptolemaic map published in 1580 AD. By applying laser-ablation experimental techniques, the absorption coefficient and the threshold-energy fluence of the paper manuscript were determined . The absorption coefficient at 157 nm was found to be one order of magnitude higher than at 248 nm. There was no evidence of oxidising of the paper substrate following laser treatment of foxing areas at 157 nm (yellowish colour). In order to investigate the basic photo-chemical mechanism of the photo-dissociation dynamics of the paper’s cellulose fibres under vacuum ultraviolet irradiation, we applied mass-spectroscopic techniques. Mass spectroscopy reveals that there is breaking of the molecular bonds of the cellulose, even at low laser energy at 157 nm. The cellulose monomer is disintegrated into small photo-fragments, which fly apart with supersonic speed. There were no photo-fragments observed for m/e higher than 32 amu (atomic mass units of m/e). Fragments with two carbon atoms have a relatively higher probability to be dissociated from the parent cellulose molecule than heavier ones. These experimental findings suggest that the bound potential excited electronic states of the parent molecule correlate with dissociative excited electronic states of the molecular photo-fragments at 7.8 eV (λ=157 nm), in agreement with previous results for various organic polymers of similar chemical structure.
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